Resistance thermometer Insulation Resistance
If the sensing element and leads are not completely insulated from the case, a shunting effect occurs in which the case becomes a parallel resistor and lowers apparent readings.
In most industrial thermometers, with specified insulation Resistances in the 100-MO ranges, error approaches zero.
The manufacturer must take care to seal water-absorbing materials. The shunting effect decreases with low resistance elements, which accounts.
The shunting effect decreases with low-resistance elements, which accounts for the use of 25.5 PRT’s in laboratory measurements.