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

[Bill Hawkins]

John,

The best way to couple a probe to a solid surface is to drill a hole in
the solid that is deeper than the probe. Put some thermal compound in
the hole that will exclude air and insert the probe. You must eliminate
air between the probe and the surface. Sadly, that won't work with the
usual thicknesses of circuit board and the size of the 2804 probe.

This is a simple exercise in heat flow. There are published figures for
thermal conductivities of circuit boards, metals and air. There are
formulas remarkably like Ohms law relating series temperature drops,
thermal conductivity, contact area, and the temperature difference
between the source of heat for the system and the sink (which would be
the probe leads and ambient air).

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 I think of time-nuts and temperature, oven temperature comes to
mind.

With a PC board, I'm OK as long as it doesn't char.

Can you be more specific about the application?

Yours for less speculation and more useful answers,
Bill Hawkins


-----Original Message-----
From: John Ackermann N8UR
Sent: Saturday, January 24, 2009 9:42 AM

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