PT chart for different elevations

[dkalasz]

Hello,
Hopefully someone can assist me I work in Afghanistan with the military and i work a wildy varying elevations 5000 6000 up to 10000 the problem i have is i cannot find either a formula to calculate the proper p/t chart or charts at any other elevation that sea level and 5000 feet. do any of you have a formula to make a pt chart with elevation being taken into concideration?

thanks
david

[wambat]

Why don't you recalibrate your gauges to "0" lbs at what ever altidude your at?
because the system doesn't care about the altidude; it's a closed system.

[nike123]

Hello,
Hopefully someone can assist me I work in Afghanistan with the military and i work a wildy varying elevations 5000 6000 up to 10000 the problem i have is i cannot find either a formula to calculate the proper p/t chart or charts at any other elevation that sea level and 5000 feet. do any of you have a formula to make a pt chart with elevation being taken into concideration?

thanks
david
P/T Chart is about relation of pressure and temperature of gas and I don't see reason to take in consideration elevations.

[NH3LVR]

P/T Chart is about relation of pressure and temperature of gas and I don't see reason to take in consideration elevations.

Dkalasz has a good point.
The problem is that that the gauges we use are referenced to outside air. As the ambient pressure changes our gauge accuracy suffers.
This has never been a problem for me. Perhaps some of our folks from Denver (Called the Mile High City here in the USA), could comment on this.
Wambat has the solution. Simply recalibrate your gauges to zero-no problem.

Now I am going to have to figure out the difference at various altitudes. Just what I needed- a homework assignment on Christmas.

[Gary]

Connect the gauge to your refrigerant tank and calibrate to the temperature on the side of the tank. The only thing that is incorrect at various elevations is the zero.

[nike123]

Dkalasz has a good point.
Now I am going to have to figure out the difference at various altitudes. Just what I needed- a homework assignment on Christmas.

Then you need to consider pressure changes in the air caused by weather change. Like day when barometric pressure of the air is 970 hPa and day when it is 1025 hPa on same elevation. I still don't see what that got to do with P/T charts for refrigerants!:confused:
This pressure only have influence when we want to find boiling point of liquids who are exposed to open air, and as you stated when we want "calibrate" our gauges.

[NH3LVR]

Then you need to consider pressure changes in the air caused by weather change. Like day when barometric pressure of the air is 970 hPa and day when it is 1025 hPa on same elevation. I still don't see what that got to do with P/T charts for refrigerants!:confused:
This pressure only have influence when we want to find boiling point of liquids who are exposed to open air, and as you stated when we want "calibrate" our gauges.

Yes, the barometric pressures do affect our gauges. And it varies every day.
If we zero our gauges at sea level on a standard day we are calling 29.92 inches of mercury "0".
(Remember the Vacuum people start at absolute 0 and count up, we start at atmospheric and count down. A fact that confused me at BOC's vacuum school.)
Now imagine we are working at at 5000 feet. The barometric pressure would be around 24.89"Hg. A small difference, but enough to require us rezero our gauges if we wanted to check superheat.
The difference on the high side would be of little concern.
The P/T chart still applies, it is just that our gauges reference to the atmospheric pressure.
All this can be avoided by looking at your gauges before hooking them to a system. Of course if your gauges are equiped with the sealing ends, you need to purge them off first.
In short the problem is gauge error induced by lower pressure at higher altitudes.

[nike123]

All this can be avoided by looking at your gauges before hooking them to a system. Of course if your gauges are equiped with the sealing ends, you need to purge them off first.
In short the problem is gauge error induced by lower pressure at higher altitudes.

Just opposite my friend.;)
Problem is gauge error induced by adjusting gauges at high level. Ideally, they should be adjusted at see level and barometric pressure of 1000 hPa to avoid influence of height (and air pressure) in order to accurately measure pressure of refrigerant in system. To that error you need to add following properties of gauge:
a. Uncertainty
b. Resolution
c. Reproducibility
d. Long-term stability
e. Temperature stability
:eek:

[Gary]

Let's imagine that you calibrate your gauges at sea level.

You then find yourself at a much higher elevation where the gauge, when opened to atmosphere, reads a couple inches of vacuum.

Does this mean that the gauge is wrong?

No, the gauge is accurately reading the reduced atmospheric pressure at the elevated location. It is SUPPOSED to read below zero.

Do you need to adjust it?

No, it is correct.

Do you need to change your P/T chart?

No, it is correct. Everything is correct. Leave it alone.

But what if you cannot calibrate your gauges to atmospheric pressure at sea level?

Calibrate them to the temperature/pressure of a jug of refrigerant. They will then be accurate, regardless of elevation.

What you should NOT do is calibrate to zero at anything other than atmospheric pressure at sea level, because your gauges will then be inaccurate.

