[time-nuts] Any experienced HP 2804A thermometer users outthere?

Didier didier at cox.net
Sat Jan 24 22:43:45 EST 2009


 

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Bill Hawkins
> Sent: Saturday, January 24, 2009 8:54 PM
> To: 'Discussion of precise time and frequency measurement'
> Subject: Re: [time-nuts] Any experienced HP 2804A thermometer 
> users outthere?
> 
> John,
> 
...
> 
> The thermal conductivity of an insulator like a PC board lies 
> somewhere between air and metal. When you measure a small 
> area of a PC board, that area must cool off unless it is the 
> ultimate source of heat. The only way to avoid that is to 
> have probe leads that are at the temperature you are trying 
> to measure, sort of like a Guard shield for a microvolt measurement.
> 

When using thermocouples, we do that by snaking the wire leading to the
thermocouple along the PWB and hold it there with small pieces of tape, as
opposed to glueing the thermocouple to the board and letting the wires go
away from the board immediately. That ensures the last few inches of wire
leading to the thermocouple are at or close to the temperature of the PWB,
so heat flow to and from the thermocouple itself is minimized.

Didier





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