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Old 19th Feb 2019, 10:30 pm   #1
GMB
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Default Thermo-galvanometers (a lost technology)

I wondered what people know about this old technology that seems to have mainly died out today, except for the odd RF ammeter in old transmitters.

If you Google this term you mainly get references to an ancient piece of lab equipment which started this off.
What I am asking about is actual panel meters of this type.

For those who do not know what I am talking about, this is where the current flows through a heater coupled to a thermocouple which then feeds a meter. It has the great advantage of giving a true RMS reading and also being able to measure RF quite happily.

Until recently I had assumed that such things were for fairly high current, in the amps region, but I now believe that about 100 years ago you could get super-sensitive meters of this type. The question is how sensitive did they get?

There is a common old meter series, the Weston 425, which I know can get fairly sensitive. I actually bought one at a ham rally recently (fsd 2A) but I have seen photos showing that 115mA fsd versions certainly existed. But how much lower did they go??

Last edited by GMB; 19th Feb 2019 at 10:42 pm.
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Old 19th Feb 2019, 11:37 pm   #2
Graham G3ZVT
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Default Re: Thermo-galvanometers (a lost technology)

The RF ones were more usually called "thermocouple ammeters", but I'm sure you knew that.

When I was very young I built a "hot-wire ammeter", it may have even been described as a galvanometer, but it was far from sensitive.
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Old 20th Feb 2019, 12:20 am   #3
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Default Re: Thermo-galvanometers (a lost technology)

I am told that these were used until at least the 1980s in the research departments of major lamp manufacturers in order to accurately determine mains supply currents in discharge lamp circuits.

The odd waveforms and likely presence of harmonics would not lead to any inaccuracy.

The instruments were regularly calibrated by known DC currents that could be measured with great accuracy.

The lamp current, as distinct from the supply current, had to be measured in two different ways.
The AVERAGE current would be related to the light output, whereas the RMS current was relevant in regard to losses and heating.

If a lamp was designed for an average current of say one amp, then it would give about the rated light output with an average current of one amp, at almost any waveform.
However if this average of one amp consisted of a very "peaky" waveform with an RMS figure of 4 amps, then the lamp life would be reduced if compared to an alternative circuit that produced the one amp average with an RMS figure of 2 amps.
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Old 20th Feb 2019, 9:41 am   #4
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Default Re: Thermo-galvanometers (a lost technology)

Bolometers comes to mind, if my memory is right, used them to measure the rf out of x band radars.
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Old 20th Feb 2019, 12:15 pm   #5
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Default Re: Thermo-galvanometers (a lost technology)

I once came across a patent from 1912 that was an almost unintelligible translation from French that referred to the use of AFAIR "sensibilsed thermobolomer of Reiss" as a radio detector. Just did a google, and unfortunately there is a later Reiss, born much later, that has developed bolometers for space exploration.
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Old 20th Feb 2019, 3:58 pm   #6
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Default Re: Thermo-galvanometers (a lost technology)

This question got the grey calls working. Not exactly on topic but we used galvometers which had mirrors on to measure the out put from thermocouples.
This was part of a bank of Chart recorders used within the instrumentation department. It was so long ago I’d forgotten about it, thank you Forum for jogging my memory.
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Old 20th Feb 2019, 4:19 pm   #7
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Default Re: Thermo-galvanometers (a lost technology)

Ah, yes! A wonderfully elegant solution to the problem of how to magnify a small movement. Obviously, the longer you make the meter needle, the greater the difference a small angle of deflection will make. If the "needle" is actually a beam of light, it can easily be several metres long, if needs be. One milliradian of movement (about 1/3142 of a semicircle) will then cause the spot of light to move by a whole millimetre for each metre from the moving mirror.
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Old 20th Feb 2019, 4:44 pm   #8
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Default Re: Thermo-galvanometers (a lost technology)

Yes Julie you are right, and we used light sensitivite paper to burn the data on.
Gosh this is taking me back!
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Old 20th Feb 2019, 7:42 pm   #9
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Default Re: Thermo-galvanometers (a lost technology)

In past-times I measured microwave-power using an octal-based 'valve' whose envelope contained a half-wave-dipole for the relevant frequency, with a resistor across the feedpoint and a thermocouple tied to it.

The feeds to the DC side of the thermocouple had quarter-wave tuned-lines attached, to decouple them. All this was included in the evacuated envelope, and was dipped into the outway from the Rhumbatron while various variable shutters were poked-into the waveguide and wiggled to optimise LO drive/RX-noise-figure.
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Old 20th Feb 2019, 8:04 pm   #10
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Default Re: Thermo-galvanometers (a lost technology)

That sounds intriguing and complicated.
The Bolometer,if i remember correctly, was based on a Wheatstone bridge, one arm was heated by the output from the X band Klystron.
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