RF Ammeter Question

[k4kyv]

I have several Weston RF ammeters with external thermocouple.  The thermocouples are all marked Type "B" Heating Element.  Some have four terminals: Two marked "METER", + and −, two others marked "HEATER", H and H.

But others have five terminals on the thermocouple heating element.  Like the others, they have the two terminals marked "HEATER", but there are three terminals on the METER side: +, − and a third one between the plus and minus, marked "H".

My question is what does the "H" terminal on the METER side connect to?

[KA2DZT]

Don,

Is it possible that two of the H terminals are bridged for some reason??  Did you try to ohmmeter the H terminals??

Fred

[k4kyv]

I looked at the matching meters that go with the thermocouples (only have a  couple).  They both have 3 terminals each: plus, minus and H. I suppose you would just leave the extra H connection unused with a thermocouple that lacks one.  Ohmmeter check shows continuity amongst all 5 terminals, something to be expected, considering how a thermocouple works.  I still wonder what is the purpose of the extra set of H terminals.

[WD8KDG]

Don,

What color are the wires for, plus, minus, & H???

All the true thermocouples I've played with only had two wires. Red was always negative. There are many types of thermocouples and the color of the two wires tells what type. I would suspect a type K, maybe a J would work best for the low temp generated by RF currents.

Now there are 3 & 4 wire RTD's.

Craig,

[W7TFO]

These are not externally heated thermocouples, like for measuring exhaust temp.

They have no wires, either.  Only binding posts.

They are usually tan, sometimes black Bakelite units that look a lot like a panel meter, but without a face.

The RF current goes thru the unit, and the remote meter reads the resultant DC.

I have one Weston with 5 binding posts: two RF, two meter, and ground.

73DG

[WD8KDG]

Ok,

Starting to make sense, sorta. Still don't understand Don's description that there is continuity amongst all 5 terminals. Trying to picture this, in what is left of my gray matter: RF goes in one terminal and out another. DC from a thermocouple goes to a remote meter via two wires. Then there is the 5th terminal.

I wouldn't think there would be contunuity between RF (or what ever it heats) and the thermocouple.

So, I'm still learning.

Craig,

[W7TFO]

Some RF couples are made by literally soldering two dissimilar wires directly to a piece of very small 'heating' wire.  It is all at slightly more than DC zero ohms.

Those are usually in self-contained panel meter types.

There are some that have isolation of the output as well, but I can't get my hands on one just yet.

Without the case, they be wee devices...

73DG

[WD8KDG]

Beginning to see the light. Years ago it seemed strange, DC & AC flowed in the same circuit. Took a while for radio to sink in.

So connecting the two dissimilar wires (thermocouple) to the heating wire sort of adds up. Thats a lot of RF, & a thermocouple generates just a fraction of a volt. Being able to read that DC fraction.......well, it works I guess.

Craig,

[Mike/W8BAC]

Don, My best guess is the extra H terminal would be to meter the heater voltage. But that leaves room for the question, "why not read the heater voltage directly from the heater input terminal?" Rather than an ohm meter it would be worth a look inside the thermocouple to see where the extra H terminal is connected in the circuit.

Mike

[k4kyv]

They have no wires, either.  Only binding posts.

They are usually tan, sometimes black Bakelite units that look a lot like a panel meter, but without a face.

The RF current goes thru the unit, and the remote meter reads the resultant DC.

I have one Weston with 5 binding posts: two RF, two meter, and ground.

That's exactly what I have, except the extra binding post is labelled "H" instead of ground. Both panel meters have the extra H terminal, but some of the heater (thermocouple) units have the extra H terminal and some don't; otherwise they are identical.

Something I never understood about those meters is that a thermocouple is a device consisting of two dissimilar metals that produce a voltage proportional to a temperature difference across the junction of the pair of conductors. I can see that a given current through a conductor will generate a certain amount of heat at the junction, but what keeps ambient temperature from affecting the reading, and what keeps the entire thermocouple junction from quickly reaching thermal equilibrium once the RF current starts flowing, so that the meter reading immediately begins dropping towards zero? It would seem that one of the dissimilar metals would have to be attached to a massive thermal reservoir that would serve as a stable reference whose temperature is unaffected by surrounding temperature or the heat generated by the current flow, in order to maintain a temperature difference across the junction that remains proportional to the rf current.

Don, My best guess is the extra H terminal would be to meter the heater voltage. But that leaves room for the question, "why not read the heater voltage directly from the heater input terminal?" Rather than an ohm meter it would be worth a look inside the thermocouple to see where the extra H terminal is connected in the circuit.

The thermocouple unit simply has two output terminals that connect to a DC meter that does directly monitor the output voltage of the thermocouple.  The typical meter movement is 0-1 milliamp DC, and it indicates voltage per Ohm's law.

The DC meter may have the thermocouple included inside the case, or they may be two separate units connected together by a pair of wires.

[W7TFO]

All thermocouples produce current as long as they are heated. 

The junction does the trick.  Some of those junctions are really small, I have one that produces measurable DC from just 7 mW input.

No thermal reservoir needed.

73DG

[Mike/W8BAC]

The typical meter movement is 0-1 milliamp DC, and it indicates voltage per Ohm's law.

Are you sure about this Don. I have done my homework on this subject and from what I have in the shack the meter movements are millivolt meters reading the DC directly from the thermocouple. The thermocouples are external, bimetallic wishbone arrangements with single wire connected to the heater. The thermocouples have either wide spacing to allow radiation to stabilize the the coupling or large copper masses to do the stabilizing.

