To All from PA0SE
Larry, VA3LK wrote:
Greetings All:
I uncovered two ancient RF Thermo Couple Meters this evening, I had put
them
away some years ago in anticipation that at some point I might want to
measure RF amps. They are identical, made in 1940, Ref No. 10A/8479.
Full
scale 2.5 amperes, noted "for Radio Frequency Only". They are of what I
would call "light construction" not the heavy duty construction that one
associates with the better products made for WW II.
How might I calibrate them? I have some excellent calibrated resistors
but
they are in general a little small to get a couple of amps into them
without
some residual damage. Is some 60 Hz a little to low "for Radio Frequency
Only"? If anyone has a simple process I would appreciate hearing about
it.
Thanks in advance.
Larry
VA3LK
As an experiment I have put two thermo couple meters in series to measure
my
antenna current on 137 kHz. The instruments were made by Weston and have a
range of 3 A; one has a square case and the other a round one.
Both read a RF current of 2.40A . I then connected one of the instruments to
read the current in the wire to the bottom end of the antenna loading coil
(the top end being too "hot") in series with a resistor of 2.8 Ohm, as
measured by a digital ohmmeter. The voltage over the resistor was rectified
by a germanium diode and the resulting DC measured by the DVM. The
thermocouple meter indicated 2.50A, the DVM 9.12V. The peak RF voltage over
the resistor must have been 9.12 V + about 0.3V = 9.42V and the RMS value
9.42V /
1.414 = 6.66V. So the RF current was about 6.66V / 2.8 Ohm = 2.38A; within
4.8% of the reading by the thermo couple meter.
I then fed a current of 2.5A at 50Hz through the meters, with a digitial
ammeter as reference. The readings were 2.55A and 2.60A, so pretty close
to the readings at RF.
DC should be avoided for calibration.
I fed a current of 2.5A DC to the meters, again checked by the DAM.
Readings were 2.55A and 2.12A.
I then reversed the direction of the current and readings became 2.55 A and
3.00 A.
When the thermo couple is OK I think there is little reason not to trust the
reading of a thermo couple meter, even when it is pretty old. My square
meter
carries a sticker that says it was checked in 1965 for the last time.
But when the thermocouple has been blown - not a difficult job I can assure
you from personal experience - the reading is only correct when the current
is zero amps.
Then a totally unconnected question to our friends in the UK and he USA.
In The Netherlands we write 5 A, but 5 ampère, 10 V but 10 volt. So in
general the unit is written with a capital when abbreviated but with a lower
case when written in full. And with a space between figure and unit.
What is correct in English or American, 3 V or 3V?. Same question for 10
amp, 10amp, 10Amp, 10Amps?, etc.
And what about rf or RF? if, IF, i.f. or I.F. (intermediate frequency) etc.
?
73, Dick, PA0SE
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