R22 Pressure to Temperature formula

[Ausccn]

I'm after a formula to work out temperature from reading R22 pressure via a pressure transducer, in metric if possible!!

[chefid]

http://www.refrigerants.com/pics/ptchart.pdf

[Ausccn]

Thanks chefid, but I am after a formula so as to convert it from pressure to temperature in software for use in a control program.

[enthalpy]

I think you can derive an approx algebraic formula by going here and entering data points. Try it. Let us know if it worked.
http://www.savetman.com/curvefit/index.html

[Ausccn]

I tried the website but
y = a(0) + a(1) * x + a(2) * x2 + a(3) * x3 + ... + a(m) * xm
doesn't mean that much to me!!!

[Andy Schoen]

Last I checked, ASHRAE prefers: Kamei, A. and S.W Beyerlein. 1992. A fundamental equation for chlorodifluoromethane (R-22). "Fluid Phase Equilibria" 80 (11):71-86. Google Linda Hall Library in Kansas City. You can order this article from them online. But you better at least understand polynomial equations. ;)

[Damo]

http://hvac-talk.com/vbb/attachment.php?attachmentid=2929&d=1184549984

This is just a rough plot I did based on 10 points from the PT chart, you could refine it a bit more if you based the formula on more data, I didn't particularly feel like typing in all 50 points from the chart.

The x values are the pressure in kpa and the resulting y values are temperature in °C, as you can see it's not exactly a linear relationship so the formula isn't very straight forward, but you did say this is for software so at least you won't have to worry about converting the values manually!

If you need a more detailed explanation of how to use the formula just let me know.

Damian.

[Damo]

I should also mention that it will only be valid for pressure input values between -64 kpa to 2325 kpa (temperature outputs of -60°C to 60°C) as that's the extremities of the data on the PT chart I used.

Damo.

[Ausccn]

Thanks for that, I will check it out..

Ausccn.

[Ausccn]

Hi Damo, not having much luck with that formula, maths wasn't my best subject!! any luck of getting a simplified version??

Ausccn

[tshort]

Try this out:

coilFaceTemp = -31.837273 + (1.2738185 * psi) + (-.0040303161 * psi^2) + (.0000051769228 * psi^3)

For 66 psi this is:

coilFaceTemp = -31.837273 + (1.2738185 * 66) + (-.0040303161 * 4356) + (.0000051769228 * 287496)

coilFaceTemp = 36.167

[codewriter]

Thats not even right Travis, Didnt I tell you that calc was not accurate a few months ago... ;) Shame, Shame... ;)

http://www.csgnetwork.com/r22presstempconv.html
View the source, and you will find the calc.

[Damo]

http://members.optusnet.com.au/~damianhome/R22kpa.gif

So for a pressure of 4 kpa for example:
temp°C=-43.2011+0.2015*(4)+-0.00032125*(4*4)+0.00000029436*(4*4*4)+-0.00000000012603*(4*4*4*4)+0.000000000000019939*(4 *4*4*4*4)


Which equals -42.4002 °C.
This value is off by 2.4002 degrees and that's because this equation isn't perfect. Being based on only 10 points from the PT chart, there is some inaccuracy and this can be seen on the graph where the black line (the formula) moves away from the blue line (the data from the PT chart).

As you can see, when written this way it's a big ugly equation with a lot of numbers.
The easiest thing to do is to put the formula into either a graphing or spreadsheet program and let that do all the maths for you.

Better?

[Ausccn]

Thanks for that Damo, I'll give it a try, not sure if the controller will be able to compute numbers with so many decimal places!!!!

[crab master]

Interesting post

I did this awhile back and I was able to get really close using two curves
and although I used PSIG and Degrees F, I am sure you could convert it and get close.

one curve was for when the pressure was below 47 psig and the other curve when the pressure was above 47 psig. (I noticed there was a pretty good difference in the curve attributes at 47 psig.) Then I simply programmed in using one curve or the other based on the pressure input with both curves able to utilize extrapolation to get the approxiamate temperature/saturated condition.

The curve for <47 psig
24, 0 (at an input of 24 psi, output 0 deg F)
46, 23 (at an input of 46 psi, output 23 deg F)

and >47 psig
47, 23.5
80, 48

I then also subtracted my suction temperature from my curve value for the approxiamate saturated condition to be able to display an approxiamate superheat.

[Ausccn]

When you say curves!! what curves??

[crab master]

I guess I assumed your input is going to a programmable controller. The manufacturer I use has curves built in that you can utilize if you want. Then you don't have to build tables.

[Ausccn]

That would be nice, but I have to do it the old fashion way!!!!!