[NH3LVR]

Sorry, but I need to disagree with the last two posts.
The Gauge measures the difference in pressure between the atmosphere and the system it is connected to. If you do not start at zero the reading is meaningless
I calibrate gauges to read zero in engine rooms, (or at least I used to), on a regular basis. I disconnect the lines to the wall mounted gauges, or valve off the connection on screw compressors and adjust them to read 0 at atmosphere.
If I zeroed my gauges at 5000 feet I would have to rezero at sea level to have a accurate reading.
US and TX Iceman, where are you articulate guys when we need you?:)

[nike123]

The Gauge measures the difference in pressure between the atmosphere and the system it is connected to. If you do not start at zero the reading is meaningless


That is not the true! Bourdon type gauges (which we use in 90% cases as a gauges for measuring pressures in AC and refrigeration equipment) and Bellow spring loaded gauges don't measure difference between atmosphere and system, and adjusting screw is needed because Bourdon type gauge, after being under high pressure for long time, hawe tendency to temporarily deform.
http://www.tpub.com/machines/9c.htm

[Andy]

Hi Guys:)

the only real answer I know of is a set of glycerine filled gauges with a second set of bourdon tubes for altitude compensation.

Refcom make good service gauges and possibly altitude compensated.

What about electronic gauges, would they not be ideal in this situation.

I would say Gary is correct, one case where zeroing of the gauges would cause a problem. A cylinder of refrigerant would be very useful for calibration.

Kind Regards Andy :)

[NH3LVR]

That is not the true! Bourdon type gauges (which we use in 90% cases as a gauges for measuring pressures in AC and refrigeration equipment) and Bellow spring loaded gauges don't measure difference between atmosphere and system, and adjusting screw is needed because Bourdon type gauge, after being under high pressure for long time, hawe tendency to temporarily deform.
http://www.tpub.com/machines/9c.htm

Well, I stand corrected. What can I say?
Even an old time NH3 Engineer can be wrong once in awhile:D

[nike123]

That is why we are here, to share our knowledge and experience. I am sure that I am going to learn from you lot of things, especially in field of ammonia as refrigerant, as your nickname suggests. ;)

[dkalasz]

what has me confused is if the pt chart doesnt require changeing why does forane have this chart

secondly I was possibly thinking that i could convert all the readings to absolute i might then get the proper reading

the problem with calibrateing to the bottle is simple other people use the gauges and i will constantly be recalibrateing i would like to find a way to use the 0 as a reference point so i dont have to murdifie any of my co workers :)

[Gary]

what has me confused is if the pt chart doesnt require changeing why does forane have this chart

secondly I was possibly thinking that i could convert all the readings to absolute i might then get the proper reading

the problem with calibrateing to the bottle is simple other people use the gauges and i will constantly be recalibrateing i would like to find a way to use the 0 as a reference point so i dont have to murdifie any of my co workers :)

That chart is for people who zero out their gauges at 5000 ft elevation. An incorrect chart to match the incorrect gauges.

If you zero out your gauges at sea level, then you can use the sea level chart (at any elevation).

If you zero out your gauges at 5000 ft, then you can use the 5000 ft chart (at any elevation).

In other words, the chart you use doesn't depend upon the elevation at which you are working, it depends upon the elevation at which you zeroed out your gauges.

[Gary]

do any of you have a formula to make a pt chart with elevation being taken into concideration?


Start with the sea level chart. Add .5 psi for each 1000 ft above sea level. Use the chart for the elevation at which you zeroed out your gauges.

Seems like the hard way, but if that's what you want...

[Abby Normal]

The PT relationship only really cares about the absolute pressure.

I have never diven from sea level to the top of Pike's Peak and observed what happens to my gauges, but I would assume they would say zero when not connected to anything.

If you were up at 5000 feet, I would say if you read off the saturation temperature for a pressure about 2 pounds or so less than what the gauges read you would be close enough.

[Gary]

The PT relationship only really cares about the absolute pressure.

I have never driven from sea level to the top of Pike's Peak and observed what happens to my gauges, but I would assume they would say zero when not connected to anything.


But your gauges are connected to something. They are connected to atmosphere and should read atmospheric pressure, which at 5000 ft would be about 5 inHg... and if they are kept accurate (5 inHg at 5000 ft) then they will accurately reflect the pressure/temperature relationship as depicted on the accurate sea level P/T chart.

Nothing needs to be changed if the gauges are not mis-calibrated to zero at something other than sea level.

[Gary]

Here is another way you could do it:

If you are working at sea level, calibrate to zero, with gauges open to atmosphere.

At 1000 ft, calibrate to 1 inHG
At 2000 ft, 2 inHg
3000 ft = 3 inHG
4000 ft = 4 inHG
etc, etc, etc.

[dkalasz]

thanks,
all this will help, alot. the elevation here has been causeing all sorts of issues and when you have 2000 units to maintain you really get tired of not getting it right the first time.. of course i work for a goverment contractor and if i want the proper tools i had to order them I held out for a year but i cant stands no more :P anyway thanks for the information

[NH3LVR]

Now I am confused. Let me take this a step at a time.
Nike123 convinced me that atmosphere pressure has no effect on Bourdon style gauges.
Do we have a consensus on that?

[Gary]

If you were in outer space and opened your gauges, they would read deep vacuum... and they would accurately read refrigerant pressures. They would be inaccurate (by about 15 psi) if you zero them out.

