Service technicians often multitask and find themselves working on more than one refrigeration or air conditioning system at a time. Many times, with today’s digital service tools, a technician will remotely monitor the system for the proper operating parameters like evaporator and condensing pressures, evaporator superheat, and condenser subcooling after servicing the system.

Figure 1 shows a student service technician setting up an ammeter and Bluetooth high- and low-side pressure gauges on a compressor after fixing a leak near the liquid line solenoid valve and charging refrigerant to a glass door reach-in freezer inside a building at Ferris State University in Big Rapids, Michigan.

Figure 2 shows the service technician’s partner remotely monitoring system pressures with a tablet and appropriately downloaded application, or app, from an outside unit two hours after refrigerant was added and freezing temperatures were achieved. (Note: This same service technician is currently working on an outdoor heat pump that is servicing a computer room at Ferris.)

The Bluetooth gauge technology allows this kind of multitasking of serviced units without going in and out of the building, saving time and money. Many times in the hot summer months, a service technician can remotely monitor system pressures from an air conditioned service van while simultaneously doing needed paper work for their company and the customer.

Bluetooth Technology

Most people are very familiar with FM radios, mobile phones, and televisions that send information wirelessly with radio waves over long distances measured in miles and kilometers. Bluetooth technology is a wireless standard that exchanges data wirelessly over short distances (100 meters) using radio wave frequency transmissions. Bluetooth technology is built into many products like computers, mobile phones, and medical devices.

The name Bluetooth comes from the 10th century Danish king Harold Bluetooth, who helped unite parties at war in Denmark, Sweden, and Norway. Thus, Bluetooth technology was created as an open standard to allow connectivity and collaboration between products within different industries.

Bluetooth technology is a combination of hardware and software. Bluetooth products contain a computer chip containing a Bluetooth radio (See Figure 3). However, software (applications or apps) is also needed to connect or interface to other products using Bluetooth wireless technology. Many of these apps can be downloaded free from iTunes or Google Play.

Bluetooth technology is bringing everyday devices into the digital world, which allows them to connect to one another without wires. Today, just about all top-selling tablets, personal computers, headsets, computer mice, keyboards, and smartphones can be bought supporting Bluetooth technology. These products have small batteries, sensors, and/or collectors that can operate for a long time without changing the tiny batteries.

Evaporator Superheat

I felt it important to bring readers up to date regarding some of the newest technologies being used by service technicians. Now, in the space remaining, I want to briefly talk about evaporator superheat, which I referenced at the beginning of this column as part of technicians’ tasks.

Evaporator superheat starts at the 100 percent saturated vapor point in the evaporator and ends at the outlet of the evaporator. The 100 percent saturated vapor point is the point where all the liquid has just turned to vapor. The temperature at this point is the evaporator temperature and can be obtained from a pressure/temperature chart once the pressure is known. The evaporator outlet is where the remote bulb of the thermostatic expansion valve (TXV) is located.

Technicians usually put a thermistor or thermocouple at the evaporator outlet to get the evaporator outlet temperature. A pressure gauge at that same point as the temperature reading will give the technician the saturated vapor temperature. Most manufacturers of larger evaporators supply a Schrader fitting at the evaporator outlet pretty close to the remote bulb of the TXV for measuring pressure (See Figure 4).


R-134a refrigeration system

• Low-side gauge reading at evaporator outlet = 20 psig or 23°F. (From a -134a pressure/temperature chart).

• Evaporator outlet temperature (thermistor reading) = 30°

Evaporator superheat calculation

• 30° evaporator outlet temperature subtracted by 23° saturation temperature at evaporator outlet equals 7° evaporator superheat

If a technician was to measure the pressure at the compressor instead of the evaporator outlet, a higher, fictitious superheat value will be read. As the refrigerant travels the length of the suction line, there will be associated pressure drop from friction and/or restrictions caused from long runs of suction piping, oil accumulation, filters, or valves. This will cause the pressure at the compressor to be lower than the pressure at the evaporator outlet. This higher, fictitious superheat reading may lead the technician to adjust the TXV stem clockwise (open) more to compensate for the fictitious high superheat reading. This could cause compressor damage from liquid flooding or slugging from too low of a superheat setting.


• Assume a 5-psi pressure drop from the evaporator outlet to the compressor.

• R-134a refrigeration system.

• Low-side gauge reading at compressor inlet = 15 psig or 15°

• Evaporator outlet temperature (thermistor reading) = 30°

Evaporator superheat calculation

• 30° evaporator outlet temperature subtracted by 15° saturation temperature at compressor inlet equals 15° superheat

The superheat changed from 7° to 15° simply by reading the pressure at the compressor inlet instead of the evaporator outlet. The correct evaporator superheat would be 7°. 

It’s best to measure the pressure at the same location you measured the temperature to exclude any system pressure drops.

The amount of evaporator superheat that is required for a certain application will vary. Lower temperature applications generally utilize lower evaporator superheats than medium- and high-temperature applications. Please consult the case and/or appliance manufacturer, if in doubt.

In the absence of manufacturer’s data, chart 1 above shows guidelines for evaporator superheat settings.

Publication date: 1/12/2015

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