Abstract
Indoor air temperature belongs to the most important climatic variables in indoor climate research, affecting thermal comfort, energy balance, and air movement in buildings. This paper focuses on measurement errors when using thermocouples in indoor temperature measurements, with special attention on measurements of air temperature. We briefly discuss errors in thermocouple measurements, noting that, for temperatures restricted to indoor temperature ranges, a thermocouple Type T performs much better than stated in “standards”. When thermocouples are described in the literature, industrial applications are primarily considered, involving temperatures up to several hundred degrees and with moderate demands on accuracy. In indoor applications, the temperature difference between the measuring and the reference junction is often only a few degrees. Thus, the error contribution from the thermocouple itself is almost immeasurable, while the dominant error source is in the internal reference temperature compensation in the measuring instrument. It was shown that using an external reference junction can decrease the measurement error substantially (i.e., down to a few hundredths of a degree) in room temperature measurements. One example of how such a device may be assembled is provided. A special application of room temperature measurements involves measuring indoor air temperature. Here, errors, due to radiation influence on the sensor from surrounding surfaces, were surprisingly high. The means to estimate the radiative influence on typical thermocouples are presented, along with suggestions for modification of thermocouple sensors to lower the radiation impact and thereby improve the measurement accuracy.
Highlights
Indoor air temperature is an essential parameter for the thermal comfort and energy balance in a building [1,2]
The thermal voltage developed by a thermocouple made from homogeneous wires will be a function of the temperature difference between the measuring and the reference junctions only, because voltages generated from gradients due to any hot or cold regions along the wires will cancel
About 50% of the total error is due to the reference junction error. Compare this to an indoor climate situation where the difference between the temperatures at the measuring and the reference junctions is small, so that most of the output signal stems from the reference junction, and the thermocouple only adds a small correction to the reference temperature
Summary
Temperature is perhaps the most measured climatic variable in indoor climate research. Thermocouples are the most commonly used sensors for indoor temperature measurements, at least within the research. They are easy to assemble, can be made very small, and they do not suffer from self-heating, which is beneficial in a low-velocity environment. Commercially available instrumentation for thermocouples is mainly optimized for industrial use and may not be the best choice for indoor temperature measurements [7]. The measurement errors for room temperature measurements are dominated by errors in the reference junction. This means that accuracy cannot be enhanced by individual calibration of the thermocouples (a common proposal for solving the problem). We highlight the applicability of thermocouples to measurements in the room temperature range, and discuss the adaption of commercial data loggers for these kinds of applications
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