Abstract
Electrical insulation resistance measurements and standing dc voltage measurements have been made on fifteen standard platinum resistance thermometers in the range 631 °C to 1064 °C. A model for the insulation leakage resistance in the thermometer-furnace system is proposed that takes into account ground leakage loops and the dc insulation battery effect. A study of the properties of the insulation battery has led to a more complete understanding of the insulation leakage process, particularly in the commonly used silica insulation. It has been discovered that both the dc and ac insulation resistance of silica-insulated thermometers can be increased by orders of magnitude by applying a dc bias voltage of about +6, 4 V between the thermometer leads and ground. This bias voltage dependence has led, in turn, to an explanation for a severe ground effect reported elsewhere that caused the insulation resistance to deteriorate for days at a time. By applying a +6, 4 V bias to the thermometer leads, it has been demonstrated for a 25 Ω thermometer that the dc and ac leakage temperature error at 962 °C can be reduced from over 90 m°C to about 0,1 m°C, and that the dc error can be reduced to about 0,1 m°C over the range 631 °C to 1064 °C. It is shown that this bias voltage technique can readily be applied to a dc commercial thermometer bridge, and can be used to estimate the leakage temperature error during a normal temperature measurement.