Reginald Neale wrote:

> One classical way to deal with transducers that are sensitive but
> nonlinear is to put them inside a feedback loop where they are driven
> to maintain a fixed operating point.

  Nice idea, but in the case of temperature sensing at least, there is
still a problem of compensating the "fixed operating point".  You can
for example, add DC power to RF to stabilise your thermocouple at a
certain temperature, but the required DC power is now dependent on
ambient temperature as well.  You need a second identical thermocouple
to compensate this, or else switch ("chop", albeit slow) your RF input
on and off.

  Either you use two matched heater/ thermocouple pairs with the
required isolation built-in (traditional RF meter thermocouples have
*no* isolation; all four wires are connected), or you will have to
switch something using mechanical (relay) switching.  I can't see any
other way to fulfil isolation, linearisation and temperature
compensation.
--
  Cheers,
        Paul B.