If you are at 5000 ft, your gauges will read 5 inHG, because they are accurately measuring the reduced atmospheric pressure... and they will accurately measure refrigerant pressures. They will be inaccurate (by about 2.5 psi) if you zero them out.

This is not because the atmospheric pressure affects the gauges, it is because the gauges are accurately measuring atmospheric pressure.

[nike123]

Now I am confused. Let me take this a step at a time.
Nike123 convinced me that atmosphere pressure has no effect on Bourdon style gauges.
Do we have a consensus on that?
Only time when there is influence of atmospheric pressure is when they are not connected to anything.;)

[Abby Normal]

But your gauges are connected to something. They are connected to atmosphere and should read atmospheric pressure, which at 5000 ft would be about 5 inHg... and if they are kept accurate (5 inHg at 5000 ft) then they will accurately reflect the pressure/temperature relationship as depicted on the accurate sea level P/T chart.

Nothing needs to be changed if the gauges are not mis-calibrated to zero at something other than sea level.

We are talking about a guage not a barometer. Gauge is going to read pressure above atmospheric.

[Abby Normal]

If you were in outer space and opened your gauges, they would read deep vacuum... and they would accurately read refrigerant pressures. They would be inaccurate (by about 15 psi) if you zero them out.

If you are at 5000 ft, your gauges will read 5 inHG, because they are accurately measuring the reduced atmospheric pressure... and they will accurately measure refrigerant pressures. They will be inaccurate (by about 2.5 psi) if you zero them out.

This is not because the atmospheric pressure affects the gauges, it is because the gauges are accurately measuring atmospheric pressure.

If you were in deep space, the manifold was inside the shuttle and the the hose outside the shuttle, I would expect the needle to be buried at 30 inches.

Pull the hose back inside the shuttle and would expect it to read 0

[nike123]

We are talking about a guage not a barometer. Gauge is going to read pressure above atmospheric.
This sentence does not deny or dispute anything what Gary said.:confused:

[nike123]

If you were in deep space, the manifold was inside the shuttle and the the hose outside the shuttle, I would expect the needle to be buried at 30 inches.

Pull the hose back inside the shuttle and would expect it to read 0

Same as before, this sentence does not deny and dispute anything what Gary said.

[Abby Normal]

This sentence does not deny or dispute anything what Gary said.:confused:
Perhaps I mistook what Gary said.

It sounded like if you were down at sea level with the gauges on the seat of your truck,'open', and began driving up the mountain, the needle would move.

[Abby Normal]

The guage measures how much pressure you have above atmospheric. The temperature scale on the gauge is based on sea level.

At the 5000 feet, read the saturation temperature that corresponds to a couple pounds less pressure.

[Abby Normal]

The gauge is doing its job, it is the fact that the temperature scales are based on gauge pressures above sea level which is the problem.

[nike123]

Perhaps I mistook what Gary said.

It sounded like if you were down at sea level with the gauges on the seat of your truck,'open', and began driving up the mountain, the needle would move.

That is right! It will move toward -1Bar!
Question is, does that move will going to be visible with naked eye regarding gauge accuracy and resolution.

[Abby Normal]

In a Bourdon tube gauge, a "C" shaped, hollow spring tube is closed and sealed at one end. The opposite end is securely sealed and bonded to the socket, the threaded connection means. When the pressure medium (such as air, oil, or water) enters the tube through the socket, the pressure differential from the inside to the outside causes the tube to move. One can relate this movement to the uncoiling of a hose when pressurized with water, or the party whistle that uncoils when air is blown into it. The direction of this movement is determined by the curvature of the tubing, with the inside radius being slightly shorter than the outside radius. A specific amount of pressure causes the "C" shape to open up, or stretch, a specific distance. When the pressure is removed, the spring nature of the tube material returns the tube to its original shape and the tip to its original position relative to the socket.

With the guage open as you went up in elevation, what pressure differential makes the needle move?

[Abby Normal]

With the manifold shut, the needle would move. Open it at high altitude and it should go back to zero as long as the ride up the mountain was not too bumpy?

[nike123]

With the guage open as you went up in elevation, what pressure differential makes the needle move?

A specific amount of pressure causes the "C" shape to open up, or stretch, a specific distance. When the pressure is removed, the spring nature of the tube material returns the tube to its original shape and the tip to its original position relative to the socket.:rolleyes:

[nike123]

With the manifold shut, the needle would move. Open it at high altitude and it should go back to zero as long as the ride up the mountain was not too bumpy?
Based on what?:confused:

[Abby Normal]

A specific amount of pressure causes the "C" shape to open up, or stretch, a specific distance. When the pressure is removed, the spring nature of the tube material returns the tube to its original shape and the tip to its original position relative to the socket.:rolleyes:

maybe re-read the part I had highlighted in bold italic, if the pressure was atmospheric 'inside and outside' it should read zero. If it reads zero at sea level, it would read zero at 5000 ft

[Abby Normal]

Based on what?:confused:based on the fact that I do not think refrigeration guages are aneroid barometers

[nike123]

I don't see what part of sentence marked red you don't understand?
You have shape on one side and pressure on other side.
Shape is changed according to pressure exerted at inlet at C shape tube. There is no influence (at least significant to measuring result and precision class of instrument) off any other pressure at C shape except that at inlet.
It could not be simplest than that!

