John, In my company, I often run into this problem and a non-contact or infrared thermometer is not always pratical. Like you, I also had to resort to momentary turn off to get accurate readings. In one instance, I had magentic fields strong enough to cause damage to the instrument. There are several things you can do and not all of these are always necessary.
1) I exclusively use "T" type thermocouple wire because it has no ferrous content and the effective temperature range more closely "brackets" the temperatures I am interested in. The reason for non-ferrous wire is strong magnetic fields not only induce erroneous readings, but they also cause heating of the thermocouple itself. In the past I typically saw this problem with the "K" & "J" types. Simply doing a momentary turn-off does not correct this problem as the wire takes some descrete time to cool off and after a point you don't know if you device under test is cooling as well. The problem is that I've never found a handheld meter that accepts "T" type wire. 2) The next thing I do is insure the thermocouple conductors are not separated, keeping loop area small. Twisting is ideal if you can do this without damaging to the welded tip. Ideally you should be able to weld your own. 3) If possible, try to orient thermocouple wires to avoid 'cutting' flux lines. 4) Try using ferrite beads to knock down any common-mode RF induced on the wire and being conducted into the instrument. The readings you are interested in are essentially DC. 5) If you are still having troubles, construct a Faraday shield by wrapping with copper foil. Grounding the shield may be helpful. The shield needs to fully enclose the thermocouple bead and surround the wires for some distance, far enough to exit the area where the fields are. Be sure the copper foil is insulated inside and out. Note, readings may take a little longer. 6) Finally, if all else fails, you can resort to the resistance method described in IEC 61558-1 (fomerly IEC 60742) clause 14.2. This takes a milli-Ohm meter. -doug ----------- Douglas E. Powell, Compliance Engineer Advanced Energy Industries, Inc. [quote]Hi John, A possible solution would be to use an infrared non-contact thermometer. They measure temperature essentially by pointing them at the object of interest. They have a few drawbacks that I know of: 1. You need to have visual access to the part or surface of interest. 2. You need to account for "spot size" since its detector essentially integrates all of the infrared in its field of view. The spot size changes with distance from the thermometer to the surface of interest. 3. If you are trying to graph temperature over time: you can't just glue it in place and hook it to a data logger. I don't know if it would be worth rigging up some kind of tripod to keep the infrared detector aimed at the point of interest. We had one in the lab for a couple of days. It was a handheld, battery powered unit about the size of a Palm Pilot. Sadly, I only got to play with it a little bit before it was taken away. I wanted to see how accurate it was at measureing component temperatures on a circuitboard. Chris Maxwell | Design Engineer - Optical Division email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315 797 8024 NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA web www.nettest.com | tel +1 315 797 4449 | > -----Original Message----- > From: Crabb, John [SMTP:jo...@exchange.scotland.ncr.com] > Sent: Monday, January 07, 2002 9:02 AM > To: emc-p...@majordomo.ieee.org > Subject: RE: EMC-related safety issues > > Seeing that we have got round to the subject of thermocouples, etc, I > often > use a Solartron SI3535D datalogger with thermocouples for measuring > component temperatures, and find quite often that it does not give > "correct" > readings when thermocouples are placed on transformers in switching > power supplies, high voltage transformers in monitors, etc. I can get > a > "correct" reading by switching off the EUT momentarily, obviously > removing the source of the problem. Note that the problem can occur > even if the thermocouple is not making an electrical connection to the > component winding involved. > > Any suggestions how to overcome this ? My previous antique datalogger > didn't have this problem, but it eventually had to be scrapped due to > lack > of spare parts - and the expectation that a more modern unit would be > better !. > > Regards, > John Crabb, Development Excellence (Product Safety) , > NCR Financial Solutions Group Ltd., Kingsway West, Dundee, Scotland. > DD2 3XX > E-Mail :john.cr...@scotland.ncr.com > Tel: +44 (0)1382-592289 (direct ). Fax +44 (0)1382-622243. > VoicePlus 6-341-2289. [/quote] Replies to this message may be posted in a public forum. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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