Good People Several (on & off line) have asked why. Mostly because some agency engineers do not believe that thermal grease accurately "transfers" thernal "information". I did some tests, and discovered that there could (occasionaly) be an atypical attachment that did not result in accurate data. And "occasional" inaccuracies are never acceptable for product safety type tests. But, for the most critical temperature measurement (iso xfmr) I have always relied on t/cs embedded during xfmr construction. On general principle, the NCB & NRTL engineers have convinced me that (assuming noise is not an issue) reliable mechanical contact is the most important requirement for a measurement where uncertainties can be reduced to device and instrument error. There is typically no "first-order" contact if thermal grease is used as t/c bead attachment. That said, poor mechanical t/c contact can be a problem when using cyanoacryalate, if the bead does not maintian UUT contact during adhesive cure. I assume that is why, during audits, some agency engineers want to see me appy a T/C. Temperature measurement in a hi di/dt enviroment can be messy; but most t/c noise problems can be fixed using hi-CMR differential inputs and correct thermocouple material. I prefer the HP34970 and type T t/c wire. Also, routing the t/c wire at right angles to mag fields may help. To verify that induced/radiated noise is not a significant source of error, operate the UUT at lo-line until well heated, observe the channel and in question, and see if the indicated temperature suddently changes when power to UUT is turned off. luck, Brian
From: Brian O'Connell [mailto:[email protected]] Sent: Monday, April 28, 2003 9:55 AM To: [email protected] Subject: RE: Thermocouple glue My use of thermal grease was discontinued several years ago by request of various agency engineers reviewing test data/technique. And more recently, during my ISO 17025 audit, the NCB auditor explicitly directed me to never use thermal grease for thermocouple application. And auditors from other NRTLs/NCBs have emphasized, at least verbally, that thermal grease is not acceptable.