[Gary]

Barometric is just another pressure, which should be accurately reflected by your gauges.

[Gary]

Perhaps I mistook what Gary said.

It sounded like if you were down at sea level with the gauges on the seat of your truck,'open', and began driving up the mountain, the needle would move.

Exactly so... and if the mountain went above the atmosphere into outer space, as you have yourself noted, the needle would read near 30inHg.

[nike123]

Barometric is just another pressure.

Prejudice has always been hard to change!;)

http://en.wikipedia.org/wiki/Prejudice

[Abby Normal]

Exactly so... and if the mountain went above the atmosphere into outer space, as you have yourself noted, the needle would read near 30inHg.

A gauge, on a manifold, open, old style hoses without check valves, when connected to nothing, will always read zero. The atmospheric pressure pushes in the tube and outside of the tube the same.

An aneroid barometer, containing an evacuated capsule in it, will notice the change in atmospheric pressure.

So with your gauge, take a pressure reading on the AC system up at 5000 ft. Measure the suction line temperature and compare it to the saturation temperature that corresponds to a pressure 2.5 psi less than what you are reading and all is well.

[Abby Normal]

I don't see what part of sentence marked red you don't understand?
You have shape on one side and pressure on other side.
Shape is changed according to pressure exerted at inlet at C shape tube. There is no influence (at least significant to measuring result and precision class of instrument) off any other pressure at C shape except that at inlet.
It could not be simplest than that!

I am arguing that you do not have to zero your gauges when you go up and down in elevation. I think you must be arguing some other point.

[Gary]

The atmospheric pressure pushes in the tube and outside of the tube the same.


The atmospheric pressure on the outside of the tube has absolutely no effect on the needle movement.

[nike123]

A gauge, on a manifold, open, old style hoses without check valves, when connected to nothing, will always read zero. The atmospheric pressure pushes in the tube and outside of the tube the same.

Very wrong. If they are calibrated at sea level at day when atmospheric pressure is 1000hPa they will read 1Bar absolute or 0Bar relative pressure. If weather change and you have 1080hPa barometric pressure at sea level, you are going to read, on same gauges, 0,08 Bar gauge or 1,08 Bar absolute pressure (depend of gauge scale).
You will not have 0bar reading because these gauges don't measure difference between outdoor and connection port pressure. They are measuring amount of distortion of shape as consequence of force exerted at C shape from connection port.


An aneroid barometer, containing an evacuated capsule in it, will notice the change in atmospheric pressure.

Also Bourdon type manometer! Question is, does the resolution of instrument allow to be clearly visible.

[Abby Normal]

The atmospheric pressure on the outside of the tube has absolutely no effect on the needle movement.
So then this discription is total BS?

http://www.refrigeration-engineer.com/forums/images/misc/quotes/quot-top-left.gifQuote:http://www.refrigeration-engineer.com/forums/images/misc/quotes/quot-top-right.gifhttp://www.refrigeration-engineer.com/forums/images/misc/quotes/quot-top-right-10.gifIn a Bourdon tube gauge, a "C" shaped, hollow spring tube is closed and sealed at one end. The opposite end is securely sealed and bonded to the socket, the threaded connection means. When the pressure medium (such as air, oil, or water) enters the tube through the socket, the pressure differential from the inside to the outside causes the tube to move. One can relate this movement to the uncoiling of a hose when pressurized with water, or the party whistle that uncoils when air is blown into it. The direction of this movement is determined by the curvature of the tubing, with the inside radius being slightly shorter than the outside radius. A specific amount of pressure causes the "C" shape to open up, or stretch, a specific distance. When the pressure is removed, the spring nature of the tube material returns the tube to its original shape and the tip to its original position relative to the socket.

[nike123]

I am arguing that you do not have to zero your gauges when you go up and down in elevation. I think you must be arguing some other point.

Maybe you think that, but you don't exactly said that!:eek:
I agree that you do not have to zero your gauges when you go up and down in elevation.;)
In fact I am totally opposed to that action!

[Gary]

So then this discription is total BS?

http://www.refrigeration-engineer.com/forums/images/misc/quotes/quot-top-left.gifQuote:http://www.refrigeration-engineer.com/forums/images/misc/quotes/quot-top-right.gifhttp://www.refrigeration-engineer.com/forums/images/misc/quotes/quot-top-right-10.gifIn a Bourdon tube gauge, a "C" shaped, hollow spring tube is closed and sealed at one end. The opposite end is securely sealed and bonded to the socket, the threaded connection means. When the pressure medium (such as air, oil, or water) enters the tube through the socket, the pressure differential from the inside to the outside causes the tube to move. One can relate this movement to the uncoiling of a hose when pressurized with water, or the party whistle that uncoils when air is blown into it. The direction of this movement is determined by the curvature of the tubing, with the inside radius being slightly shorter than the outside radius. A specific amount of pressure causes the "C" shape to open up, or stretch, a specific distance. When the pressure is removed, the spring nature of the tube material returns the tube to its original shape and the tip to its original position relative to the socket.

