[ARC5] RE meters

[J. Forster]

If you want any kind of accuracy, you have to use the one it's designed for.
Likely any DC voltmeter or 1 ma with the series or shunt R pulled will work.
Basically you need a meter that will go full scale on millivolts at a milliamp
or two.

Apply a monitored current to your TC (less than the max allowable) and measure
how much voltage it puts out with a DMM. For example, if the TC is rated at 5
amps, RF, put 5 amps DC through it and measure the TC output. You also need to
measure the resistance of the TC (usually a few ohms) but use a low current
ohmmeter so as not to blow it.

You now know the OC voltage and source resistance and can select a meter and
resistor that will match those values.

For example, if your TC puts out 10 mv and has a 20 ohm R and you have a 30 ohm
meter that is 1 ma FS:

Circuit R = 20 + 30 = 50 ohms

Voltage = 0.01

So Current = V/R = 0.01 / 50 = 2 ma, but your meter is 1 ma FS, so you need 50
ohms more in the circuit.

Good luck,
-John

Rev. Don Sanders wrote:

> Anyone have a thermocouple I can use with the meter I have?
>
> Healthfully yours,
>                           DON
> ----- Original Message -----
> From: "J. Forster" <jfor at quik.com>
> To: "philip" <signetics at netzero.com>
> Cc: <ARC5 at mailman.qth.net>
> Sent: Monday, April 18, 2005 4:25 PM
> Subject: Re: [ARC5] RE meters
>
> > That's not a surprise. The meter is actually linear, but the scale is
> calibrated
> > to correct for the non-linearity in the current sensor.
> >
> > The thermocouple unit has a series resistor in the RF circuit (The current
> > transformer just isolates and scales.. it is linear). The power dissipated
> in
> > the R of the sensor is current squared x R  (I**2 x R) and is non.linear.
> > Assuming the heat losses in the thermocouple assembly are by conduction
> only
> > (good approximation) the temperature rise will be proportional to the
> power
> > dissipated, or the square of the RF current. The thermocouple produces a
> voltage
> > proportional to temperature, and so is linear.
> >
> > So, in short, the non-linearity is due to the heating effect of the RF
> current,
> > which is proportional to the current squared. The scale corrects for this.
> >
> > -John
> >
> > philip wrote:
> >
> > > One thing I do know about the antenna relay meter is that
> > > it is nonlinear. The meter movement that is.
> >
> >
> >
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