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

Neville Michie namichie at gmail.com
Sat Jan 24 18:20:31 EST 2009


On 25/01/2009, at 6:05 AM, Robert Atkinson wrote:

> Hi John,
to measure a solid surface I would use a very thin copper/constantin  
thermo-couple
and put the "cold " junction on your thermometer probe in an  
insulated shield.
The "hot" junction goes on the part of the board to be measured.
A microvolt DVM is needed to measure the thermocouple, a 1 microvolt  
difference is about .025 Kelvins.
Here you get the lack of self heating of the thermocouple and very  
small down-lead conduction because of the thin wire,
but you get the stability and accuracy of the quartz thermometer  
underpinning your measurement. There is
a neat little quadratic that converts microvolts from the  
thermocouple to temperature difference.
The main difficulty with using thermocouples is the accuracy of the  
cold junction and your quartz thermometer solves that problem.
If you have doubts about your microvoltmeter you reverse the  
thermocouple connections to get twice the error difference.
If you want more bang for your buck wire 5 thermocouples in series,  
insulate the junctions from each other, and use the
composite junction on your board to get a resolution of 0.005  
Kelvins,. The reversed polarity trick also works here to remove DVM  
offset.
I know this is not using the quartz thermometer directly but it would  
be a very good way to measure a board to 1millikelvin.
For theoretical reasons a thermocouple can not have a temperature  
offset.
cheers, Neville Michie

> --- On Sat, 24/1/09, John Ackermann N8UR <jra at febo.com> wrote:
>
>> From: John Ackermann N8UR <jra at febo.com>
>> Subject: [time-nuts] Any experienced HP 2804A thermometer users  
>> out there?
>> To: "Discussion of precise time and frequency measurement" <time- 
>> nuts at febo.com>
>> Date: Saturday, 24 January, 2009, 3:42 PM
>> The temperature probes for the 2804A quartz thermometer seem
>> primarily
>> intended for liquid immersion.  I'm looking for
>> practical tips on how to
>> couple the probe to a solid surface (e.g., a PC board) for
>> accurate
>> temperature measurements of the surface.
>>
>> Anyone know the best way to do this?
>>
>> Thanks,
>>
>> John
>>
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