Hmmm... you may have a point. :) Perhaps the OP could settle this. He would know if his gauges maintain zero at different elevations.

Nonetheless, if you adjust an open gauge to read 5" at 5000', it will accurately read all refrigerant pressures and a standard P/T chart can be used.

[Abby Normal]

Very wrong. If they are calibrated at sea level at day when atmospheric pressure is 1000hPa they will read 1Bar absolute or 0Bar relative pressure. If weather change and you have 1080hPa barometric pressure at sea level, you are going to read, on same gauges, 0,08 Bar gauge or 1,08 Bar absolute pressure (depend of gauge scale).
You will not have 0bar reading because these gauges don't measure difference between outdoor and connection port pressure. They are measuring amount of distortion of shape as consequence of force exerted at C shape from connection port.



Also Bourdon type manometer! Question is, does the resolution of instrument allow to be clearly visible.

Take a cylinder of refrigerant. Put a guage on it. Keep the cylinder temperature constant.

If atmospheric pressure fluctuates, the needle will fluctuate. Take the guage offf, the gauge will read zero, does not matter if it is a high or low pressure system moving through.

You need to really just look at the definition of 'gauge pressure' and 'absolute pressure' again.

The guage always measures pressure with respect to the atmosphere.

[Abby Normal]

Hmmm... you may have a point :)

Nonetheless, if you adjust an open gauge to read 5" at 5000', it will accurately read all refrigerant pressures and a standard P/T chart can be used.

Or subtrract the 2.5 pounds and read off the temperature :)

six of one, half a dozen of another

[nike123]

So then this discription is total BS?

http://www.refrigeration-engineer.com/forums/images/misc/quotes/quot-top-left.gifQuote:http://www.refrigeration-engineer.com/forums/images/misc/quotes/quot-top-right.gifhttp://www.refrigeration-engineer.com/forums/images/misc/quotes/quot-top-right-10.gifIn a Bourdon tube gauge, a "C" shaped, hollow spring tube is closed and sealed at one end. The opposite end is securely sealed and bonded to the socket, the threaded connection means. When the pressure medium (such as air, oil, or water) enters the tube through the socket, the pressure differential from the inside to the outside causes the tube to move. One can relate this movement to the uncoiling of a hose when pressurized with water, or the party whistle that uncoils when air is blown into it. The direction of this movement is determined by the curvature of the tubing, with the inside radius being slightly shorter than the outside radius. A specific amount of pressure causes the "C" shape to open up, or stretch, a specific distance. When the pressure is removed, the spring nature of the tube material returns the tube to its original shape and the tip to its original position relative to the socket.

Try to throw away picture you have in your had and read carefully without prejudice. Where in this paragraph you see influence of any other pressure except that on connection port.

I preaching you to throw away picture, and same time I don't read what really is in that paragraph!:o
I only could say, that outside pressure don't have significant influence (unlike as height difference ) at measured results.

P.S. I was referring at this type of gauge which is what we use in our measurements:
http://tinyurl.com/2nk68o

[Abby Normal]

the line in bold italic, pressure difference inside the tube with repsect to what is outside the tube is what makes it move.

The atmosphere is outside the tube

So when the inside of the tube is only exposed to atmosphere the needle will not move from its starting point.

[nike123]

Kids are playing with toy similar to that of Bourdon gauge.
They blow air in paper spiral and as they do that, spiral flattens. When they stop blow, paper rolls back. That is same principle as Bourdon gauge. How much influence you think that atmospheric pressure have on that action? Do you think that is significant to function of that toy, and accordingly, to Bourdon gauge.

[Abby Normal]

ask yourself, how does it indicate vacuum? Something bends it past its original state.

[Josip]

Hi, Abby Normal :)


Take a cylinder of refrigerant. Put a guage on it. Keep the cylinder temperature constant.

If atmospheric pressure fluctuates, the needle will fluctuate. Take the guage offf, the gauge will read zero, does not matter if it is a high or low pressure system moving through.

:confused::confused: are you sure? In the same volume without change of temperature the pressure will remain the same...it is closed system..and that pressure we call potential energy.....or energy of state....


You need to really just look at the definition of 'gauge pressure' and 'absolute pressure' again.

The guage always measures pressure with respect to the atmosphere.


I must say I do not agree with this...only the scale on the gauge for refrigerants is in connection with atmospheric pressure....but the gauge is enclosed instrument and atmospheric pressure has no affect on...we can only say we have 5 bar relative pressure or 6 bar of absolute pressure....

that 1 (one) bar of pressure shown on the gauge for refrigerants is only to help us to show which pressure bellow/under atmospheric pressure we have in the system....we can say we have o,4 bar of absolute pressure or we are at -0,6 bar of relative pressure...it is the same pressure....there in the space is no pressure at all...it is vacuum, but we are on the earth and we have to modulate our instruments to be able to read some pressures we face here for practical purposes...

I'm sorry, my english is not that good and maybe my explanation is not clear...

Best regards, Josip :)

[Abby Normal]

Hi, Abby Normal :)



:confused::confused: are you sure? In the same volume without change of temperature the pressure will remain the same...it is closed system..and that pressure we call potential energy.....or energy of state....

