On Wed, 22 Jun 2005 22:59:00 +0100, Ian White GM3SEK
wrote:

Certainly. A thermal wattmeter determines the power delivered into a
load resistor without making any assumptions about how and why it got
there. It only involves measurements of mass, time and temperature rise,
and a knowledge of specific heat capacity, so it is completely
independent of any assumptions related to RF transmission line theory.

Hi Ian,

So here I will follow your plunge into the rabbit hole.

That thermal wattmeter, and I know a variety of them, each very
different from the other, has a scale, just like the Bird. Every
meter has scaling circuitry for that scale, just like the Bird.
Accurate Thermal wattmeters use AC references and need to transform to
DC to drive the scale, just like the Bird. Accurate Thermal
wattmeters don't even directly measure mass, time, temperature rise,
or specific heat capacity - they infer them by comparison. The
measurement is balanced against a simpler substitute - one difference
from the Bird that is of no consequence.

Every step of the way, there is a conversion performed to meet the
needs of displaying a result, just like the Bird.

Further, the best and most accurate thermal wattmeter is as restricted
as a Bird Wattmeter because it (they) too is (are) load specific. A
50 Ohm thermal wattmeter is no more correct on a 75 Ohm line than a
Bird Wattmeter. Those same thermal wattmeters all quite deliberately
employ the same printed restrictions of operation at a known load
without reflections present.

If you are trying to make an appeal to a calorimeter, with thermometer
in hand, you are simply exacting the algorithms you must use, compared
to the already quantified results that the Bird will offer by the
similar math being embedded in the coupling and scaling of a tensioned
needle indicator in a magnetic field. Current, field, mass, tension,
deflection, time - still in the jumble, they evaluate to power. I've
calibrated meter movements, balanced needles, replaced springs,
adjusted trim pots, tuned capacitance, replaced resistors - and I have
worked and calibrated calorimeters, bolometers, thermistors,
thermocouples, barreters, Wollaston wires, diodes, thermopiles, black
bodies.... But I've said all this before, and it cannot have escaped
your attention. So just what is it about this list of thermal
technology that is so decidedly uncircular that it trumps the Bird?

There are any number of ways to measure power, none of them are
exclusive, and certainly none can claim to achieve this feat through
other than ordinary transformation of physical actions.

73's
Richard Clark, KB7QHC