January 1, 2007
The problem with measuring temperatures in an HVAC environment is that there is no single instrument for the job.
Yet some HVAC technicians and teachers think the digital thermometers seen on every distributor’s counter are all they need.
That kind of thinking leads to problems.
In addition, there are some significant problems with common measurement techniques.
First, those pocket thermometers. You can strap them to cold pipes and record temperatures 15 degrees higher than the actual temperature. And while performing temperature-shock testing, you can take them from a 32ºF environment to a 120ºF one and see inaccuracies of 20 degrees. The digital, pocket thermometer is an inexpensive solution for taking air-temperature measurements and can deliver good performance, provided the ambient conditions are stable and you can leave them in place long enough. But their inability to make good thermal contact with pipes makes them a poor choice for pipe-temperature measurements.
SuperheatTemperature measurements for determining the target superheat are challenging and critical for HVAC technicians. Four measurements are required: refrigerant boiling point (which is a direct function of pressure), suction line, indoor wet bulb and outdoor dry bulb.
The temperature of the refrigerant boiling point is the easy part. The technician can measure the pressure at the evaporator with a pressure gauge and read the boiling point right on the gauge. Subtract the boiling point temperature from the suction line temperature and you have the actual superheat for the system. The outdoor dry bulb and indoor wet bulb are needed (along with the manufacturer’s chart) to determine the target superheat. Manufacturers specify a target superheat as a safety factor. If the actual superheat is too low, the refrigerant could enter the compressor and damage it. If it is too high, efficiency and comfort declines.
Air temperature is the most common measurement that technicians make. Whether it’s of the supply and return air of an air-conditioning system or furnace, at the condenser on a split system, or in an occupied space for target temperature or comfort level, accuracy is extremely important.
It is even more important in measuring superheat where three accurate measurements - outdoor air, indoor air and suction line temperature - are critical.
Outside airWhen measuring the outdoor temperature, for example, temperatures can vary considerably in the areas around the condenser. The only reliable place to take the outdoor temperature is right in front of the condenser coils and the thermocouple should remain there until stable. For a thermocouple, this can be as little as a few seconds. Holding the lead there for even a few seconds can be a problem for busy technicians. Some manufacturers have developed a thermocouple designed specifically for HVAC techs with an “alligator” clip on the sensor so it can be easily attached directly onto the condenser grill and stay in place as long as needed. They work with any meter that accepts a K-type thermocouple.
One of the critical measurements for HVAC technicians to do is wet bulb temperature. There are two ways to get an accurate measurement. The first is the common “wet sock” approach. The second is a dedicated wet bulb measuring instrument.
With the wet sock method, there is a hard way and an easy way. The hard way is what most technicians are used to - constructing one’s own wet sock out of toilet paper, paper towels, or shoelaces. The easy way is to use a thermocouple with the wet sock built in.
Here again, an alligator clip on the thermocouple makes a valuable contribution to accuracy by enabling technicians to clip the instrument directly on the indoor return grille or directly to the evaporator coil on the incoming-air side. It can be left in place until it completely cools down in the airstream. It’s easy and reliable and can be used with any thermometer using K-type thermocouples. By using the “min” function available on many instruments, technicians don’t have to watch the display to determine the lowest point.
PlacementWhen taking wet bulb for determining superheat, the best place to take the measurement is in the air returning to the evaporator. After the system has been stabilized, the technician should remove the filter, clip the thermocouple to it using the alligator clips, replace the filter and shut the door. This enables the technician to take the reading without interfering with the operation of the system and assures maximum accuracy.
There’s a second way to take wet bulb measurements. One recently introduced thermometer features electronic psychrometrics, which contain sensors that measure air temperature and relative humidity and microprocessors that calculate both wet bulb and dew point temperatures.
Pipe temperature, also one of the critical measurements HVAC technicians must make, presents some unique challenges. Although many methods exist for measuring pipe temperature, testing and research proves that many methods in use today are just plain inaccurate. This is because a sensor that is not properly isolated will be affected by the temperature of the surrounding environment. When the ambient temperature is higher than the pipe temperature, a false measurement will provide a reading higher than the actual pipe temperature, indicating a falsely higher superheat. This may cause the technician to overcharge the system and put the compressor in danger.
