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Temperature Measurement with Smart Controllers
The sluggish thermal response of these controllers results in a measurement “delay” or lag of 5 to 10 minutes
A Basic Challenge
Although conceptually simple, smart wall-mount controllers such as rooftop controllers, PTAC controllers, baseboard controllers etc all have issues measuring space temperature because of self heating in the controller itself and sluggish thermal response of the controller package. In our development of a CSA approved baseboard thermostat and other controllers we needed to meet stringent requirements for response and repeatability. This required development of temperature predictor software in our devices.
As a note: With wireless controllers one could overcome this problem by using devices that “sleep” most of the time, so they would not self heat, however this would add additional cost because the system would still need a backbone of always live devices spaced in the 30 to 80 foot range from each other. This would increase system cost.
The sluggish thermal response of these controllers results in a measurement “delay” or lag of 5 to 10 minutes on rising and falling temperatures. There is nothing in a process control loop that is harder to correct than pure lag. This lag causes the associated controller to overshoot and/or cycle which is undesirable for both comfort or energy savings.
Some manufacturers attempt to correct these problems by mounting the temperature sensor (thermistor) on a short lead extending from the case, or mounting it on extended leads above the PCB. The intent is that the thermistor is put more in the stream of air flowing around the controller. However, our results show that this has little or no effect unless the leads are a foot or more in length. The reason is simple. The copper wire connecting the thermistor to the PCB is an excellent conductor of heat and readily conducts the internal temperature of the printed circuit board and case to the thermistor material (usually a small bead). The thermistor still measures the temperature of the printed circuit board (PCB) and internal environment of the thermostat controller, not the air temperature of the space.
It is possible to compensate for the self heating of the electronics with a fixed offset for a given static room temperature. Probably most manufacturers do this. For example, if the internal electronics controller always heats the air flowing through it by say, 5 deg., we could set this offset to 5 deg to compensate so it will control around the correct temperature. However, the lag in accurate readings caused by the thermal mass of the sensor will cause havoc for the control system if the room temperature changes, even by a small amount. The control system will overheat when heating the space because the reading is too low, and overcool when cooling because the reading is too high. This will result in the room temperature cycling continuously.
The graph below shows the comparison between measured room temperature BLUE “thermistor” temperature (that measured by the controller) for a typical smart controller (RED). It also shows the temperature as calculated using Walker’s predictor software (YELLOW).
Walker Technologies has implemented a sophisticated temperature predictor in the firmware of its smart thermostats. This predictor dynamically calculates the value of room temperature by accurately measuring the rate of change in temperature of the PCB in the controller and relating this to thermal mass to instantly predict the actual air temperature.
controllers are unique in their ability to accurately detect small
changes in external temperature and make immediate small adjustments to
control outputs. The result is control accuracy to within a few
10ths of a degree as compared to several degrees as compared to
About the Author
Al Walker, President of Walker Technologies, got his degree in Honours Physics at the University of British Columbia. After University he taught in the Instrumentation Department at British Columbia Institute of Technologies (BCIT) before he formed Walker Technologies. His design of embedded controllers were instrumental in the early stages of the development of several companies including Reliable Controls, KMC, and Delta Controls (Energrated systems) and was the developer and manufacturer of the Honeywell Custodian and Custodian Plus control systems installed and still operating in Canada. He developed the GCL control language used by several control companies. Mr. Walker has also been involved in several other leading edge advancements in the HVAC industry and has spent the last 5 years developing a viable ZigBee based wireless control system. Al can be reached directly at Al Walker <firstname.lastname@example.org>
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