How to Test the Insulation Performance of Sheathed Platinum Resistance Meters

Testing the insulation effectiveness of encased platinum resistance meters is an important step in making sure they work well in industrial settings with high temperatures, high humidity, and corrosive materials. Insulation breakdown can directly cause measurement drift, signal interference, and even sensor failure. This is why it is important to test regularly using approved methods. Based on industry standards, the following is a thorough testing process and important technical points:




I. Getting Ready for the Test: Making Sure the Measurements Are Correct



Environmental Control: Testing should be done in a place that is dry and free of dust so that readings aren't affected by moisture or dirt on the surface. The best temperature for the room is between 15 and 35 degrees Celsius, and the humidity should be less than 75%.



Before using the megohmmeter, do a self-test and calibration to make sure the output voltage is steady (usually DC 500V) and the measurement accuracy is good enough.



Confirmation of Test Component Status: The encased platinum resistance meter should be at a steady temperature. Don't test right after the temperature changes a much. To get rid of thermoelectric potential and residual charge interference, it is best to let it sit in an atmosphere with a steady temperature for at least 30 minutes.







II. Standard Testing Method: Using a megohmmeter to measure



How to Wire



Connect the two leads of the armored platinum resistance thermometer to the "L" terminal (line terminal) of the megohmmeter by short-circuiting them.



Attach the metal sleeve that protects the wires to the "E" terminal (ground terminal).



Make sure there is excellent contact and that there is no oil or oxidation that could impact conductivity.



Put on the test voltage



Turn on the megohmmeter and give it a 500V DC voltage for one minute. Get the stable insulation resistance value. This voltage effectively excites possible leakage routes without harming the insulating material.



Standards for Acceptance



Standard Industrial Environment: Insulation resistance must be at least 100 MΩ.



Applications with high requirements, such nuclear power and pharmaceuticals, need insulation resistance of at least 500 MΩ.



If the measurement is less than 10 MΩ, it means that the insulation has been severely damaged and that water may have gotten in, contamination may have occurred, or the material may have aged.



Technical Basis: The IEC 60751 standard says that a platinum resistance thermometer's insulation resistance should be at least 100MΩ when measured at 500VDC to make sure it stays stable over time.







III. Important Things That Affect Test Results and Safety Measures



Surface Contamination and Moisture: If the surface of the armored tube has oil, salt, or moisture on it, a conductive coating may form, which will make the resistance seem lower than it really is. Before testing, you should wipe the surface with anhydrous alcohol and leave it in a dry area for a while.



Treatment of the lead wire and junction box: To avoid parallel routes impacting the measurement findings, the secondary instrument connection should be unplugged during testing. If there are terminal blocks in the junction box, other circuits must be kept separate.



Temperature Compensation: As the temperature rises, the insulation resistance goes down. If testing is done in a hot place, the temperature of the room should be written down and included in the report. If necessary, data correction should be done.



To check for repeatability, it is best to take two measurements at different times. If the result divergence is more than 10%, you should check the contact state or the stability of the equipment.







IV. How to Handle and Manage Trends After the Test:



If the test passes, it should be appropriately labeled, the results should be recorded, and the test should be included to the equipment maintenance file.



If it doesn't work, you can try the following fixes:



Use an alcohol swab to clean the surface and then test again.



To get rid of moisture inside, bake it in the oven at about 150°C for 2 to 4 hours. Then let it cool and test it again.



If it still doesn't work, the internal insulation layer is broken, and the sensor needs to be replaced.



Industry Practice: It is best to do insulation resistance testing every six months to see how things are changing and go from "passive replacement" to "predictive maintenance."