Technical Note:
Rate Compensated Heat Detector Testing

Interpretation of test results
The purpose of this note is to explain detector set point, rate compensation, and how this should affect the interpretation of test results observed on remote temperature readout of the HST heat detector tester. The HST remote temperature output is taken directly from the heating unit. Detector trip points will often be observed to be lower than the manufacturer rated set point. This is due to the unique way rate compensation works to self-adjust the set point based on how quickly the temperature is rising from the starting temperature.

Set Point verses trip point- “virtual set point”
The unique feature of Rate Compensated heat detectors: "virtual” set point.
The detector has two parts which expand or contract with temperature changes.

First is the outer expansion tube or shell.
Second is a pair of expansion "struts" or metal strips. This interior part also contains the electrical contacts.

These two parts work together to cause the detector to behave in 2 different ways:
1st- A slow rate of temp rise- Heat is allowed to penetrate into the struts, both parts expand together and detector trips close to the rated set point.
2nd- A high rate of temp rise- The shell rapidly expands before heat can penetrate to struts and the detector trips at a temperature that would appear to be lower than the rated set point.

Manufacturer Calibration (UL testing) vs. Compliance Testing
New heat detectors are calibrated (tested to UL521 standard) to a rated +/- set point.
This is done by raising the temperature in very slow, controlled manner (varying rates depending on the temperature of the detector).
For example: the temperature could be raised at 50 degrees per minute until 150 degrees is reached. Then continue at 1 degree per minute until the detector trips at 225°. Ambient temperature also needs to be accounted for by heating to a pre-determined starting temperature for a set period of time. The detector should then trip within the +/- range of manufacturers set point specification. This type of testing is not required for normal detector compliance testing.
Compliance testing is done after installation of the detector in the intended area of use and is required by the authority having jurisdiction. There is no requirement to test for a specific temperature. However, the test should not damage the detector.
The HST can be custom programmed to raise temperature at a slower rate to trip detector closer to the rated set point. (see Rate Control option Appendix B) However, the HST’s intended application is to compliance test detectors in a non destructive manner, identify shifts in set point, and help ensure that the detector continues to operate as it was originally calibrated by the manufacturer. Per the UL standard, there is no way to field re-calibrate a heat/fire detector.