How To Calibrate BEMS Temperature Sensors
What is a Thermistor?
A thermistor is a type of temperature sensor that relies on the relationship between electrical resistance and temperature. Unlike standard resistors, a thermistor’s resistance changes significantly with temperature variations, making it highly sensitive for precision applications.
Thermistors are commonly used in Building Energy Management Systems (BEMS) to monitor environmental temperatures and adjust heating, ventilation, and air conditioning (HVAC) systems accordingly. The most typical thermistors are Negative Temperature Coefficient (NTC) thermistors, meaning their resistance decreases as temperature increases.
Importance of sensor accuracy
Accurate temperature readings ensure efficient energy management, prevent unnecessary HVAC adjustments, and maintain occupant comfort. If a thermistor reports incorrect temperatures due to cable issues, environmental interference, or other errors, the system may compensate improperly wasting energy, causing discomfort, or leading to malfunctions.
Thermistor longevity and calibration needs
Thermistors degrade slowly, often taking decades before experiencing meaningful resistance shifts. Because of this, direct sensor recalibration is unnecessary. Calibration should focus on ensuring that data transmission from the sensor to the Building Energy Management System (BEMS) is correct and free from interference.
Calibration method - using a known resistance
Rather than testing the thermistor itself, calibration is performed using a fixed resistor with a known resistance to verify system accuracy. A 10K3A1 thermistor should read 10KΩ at 25°C.
To test:
- Attach a 10KΩ resistor to the sensor cable in place of the thermistor.
- Check the reading at the BEMS—it should display 25°C.
- If readings are incorrect, investigate cable integrity and interference sources.
Factors affecting sensor readings
Cable length and Electromagnetic Interferance (EMF)
Long cable runs can introduce resistance, causing minor temperature shifts. If necessary, software offsets can correct these discrepancies to maintain accuracy.
Electromagnetic interference (EMF) from nearby electrical wiring can also distort sensor values. The best way to reduce interference is by using screened cable, earthed at one end, to drain unwanted electrical noise.
External influences
Environmental conditions can affect readings. Common influences include:
- Solar Gain; Direct sunlight raises sensor temperature artificially.
- Nearby Heat Sources; Temporary radiators, industrial equipment, or electrical devices may skew readings if sensors are placed too close.
- Airflow Disturbances; Strong air movement can cause fluctuating temperatures, leading to unstable readings.
It’s critical NOT to use offsets to compensate for these influences, as they change unpredictably. If external factors affect readings, the correct approach is to relocate the sensor rather than apply adjustments that remain even after the influence is gone.
Ensuring reliable sensor performance
Thermistor accuracy is essential for maintaining precise building control. By validating readings via a known resistance, correcting for cable related errors, and ensuring proper sensor placement, systems remain efficient, reliable, and energy conscious.
