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Help a noob - understanding the gauge scales

rojeho on Sun August 02, 2015 3:28 PM User is offline

The Harbor Freight (insert pause for laughter and eye rolls) gauge set has a scale for PSI and one for R134a. On the low side they are close to the same scale but on the high side the R134 scale is close to half the PSI scale. Why is this? For example a high reading of 250PSI would max the gauge on the R134 scale. Which one should I use and why?

Thank you for your help!

AC_Doc on Sun August 02, 2015 5:15 PM User is offline

because, if you look at the base of the scale for r-134a you'll see that the numbers on that scale are the TEMPERATURE of R-134a at the indicated pressure.

Sometimes you must accept things at faith value!

rojeho on Sun August 02, 2015 9:16 PM User is offline

Thank you! I missed that and figured that they were all PSI. So now I'm curious - what value is there in the temperature scale for a professional (i.e. not me).

mk378 on Sun August 02, 2015 11:08 PM User is offline

The temperature scale shows the temperature at which the refrigerant will evaporate (or condense) at the corresponding pressure.

The main use of this scale is under static conditions-- engine off and compressor has not been running for some time. If the pressure is below what the temperature scale shows, the system is practically empty, there is no liquid refrigerant present. If the pressure is above what the temperature scale shows, there may be contamination with air.

rojeho on Tue August 04, 2015 5:17 PM User is offline

Interesting - thank you!

AC_Doc on Fri August 07, 2015 12:28 PM User is offline

The Temperature Scale is also a handy way to determine the temperature of the main players (evaporator and condenser) during the A/C operation without having to use the refrigerant pressure-temperature relationship data charts!

Looking at the temp scale on the low side gauge will tell you the temperature of the refrigerant entering the evaporator. On a functional system, add another 5 to 10 degree evaporator to air temperature transfer factor and you have the best vent output temperature available.

Example: On the low side gauge, R-134a Low side 30 psi = 35 degrees refrigerant = +5 degrees = 40 degrees vent output air temperature.
If the low side is 60 psi, then don't expect 35 degree vent air. Can't happen.

On the other hand, if the low side pressure is less than 27 psi (32 degrees) , the refrigerant temperature is below freezing, and any moisture on the evaporator fins will start to freeze, blocking the air flow through the evaporator. Less vent air flow will result.

Likewise, the condenser temperature needs to be 30-40 degrees hotter than the ambient incoming air going through it to provide a good heat dump transfer from the condenser to the cooling air. [Heat always travels from hot to cold.] The greater the difference, the more heat exchange takes place.

Example: On the high side gauge, R-134a High side 200 psi = 130 degrees refrigerant going into the condenser. If the condenser temperature is close to the ambient air temperature, much less heat dump out of the condenser occurs (no matter how much of the ambient air you blow through it). In this example, the condenser will shed maximum heat up to about a 90 degree ambient air temperature. As the ambient air increases above 90 degrees the efficiency of the condenser at 200 psi high side will start to decrease as well.

You will notice the inability of the compressor to hold the low side down during idle conditions. The low side pressure increases, then so does the evaporator temperature, resulting in warmer vent air. That's why systems are tested at about 1500 RPM (cruise condition).


Sometimes you must accept things at faith value!

Edited: Fri August 07, 2015 at 12:41 PM by AC_Doc

rojeho on Tue August 11, 2015 12:34 AM User is offline

Thanks for taking the time to type that AC_Doc. That's really interesting stuff. I love being able to learn from you guys. Much appreciated.

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