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!
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!
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

[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

[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.

[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.

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.

[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?

[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.

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?

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?

Quote: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.

[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!
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?

Quote: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.

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.