The efficient running of industrial chillers and HVAC systems requires the refrigerants and lubricating oils to be at optimum temperatures throughout refrigeration.
The most frequently used sensors for this application use negative temperature coefficient (NTC) thermistors of varying resistivities to measure the temperature. These devices display a lower resistance at higher temperatures.
Thermistors vs. RTD-Type Sensors
RTD-type or thermistors can also be used for refrigerant monitoring, with thermistors being the more popular option due to their cost efficiency and the fact they can mix with the media being sensed. RTD-types are usually more expensive and must be enclosed from media to avoid failure.
Thermistors are more rugged than RTD-type sensors and can be directly placed in any non-conductive fluid for temperature monitoring – this direct contact means changes in temperature are sensed rapidly. However, thermistors must be temperature and resistance calibrated, as they show non-linear output. Thermistor suppliers can supply Resistance-to-Temperature curves for calibration.
RTD Tempertature Sensor
Open Thermistor Temperature Sensor
Closed Thermistor Temperature Sensor
It is the combination of a thermistor with a housing that determines which sensor assembly is better than another for HVAC monitoring applications. Thermistor housings can be classed as either exposed or enclosed.
Exposed thermistors are in contact with the fluid they are monitoring. In the case of HVAC’s, these fluids can be oils, refrigerants, emulsions of refrigerant and oils. Exposed thermistors can also work with any other non-conductive fluid.
Direct contact between fluids and the sensor means that readings are more responsive and more accurate. However, fluid can leak deeper into the housing through the thermistor connections, which can cause operational problems, breaches of warranty, or require maintenance. This issue is particularly prevalent in high-pressure applications.
Enclosed thermistors contain the probe inside the housing, keeping it separate from the fluid. Whilst the system is at this point protected from leakage, it is also less responsive and accurate as it is surrounded by air, as opposed to the fluid it is measuring.
A New Third Possibility
Gems Sensors & Controls has designed a new type of housing called the TM-950 Series, which delivers the benefits of a hermetically sealed sensor housing with the responsiveness of an exposed thermistor. This sensor series was built to meet the needs of the HVAC and chiller industries.
10 ton chiller
Sensors in the TM-950 Series have hermetically sealed sensor housing, which is achieved using a fused-glass technique where molten glass is placed inside the heated housing. This glass is compressed as the system cools and bonds at an atomic level with the metallic surface. The thermistor hardware is set up through tubing prior to fusing taking place.
The TM-950 Series is highly flexible and is able to adapt to any type of sensor that suits the required temperature profile. Following installation of the sensor and associated leads, the entire tubing is filled via induction soldering to provide a seal that is completely leak-proof up to 450 psig. The end result is a sensor which has all of the advantages of a direct contact sensor, along with the leak-proof behavior of a closed sensor.
This information has been sourced, reviewed and adapted from materials provided by Gems Sensors and Controls.
For more information on this source, please visit Gems Sensors and Controls.