I am really just talking about something very basic here.

Absolute Pressure, measured above a perfect vacuum and Gauge Pressure meassured above atmosphere.

Inside the cylinder in my example the absolute pressure is the same.The gauge on top of the cylinder tells us how much pressure we have inside the cylinder with respect to atmospheric pressure.

If atmospheric pressure drops a little bit, the gauge reads a little higher, If atmospheric pressure increases, the gauge reads a little lower.

wiki is never the gospel but http://en.wikipedia.org/wiki/Pressure



For gases, pressure is sometimes measured not as an absolute pressure, but relative to atmospheric pressure (http://en.wikipedia.org/wiki/Atmospheric_pressure); such measurements are called gauge pressure (also sometimes spelled gage pressure).[1] (http://en.wikipedia.org/wiki/Pressure#_note-0) An example of this is the air pressure in an automobile (http://en.wikipedia.org/wiki/Automobile) tire (http://en.wikipedia.org/wiki/Tire), which might be said to be "220 kPa (http://en.wikipedia.org/wiki/Pascal_%28unit%29)", but is actually 220 kPa above atmospheric pressure. Since atmospheric pressure at sea level is about 100 kPa, the absolute pressure in the tire is therefore about 320 kPa. In technical work, this is written "a gauge pressure of 220 kPa". Where space is limited, such as on pressure gauges (http://en.wikipedia.org/wiki/Pressure_gauge), name plates (http://en.wikipedia.org/wiki/Name_plates), graph labels, and table headings, the use of a modifier in parentheses, such as "kPa (gauge)" or "kPa (absolute)", is permitted. In non-SI (http://en.wikipedia.org/wiki/SI) technical work, a gauge pressure is sometimes written as "32 psig", though the other methods explained above that avoid attaching characters to the unit of pressure are preferred.[2] (http://en.wikipedia.org/wiki/Pressure#_note-1)
Gauge pressure is the relevant measure of pressure wherever one is interested in the stress on storage vessels and the plumbing components of fluidics systems. However, whenever equation-of-state properties, such as densities or changes in densities, must be calculated, pressures must be expressed in terms of their absolute values. For instance, if the atmospheric pressure is 100 kPa, a gas (such as helium) at 200 kPa (gauge) (300 kPa [absolute]) is 50 % more dense than the same gas at 100 kPa (gauge) (200 kPa [absolute]). Focusing on gauge values, one might erroneously conclude the first sample had twice the density of the second.

[Josip]

Hi, Abby Normal :)


I am really just talking about something very basic here.

Absolute Pressure, measured above a perfect vacuum and Gauge Pressure meassured above atmosphere.

Inside the cylinder in my example the absolute pressure is the same.The gauge on top of the cylinder tells us how much pressure we have inside the cylinder with respect to atmospheric pressure.

If atmospheric pressure drops a little bit, the gauge reads a little higher, If atmospheric pressure increases, the gauge reads a little lower.

wiki is never the gospel but
http://en.wikipedia.org/wiki/Pressure


:):)...ok, now I catch what you mean to say......but for that reason we have to calibrate our gauge/s to 0 (zero) to be able to read exact pressure within cylinder/s otherwise we are reading wrong values...I have used a digital pressure gauge and always I had to set it to (local) zero pressure to obtain exact internal pressure readings...

Refrigeration gauges are coming with that difference...there we have dials showing -1bar...what is nonsense but very useful in practice...in absolute measuring systems we do not have - (minus) only 0 (zero i.e. absolute vacuum) and more...the same like with Kelvin degree

I was thinking about standard atmospheric pressure...it is agreed measure and.......my english is not so good, but here is the link...not the gospel like you said...but...hope all of us will learn something...;)

http://en.wikipedia.org/wiki/Atmospheric_pressure

Best regards, Josip :)

[Abby Normal]

well Josip, your and nike's english is a light year beyond my croat.

I usually have a hard enough time talking to Scots and Jamaicans :)

[nike123]

well Josip, your and nike's english is a light year beyond my croat.

I usually have a hard enough time talking to Scots and Jamaicans :)

Yah mon! :D
I need fifteen minutes for each sentence, and my Firefox spell checker constantly marks almost every written word that is wrong. That is how is god my English. If we were face by face, most of our communication will be with hands and legs.:D

Needless to say is, that most of meaning is lost in translation!:o

I am now only very, very, very..... sad, because we could not go together with Frank on some of his great beers.
Maybe then, our language difficulties could be little smaller.:):)

[thermo prince]

A lively discussion from an pleasant group of people, no doubt - the whole debate centres on whether "altitude compensation " is inherent in the mechanical gauge or needs to be manually zeroed.
All of you were coming at the same point from differing directions. Good to see all still friends , no black eyes or spilled kegs of (Frank's ) beer ! :D :D

Dare I say, just like the old Bourdon gauges themselves, some error was creeping int the discussion from parallax, angularity or hysterisis offsets perhaps ;) :D

This has been quite entertaining while we eagerly await the next episode of the " Flyboy 404a unit <->Gary " bestseller ! :D :D :D:rolleyes:

have a good w/e all :) :)
T-P

[Abby Normal]

Hmmm... you may have a point. :) Perhaps the OP could settle this. He would know if his gauges maintain zero at different elevations.

