Ask Me Help Desk - Pressure/Temp Gauges

How do you read manifold gauges? i.e.. On a 70 degree day what pressures should you expect on a normal running unit? Low pressures and high pressures.I know it has to do with the ambient temp. I was told that you add 35 to the ambient and that should shoud translate to high pressure. Is this correct? How do you derive at the low pressure?

Quote:

Originally Posted by esquire1

How do you read manifold gauges? ie. on a 70 degree day what pressures should you expect on a normal running unit? Low pressures and high pressures.I know it has to do with the ambient temp. I was told that you add 35 to the ambient and that should shoud translate to high pressure. Is this correct? How do you derive at the low pressure?

Ithe pressure/ temp coming off the discharge side should be constant ( the temp after the condenser coil depended on outside air temp)... each freon have its owns temp/pressure... R- 11 40f= 15.6 psi (vac) 100f = 8.8 psi, R12 40f = 37 100f = 117, R22 40f =69 100f = 196... normal gauge has about 3 rings on it, each ring have numbers on them. Read the temp/pressure corresponding to the freon... the outer ring uasslly pressure... ex the needle is at 65psi R22= 38 F, R502= 30f

The pressures and temps can change from hourr to hour depending on several different things. One is the temp inside the house, if the house is hot then the suction line is going to pick up more heat there fore the pressure and temp will be higher on the low pressure gauge. If the humidity is high outside the high pressure gauge will red less that it would on a dryer day. If the Evap coil has any restriction or if there are grills that are shut or partially shut the low pressure gauge will read lower. If the coil on the condensing unit is dirty or blocked by an overhang or bushes the gauge will read high.

Now about the 35 degree difference int temps you quoted. At one time that was a way that some people used to charge a unit but they measured the air going into the condenser coil and the air coming out of the condenser coil if it was in a 30 to 35 degrees range that was acceptable. But there were other things that were considered. Was the suction line very cool to the touch and was there moisture forming. Also the temp of the air passing over the evap coil. A ideal temp drop is 18 to 20 degrees. Tjhen the last thing thing is what kind of metering device do you have on your eguipment?