Good insulation resistance in a metal sheathed platinum resistance thermometer (PRT) probe is important for obtaining accurate and stable temperature measurements. If a probe is cycled frequently between hot and cold temperatures, its components become fatigued over time, and moisture can leak through the sheath seal at the probe handle. This is a common cause of decreased insulation resistance and noisy measurements.
Cold temperatures allow moisture to leak through the sheath seal into the probe’s insulation. This leakage corrupts the dielectric property of the probe, and resistance shifts occur. A probe can be taken to high temperatures to dry the insulation and temporarily correct the dielectric property. However, this does not repair the broken seal, and the problem can be expected to return when the probe is reintroduced to cold temperatures.
The resistance of the insulation between the metal sheath and any of the four PRT leads should be greater than 500 MΩ at room temperature for most 25 Ω metal sheathed PRTs.
The diagram at right is an amplified, cutaway view of a typical 4-wire PRT that illustrates the key components: PRT element, sheath, insulation, and leads.
Testing PRT insulation resistance
Insulation resistance should be checked with a multimeter or insulation tester with resistance or insulation test ranges of 0 to 500 MΩ or more. Be sure to abide by the manufacturer’s recommended safety and operating procedures for any equipment chosen.
As a sample test, I examined a Hart Scientific 5628, 25-ohm PRT, at room temperature using a Fluke 189 DMM, which can check resistance in the “ohms” function up to 500 MΩ. Please see the photo for the setup connections and tester indications.
Conclusion
Low insulation resistance can inject errors in your temperature measurements because the insulation can act as a parallel resistance to the actual element resistance. If, for instance, the insulation resistance is a mere 1 kΩ at room temperature (25 °C), then this resistance, when acting in parallel with a 27.5 Ω PRT element resistance, yields the following equivalent resistance:
Considering the sensitivity of a 25 Ω PRT is 0.1 Ω per °C,the decrease in element resistance from 27.5 Ω to 26.76 Ω corresponds to about a 7.4 °C lower measured value at room temperature.
Therefore, when you observe noisy readings or shifts in readings from your metal sheathed PRT, refer to the PRT manufacturer’s specifications and measure the PRT insulation resistance to determine whether it is causing the problem.
Using a Fluke 189 DMM, one lead is clipped to one of the probe leads, and the other lead is clipped to the sheath near the handle. The 189 indicates "OL" on the Ohms Range at room temperature. This signifies an insulation resistance greater than 500 MΩ and is considered good.