OCR Text |
Show successive thermistor as it is connected by plug and switch, and the dial readings noted. These are later compared with the thermistor calibration curves to determine snow temperatures. The thermistors must be individually calibrated prior to installation. Although their temperature- resistance curves are very similar, enough variation exists from one thermistor to another that a separate curve must be obtained for each one. Thermistor resistance is a non- linear function of temperature. When the Wheatstone bridge is connected in the fashion shown, dial readings are also a non- linear function of measured resistance. In this case the non- linearity feature tends to cancel with the result that dial readings are almost a straight-line function of temperature over the range of 0° to - 15° C. commonly found in the snow cover. After the thermistors have been attached to their lead wires, they must be coated with an insulating material. This affords protection from weathering in the field, and provided electrical insulation in the calibrating bath. Encapsulation in RTV silicone rubber is the best form of coating, but repeated coatings with white acrylic plastic spray enamel is satisfactory. Once the thermistors have been coated, they can all be placed as bundle in the calibrating bath and the lead wires brought out for convenient connection to the test set. A wide- mouth thermos bottle filled with an ice- salt or ice- alcohol mixture will provide a satisfactory calibrating bath. The calibrating standard may be a glass or a dial thermometer inserted in the bath. The latter is more convenient. The temperature of the bath is successively adjusted to different values within the expected operating range by addition of ice, water, or freezing point depressant as required. After the bath has achieved equilibrium at each new temperature, the corresponding dial reading of the test set for each thermistor is recorded. From the record thus obtained a calibration curve can be constructed for each thermistor. - 3 - |