Preparation of thermistor cables used in geothermal investigations

Abstract

Thermistors are small rugged beads, disks, or rods of sintered oxides, which can be used to measure temperature with a precision of ±0.01°C with a simple four-dial Wheatstone bridge. Thermistors of number 1 material have a spinel crystal structure, the approximate formula being Ni0.6Mn^4Mn+2O4. Thermistors of this material generally are used in temperature sensing, and those with a nominal resistance of 1,000 ohms at 20°C have a negative temperature coefficient of resistance of about 5 percent per deg C at that temperature. Most thermistors differ in resistance and in change of resistance with temperature and so are not interchangeable.An empirical adaptation of the equation for electrical resistivity of semiconductors fits the resistance-temperature data for thermistors very well:R=A exp where R is resistance, in ohms; Tis temperature, in degrees Celsius; and A, B, and C are constants. For this equation, interpolations between 15°C intervals are good to ±0.01°C. The effect of pressure on thermistor resistance is negative and very small, about 10~7 bar"1 ; at the bottom of a 3,000-meter hole, in a logging cable, a thermistor (nominally 50,000 ohms at 20°C) would have a change of resistance of 0.1 ohm, equivalent to + 0.0002°C, due to the pressure of the water column in the hole.In a separate study of precision of measurement, a platinum thermometer was used to determine the absolute temperature of 23 carefully made ice baths (nominally at 0°C), and the mean was + 0.0004° C, the standard deviation being 0.0014°C. These values constitute, respectively, the systematic and the random errors of both the thermometer measurements and the bath temperatures. Five thermistors in the same baths showed a standard deviation of temperature measurement of 0.003°C; however, the coherent variation of the resistances of all five thermistors from bath to bath shows that relative-temperature measurements with thermistors can inherently be as precise as ± 0.0001° C.In the calibrations of 41 thermistor cables at the ice point (0°C) in a large box, the mean temperature as measured by the platinum thermometer was 0.0018° C, and the standard deviation was 0.0031°C. Thermistors in cables can be accurately calibrated in such baths.