TricksThe trick is for technicians to find a way to measure only the pipe temperature without being thrown off by the environmental temperature. One way involves the use of a standard beaded thermocouple with a fabric fastener strip. The technician wraps the bare wire around the pipe and holds it in place with the strip. Some thermocouple instruments come with a beaded thermocouple and fabric strips.
Another method requires the technician to push the beaded thermocouple under the foam insulation. The insulation must be dry and fit tightly.
However, the most effective, reliable, and repeatable way to accurately measure suction line temperature requires a pipe-clamp thermocouple. They work with any meter that uses K-type thermocouples and provide excellent thermal contact by squeezing directly onto the pipe and providing good isolation from the ambient air. This style of thermocouple snaps quickly and easily onto a pipe and maintains a reliable thermal contact.
Temperature measurements for dry bulb, wet bulb and pipe temperature provide vital diagnostic data and the instruments and techniques used in taking these measurements should be carefully considered to assure maximum accuracy.
SecretsMost thermocouple instruments come with a beaded thermocouple. From a thermal transfer perspective, the rounded head at the end is one of the worst shapes for HVAC surface probe or pipe clamp applications. There is, however, a trick to make these thermocouples work well: Simply strip back the insulation about three quarters of an inch. By putting the bare wires in contact with whatever is being measured, whether it is a flat surface or a pipe, the exposed thermocouple wire will transfer heat much more efficiently. Readings will be both more accurate and faster.
When you add the accuracy specifications for a thermocouple and the instrument it connects to, the results are an accuracy specification of approximately plus or minus six degrees Fahrenheit. That simply is not good enough for HVAC work. But there is a solution. The secret is to calibrate your instruments as a system. Every thermocouple thermometer comes with a calibration pot. All you need is a known temperature. A stabilized glass of ice water works best. Stir vigorously, take a measurement, and set it for 32ºF. Now your total accuracy will be in the neighborhood plus/minus one degree all through the temperature ranges you need. Whenever you change thermocouples, repeat the process.
Thermal shockMost temperature instruments have a serious problem when the ambient temperature changes dramatically - just like the pocket thermometer mentioned at the beginning of this article. These changes occur constantly on the job - when your instrument goes from an indoor air temperature of 70ºF to the exhaust from the condenser (about 130ºF), or from a freezer to the top of a roof, or from the cab of your truck to a cold basement. Most instruments recover accuracy if allowed to stabilize. The time can vary from a few seconds for the very best instruments to a couple of hours for instruments with poor thermal shock capabilities.
Thermal shock recovery time varies with different brands of thermocouple thermometers as well. In these instruments, an electronic thermometer measures the actual temperature inside the meter. With the K-type thermocouple thermometers, there is a temperature reference junction inside the meter and a junction at the meter end of the thermocouple wire. Both of these junctions need to be at the same temperature to ensure an accurate reading. Some meters employ an adapter. The reference junction is inside this adapter. Simply by holding this handle in your hand, you can alter the reading that the meter displays. One manufacturer places both the reference junction and the internal thermometer inside the meter on a highly conductive piece of ceramic substrate so they are at the same temperature resulting in optimum thermal recovery.
InfraredWhen technicians have to take a measurement in areas that can’t be reached with a thermocouple, the only alternative is infrared. While these instruments have been around the industry for a while and are very convenient, three pitfalls must be considered when using one.
First, an infrared instrument takes the average temperature of everything in its cone of view. If that cone includes a pipe and the wall behind it, the instrument will register a combination of the two temperatures.
Second, infrared radiation behaves similar to light in that it reflects off any surface that also reflects light. If the surface being measured is reflective, the resulting temperature reading will be too low. Spray painting the surface with a dark dull paint provides the best solution.
Third, the infrared instrument requires a stable environment to get a stable reading. If the technician moves the instrument from one environment to another, the reading may change.
Infrared instruments provide a useful solution to difficult problems, but must be used with discretion.
Temperature measurements for dry-bulb, wet-bulb, ambient-air and pipe temperature provide vital diagnostic data and the instruments and techniques used in taking these measurements should be carefully considered to assure maximum accuracy. The alternative is botched jobs and lost customers.