PT chart for different elevations

[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?

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.

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!
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.

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



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; such measurements are called gauge pressure (also sometimes spelled gage pressure).[1] An example of this is the air pressure in an automobile tire, which might be said to be "220 kPa", 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, 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 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]
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!
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.

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

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 !

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

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

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.

[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.

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.

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