Nonetheless, if you adjust an open gauge to read 5" at 5000', it will accurately read all refrigerant pressures and a standard P/T chart can be used.
I was thinking Gary, since you abandoned the cold barren northern wasteland and moved to Florida, that you may know some scuba types.

You could donate an old gauge off of a manifold and let someone take it with them on a nitrox dive and see if the needle moves :)

[Gary]

I was thinking Gary, since you abandoned the cold barren northern wasteland and moved to Florida, that you may know some scuba types.

You could donate an old gauge off of a manifold and let someone take it with them on a nitrox dive and see if the needle moves :)

I'm thinking it might be easier to seal it in a Mason jar, solder a fitting to the lid and evacuate the jar.

[Abby Normal]

I'm thinking it might be easier to seal it in a Mason jar, solder a fitting to the lid and evacuate the jar.
that would work

[Franzfernando]

it makes no difference as the system is sealed

[Abby Normal]

it makes a difference as to what the gauge reads

[Josip]

Hi, all :)


it makes no difference as the system is sealed


it makes a difference as to what the gauge reads


....seems PART II;) of this thread....

Best regards, Josip :)

[Lord Kelvin]

Convert the P/T chart you have to Absolute pressure ( Psi ,not Psig).You can do this by adding the normal atmospheric pressure in Psi to the data displayed in your chart.When you read your gauge at 10,000 feet substract the actual atmospheric pressure in psi .Go to your modified P/T chart and do what you want.

Are you repairing fridges at that altitude?

[Abby Normal]

Convert the P/T chart you have to Absolute pressure ( Psi ,not Psig).You can do this by adding the normal atmospheric pressure in Psi to the data displayed in your chart.When you read your gauge at 10,000 feet substract the actual atmospheric pressure in psi .Go to your modified P/T chart and do what you want.

Are you repairing fridges at that altitude?

Can't go wrong with absolute pressure :)

[HVACGod]

The atmospheric pressure on the outside of the tube has absolutely no effect on the needle movement.

Gary, you are incorrect in this assessment - external pressure on the tube is relevant to indicator position.

[get the gauges]

Can some one just confirm the fact that if a system is reading 30 psi suction on a gauge it will also read 30psi on the gauge if your up a mountain or at the bottom of the ocean. If your gauges come off the system and are open to atmosphere they won't read zero unless at the same altitude you originally zero'd them(so don't touch them,they may read a partial vacuum) I can't see how a different pt chart could exist for refrigerant at different altitudes,the process is going on in a copper incased environment, am i missing something??.( i do get the bit about having a bottle of gas around to calibrate the gauges.

[nike123]

Debate here is if the deformation of c shape of Bourdon type manometer is going to be same or somewhat different on sea level then at high altitude because of the reduced pressure of air counteracting at c shape. Some of us think that that's not the case, and some that that's the case.

[NH3LVR]

ask yourself, how does it indicate vacuum? Something bends it past its original state.
I am still having problems getting this one in my head.
I am about to invest in a Bell Jar and experiment.

[SteinarN]

It has been very entertaining to read this topic. I feel i must throw in my opinion.

First of all, the normal mecanical gauges we use on our gauges set(?) read the difference between outside air pressure and innside pressure in the hose/gauge.

Thus a gauge with the port open to the outside atmosphere will show no change to the reading whatsoever when the outside pressure (and the inside pressure in the gauge/hose) changes as it does when we travel up a mountain.

When we have calibrated the gauges at sea level, the calibration will be correct in that sense that the gauge will stay in the same position at any level above or below sea level also.

When we are to use the gauges at high altitudes where the atmospheric pressure is lets say 0,7 bar, ie 0,3 bar below the pressure at sea level, then we shall calibrate the gauges to read -0,3 bar with ports open to the atmosphere. We can calibrate at sea level or when we reach that altitude. As stated above, the calibration doesnt change with altitude. If we do that then the temperature reading of the gauges will be correct at that specific altitude. The relative pressure reading is strictly speaking not correct, it is indicating 0,3 bar to little, but this error is exactly equal to the error of the relative pressure on a P-T diagram not corrected for this high altitude with an atmospheric pressure of 0,7 bar. Thus we can also take the pressure reading from the now calibrated gauges and convert them to temperatures in a normal, ie not altitude compensated P-T table to find the temperature.

This it how it is, and it is absolutely correct. ;)

[Doug Lockhart]

The Pressure/Temperature relationship is ONLY accurate IF the gauge is 'zeroed' to ABSOLUTE zero. If you zero the gauges to atmospheric pressure then your relationship is off by the difference that day between 14.696 psia and what you zeroed it too.:eek:

[SteinarN]

The Pressure/Temperature relationship is ONLY accurate IF the gauge is 'zeroed' to ABSOLUTE zero. If you zero the gauges to atmospheric pressure then your relationship is off by the difference that day between 14.696 psia and what you zeroed it too.:eek:

I was talking about normal relative gauges. When we zero them at sea level they measure absolute pressure - 1 bar. That is, lets say, 11 bar absolute pressure - 1 bar (sea level pressure) gives 10 bar "relative gauge pressure" measured at sea level.

