Testing Challenges
Unfortunately, as many technicians can attest, it sounds a lot easier than it is. The problem is the three temperature measurements. They're not as easy to take as it would seem.

For example, while outdoor dry bulb is the most obvious and easiest super-heat measurement to take, temperatures can vary considerably in the area around the condenser.

"We used to use an analog thermometer in class," says Featherstone. "But it took too long to get the right reading and it was a chore to hold it in position. We started using a stick meter with a K-type thermocouple and got the job done faster and more accurately."

With some K-type thermocouple thermometers, there is a temperature reference junction inside the meter (the "cold junction") that is monitored by a thermometer inside the meter.

Both the reference junction and the thermometer need to be at the same temperature to ensure an accurate reading. Some meters employ an adapter, with the reference junction in the adapter, not in the meter, and not thermally close to the thermometer inside the meter. Any difference in temperature between the external reference junction in the adapter and the internal thermometer inside the meter will show up as an error.

By simply holding this adapter in your hand, you can alter the reading that the multimeter displays. And be aware that if you take a warm adapter from your pocket and put it in a cold meter from the truck, it will be several minutes before you can get an accurate reading.

Indoor wet bulb temperature measurement also presents a problem. Technical articles, manuals, and educational texts suggest technicians use such things as moistened toilet tissue and paper napkins wrapped around the bead. This nuisance factor is one of the reasons some techs take a short cut here, and simply assume a relative humidity of 50%.

To make it easier to take wet bulb temperatures, there are K-type thermocouples available with a "sock" attached and alligator clip for attaching to grids and coils. These function with any K-type thermometer. Some manufacturers also offer test heads that fit onto stick meters and dataloggers, that display both temperature and wet bulb temperature. These self-contained units measure relative humidity, temperature, and dew point with no moistening, no sock, and no external sensor.

The last measurement presents an even bigger source for potential errors. The suction line temperature often can't be measured with a simple pocket thermometer because the thermal contact with the pipe is not good enough and the thermal contact with the ambient environment is too good.

The resulting temperature would be somewhere between the pipe temperature and the air surrounding it. The trick is to find a way to measure only the pipe temperature.

One way involves using a standard beaded thermocouple, and pushing it under the pipe insulation. Unfortunately, this only works if the insulation is dry and fits tightly.

Some thermocouples feature about an inch of insulation cut back from the bead. This allows you to wrap the whole inch of bare wire around the pipe and isolate it from the environment with a Velcro strip. Other thermocouples clamp directly to the pipe and seal out ambient air.

The newest solution to this challenge is an accessory head for thermocouples that measures both suction line pressure and suction line temperature simultaneously. It converts the pressure measurement to boiling point, subtracts it from the suction line temperature, and displays the actual superheat. This eliminates the need for reference charts and math.

By keeping in mind the pitfalls involved in dry bulb, wet bulb, and suction line measurements and by having the best tools available to get accurate results, you can easily determine the appropriate refrigerant charge for an air conditioning system, thereby assuring optimal operation and preventing serious damage to compressors.

Adolfo Wurts is engineering manager for Fieldpiece Instruments, Anaheim, CA, a manufacturer of measuring instruments for the HVACR industry. Wurts can be reached at 714/634-1844.