Richard Clark wrote:
On Fri, 10 Dec 2010 19:52:55 +0000, Ian Jackson
wrote:

Do a Google on "Grease Spot Photometer" (back to school physics, over 50
years ago!). Very simple to make, and pretty accurate.

Hi Ian,

A very good suggestion which immediately led me to:
http://www.phy6.org/outreach/edu/greaspot.htm

A variation of this that I calibrated in the lab is an Optical
Pyrometer:
http://www.pyrometer.com/Pyro_Optical.html

Comparison measurements can be very accurate iff what you are
comparing to (aka standard) is known to sufficient accuracy. Both
references provide more than enough to inform the reader with the
essential details.

Slightly more elaborate methods of measuring RF power fall into the
Calorimetric method (actual heat). Hewlett Packard made the ultimate
heat transfer standard - the HP 434A power meter DC to 12GHz!


I think HP made a number of DC substitution/transfer sorts of power meters
The 434 was unique in using flowing oil and it could directly measure
watts, without needing attenuators (which have their own calibration
issues).

Didn't the 432 used the idea of DC power substituting for RF power to
bring the sum to a fixed temperature? The difference between the 432
and the 434 is that the thing measuring the temperature is also the RF
load in the 432. The 434 just uses the RF as a heater, and relies on
the DC powered heater and RF powered heater being matched.



A NIST Type IV power meter (like those from Arbiter) definitely does DC
power substitution, and uses the HP/Agilent thermistor heads.

Once you get away from "replacement heat" sorts of schemes, you'd be
into the classic calorimeter.. measure the temperature change over time,
and then turn that into energy. You'd calibrate it by putting DC on the
same sensor, essentially measuring the thermal capacity.


The thermocouple heads (8481A, 8482A for instance) work pretty much the
same way as the thermistor mounts.. measuring the heat dissipated by the
RF power coming in. They're not a substitution measurement though..
The actual sensor changes voltage in response to temperature (with a
clever compensation scheme so that overall temperature doesn't affect it)

But these are all basically thermal sensors (as opposed to, say, RF
voltmeters, like a diode detector as in a 8481D or 8484A head) and they
dissipate the RF power being measured.



73's
Richard Clark, KB7QHC