5.1.1.2. Calibration

It is an intrinsic property of thermocouples that their characteristic curve is neither linear nor does it follow a persistent function throughout the measurement range (Fig. 5). Modern thermocouple thermometers, however, usually apply appropriate corrections automatically. If an accuracy of better than 0.3 °C has to be guaranteed, individual calibration of each thermocouple is necessary (e.g. in a water bath against an accurate laboratory thermometer). In the temperature range relevant in honey bee life, of about -30 to 100 °C, linear correction functions are sufficient. Advanced data loggers provide the possibility to store the corrections permanently in a device or connector memory (e.g. Ahlborn Messtechnik, http://www.ahlborn.com).

For air temperature measurements in direct sunlight, heating of the thermocouple junction by solar radiation has to be taken into consideration. To avoid any resulting measurement error, a calibration comparing the readout between two thermocouples, one in sunshine and one in the shade, has to be applied (Stabentheiner et al., 2012). To do this properly, the intensity of solar radiation also has to be measured (see section 5.4.).

Fig. 5. Characteristic curves of the main types of contact thermosensors (schematic), with main benefits. For main constraints see text. Measured parameters: Voltage (V) or resistance (R, in Ohm). Graph by Anton Stabentheiner.

Figure 5