When at high altitudes with atmospheric pressure of lets say 0,7 bar we have to calibrate gauges to -0,3 bar.
That is those same 11 bar absolute pressure - 0,7 bar (high altitude pressure) - 0,3 bar (calibration offset) gives a 10 bar "relative gauge pressure". In that way the same absolute pressure gives the same reading on the gauges and the temperature scale will be corect, also we can take the gauge relative pressure and convert it in a P-T table for sea level pressures and still obtain corect temperatures.

[nike123]

The Pressure/Temperature relationship is ONLY accurate IF the gauge is 'zeroed' to ABSOLUTE zero. If you zero the gauges to atmospheric pressure then your relationship is off by the difference that day between 14.696 psia and what you zeroed it too.:eek:

In other words, once zeroed to accurate value, they don't need to be "zeroed" to compensate atmospheric differences. Is that correct interpretation of your words?

[Doug Lockhart]

In other words, once zeroed to accurate value, they don't need to be "zeroed" to compensate atmospheric differences. Is that correct interpretation of your words?

That's basically correct. At Digi-Cool, we elected with the first generations of DRSAs (Digital Refrigeration System Analyzers) to allow the tech to 'zero' at any point because we effectively have done this in the industry for 50 years. That being said it is technically WRONG. We should have an internal accurate barometric pressure transducer that will give us the real barometric pressure and then will add or subtract that difference into what is displayed as referenced to atmospheric....
What happens then inside the system is not affected by anything that happens barometrically EXCEPT to bellows of controls etc....
Hope that helps and yes Digi-Cool will have the ability on the next generation of units for the purists.:)

[nike123]

Ok, that is what I taught. Coming from your mouth, I am sure that I am not wrong.:)

[Abby Normal]

a couple of pages of posts really about the definition of gauge and stmospheric pressure readings, amazing

If you are close to sea level, I doubt any normal fluctuations would put you off by more than 1F on superheat or subcooling.

The eye of Hurricane WIlma had the lowest recorded pressure in history of the atlantic basin, and it caused atmospheric pressure to drop by about 2 psi. I don;t think anyone will be out on a service call as a hurricane passes.

I could see guys at big plants with the large face gauges noticing some needles bouncing around, maybe some chiller designed to run on next to no superheat could get problematic

[Refrigerologist]

Being someone who has lived and worked at sea level or at a maximum of 300ft high I am struggling slightly, (completely in reality), with this concept of whether to recalibrate or not. As I understand it (and I could very easily be wrong), physics tell us that if we were to be weighed at at sea level and then again at 1000ft we would weigh very slightly less at the higher level. Would this apply to the density of the refrigerant? And therefore at a higher level might it interfere with the specific weight of the refrigerant and thus the pressure excerted. And maybe Gary is correct that a gauge calibrated at sea level should be left alone as it would change in line with what is happening at the higher elevation. Probably a load of cobblers but it is all my own work!

I agree with Gary that if you have a cylinder containing some liquid refrigerant, allow it to reach ambient temperature, fit a gauge on it, read the ambient and calibrate the gauge to the saturated temperature. The gauge is calibrated. Very simple solution, the best ones usually are!

[dogma]

Guages should be zero'd at sea level. Yes units are sealed but atmospheric pressure acts on the system/vessel as well. Guages should only be zero'd for absolute pressure readings.

In manufacture and design of domestic/commercial refrigeration, everything is designed for sea level pressures.

There can be miscalculations if the unit is designed at 3000ft and undergoes testing/ operation at sea level.

These differences can be minor but for MEPS testing, it can vary significantly.

[NH3LVR]

Back to this thread which always had me confused!
As I threatened in a earlier post, I wanted to procure a Bell jar and expose my gauges to it.
And into the shop comes a clear plastic tank with a vacuum pump attached!
After the repair I befuddled my mechanics with insisting we do another test, which they did not understand.
I placed a low side refer gauge in the tank (with the connection open to atmosphere). I pumped it down to 25 inches (-.85 BAR I believe) of vacuum and as I expected the gauge did not move.
I relieved the vacuum and placed a cap on the gauge, and pumped it down again. The gauge reading went up to 11 Lbs (.8 BAR). Apparently the outside pressure does affect the gauge.
I did not put a liquid filled gauge in as the other guys were convinced I was crazy already. There suspicions would have been confirmed if I had blown gauge fluid over the inside of the tank.

[Abby Normal]

thanks for confirming the definitions of absolute and gauge pressure. That a gauge measures pressure with respect to atmospheric

your first pump down with the open guage measured no difference in pressure. You broke the vacuum sealed atmospheric air in side the guage when you capped it, then you pumped it down and the guage showed the difference in pressure between what was trapped inside and the partial vacuum outside of it.

[Abby Normal]

another cold case file solved NH3, well done :)

you or the RSES should lobby for your procedure to be the very first lab taught in HVACR schools