[W7TFO]

Meter movements..yeah...I have some Westinghouse sets that are 30uA...pretty sensitive.

I've never seen a 'split' RF ammeter using millivolts as end scale.  Only uA.

But then again, I'm not anywhere through seeing things.

73DG

[W2PFY]

38.7 The RF Ammeter and True rms
Current to a radio transmitting antenna is commonly passed through a small resistor, a shunt, that is
thermally connected to a thermocouple or other thermometer and mounted in a thermally insulating
blanket. The rise in temperature is a measure of the current and is often sensed with a thermocouple.
This is an example of true rms indication. Root mean square current is the square root of the integral
of the square of the instantaneous current over an unspecified time divided by that time. It is intended
to represent a stationary ac waveform by a single value that is equal to the direct current which would
dissipate the same power in a resistive load. The RF ammeter does that precisely. The indication is not
particularly linear, but it can easily be calibrated by applying dc to the input.
Other schemes for measuring rms current depend on analog multipliers and subsequent integration.
They are limited by crest factor, the ratio of highest instantaneous peak to the rms over a measuring
period. Inexpensive meters simply measure the peak, assume a sinusoidal waveform, and scale to rms

[W7TFO]

Here are a few shots:

#1 is an old WECo model.

#2 is a Westinghouse assembled.

#3 is #2 opened to show the little glass job with a very tiny inverted 'V' shaped wire forming the couple.

BTW, none of these is a Weston like Dons'.

73DG

[WD8KDG]

On the topic of thermocouples: The curve for each "Type" is voltage vs temperature. Each "Type" has it's own curve. Yeah, there is current flow, but the meter responds to the voltage input.

Another item to consider is reference junction error. Just as the two dissimilar metals in the thermocouple produce a voltage, if the metals in the terminals and the wire between the thermocouple & meter are different, error is induced. When I use to work for a living, my black box of calibration equipment had short lengths of thermocouple wire of various "types". So when working with a, for example "type K" thermocouple, type K wire was used between the thermocouple in question & calibration equipment.

Pick'en belly button lint made a difference.

Craig,

[Mike/W8BAC]

Hi Dennis,

Thanks for the pictures. The vacuum tube TC is cool. Below is a PDF showing a 1937 patent of a thermocouple. It's just a hunch but I think if you look close at the Westinghouse TC you will see a single wire (+ to the meter) welded to the heater and a second wire branched off that single wire (- to the meter) as shown in the patent drawings.

The (+) wire welded to the heater is a chromel (non magnetic 90% nickel, 10% chrome). The branch wire off the positive wire (-) is alumel wire (95% nickel, 2% manganese, 2% aluminum and 1% silicon) and slightly magnetic.

[Mike/W8BAC]

Here are the two main thermocouples I have been working with.

[Mike/W8BAC]

The second thermocouple above is from a 20V-3. If my arithmetic is right 1000 watts into a 50 ohm load should produce about 4.5 amps of current. My millivolt meter movement requires .022 volts for a 4.5 amp indication. That's not much and the current produced in a TC is low. So, that's the reason I asked Don if he was sure.

I know that many other methods of measuring current are available but this is what I have been exposed to. Thanks

Mike

[AB2EZ]

Don et al.

Whenever you use a thermocouple, you are really using three, physically separated thermocouples

The 1st is the obvious one where the two dissimilar metals (wires) are welded together and in thermal contact with the heater resistor.

The 2nd is where one of these two dissimilar metals (wires) is joined/welded to one of the copper wires that leads to the meter. That is also a junction between two dissimilar metals (one of which is copper).

The 3rd is where the other of the two dissimilar metals (wires) is joined/welded to the other copper wire that leads to the meter.

If all three of these  junctions of dissimilar metals are at the same temperature, then no current will flow around the closed loop (that includes the meter) [If any current did flow, that would violate the 2nd law of thermodynamics]

If one the these junctions is at a higher temperature than the other two, then current will flow. [This does not violate the 2nd law of thermodynamics, because heat (energy) is moving from the higher temperature junction to the lower temperature junctions... i.e. the flowing current is associated with power flowing from the higher temperature junction to the other junctions, etc.]

Therefore, what determines the amount of current that can flow is how much hotter the junction at the heater resistor is than the junctions where the dissimilar metals connect to the copper wires.

Stu

[Mike/W8BAC]

Don, I found this today and thought it might help answer a few questions. If you save the .jpg pictures to a file and open them with a picture viewer you can zoom in and read them better.

Mike

[W7TFO]

Ah, 

This bears out my memory, the 'H' is grounded in some instances.

73DG

[Opcom]

Thanks for explaining the heaters on RF ammeters.

The discussion about heating the measurement TC brings up a general TC note that might be interesting to car buffs.

Common (e.g. Isspro) TC-driven pyrometers used with diesel engines have a very wide full-scale range such as 0-1500 deg F.

The point is that the vehicular instrument usually has a zero adjustment like any electronic meter and the user ought to set this to the highest expected ambient temperature.

Those who soup up military 1960's era turbocharged Diesels will acknowledge that the exhaust temp ought not to be allowed to reach 1500 degrees because the eutectic aluminum pistons will begin to melt. 1200 degrees F in the exhaust manifold is safe for short periods such as going up a hill or getting on the highway.

In the usual climate variations, a seasonal difference of 100 degrees is possible under-hood on a cold engine, but the instruments are not very sensitive or accurate at the low end of the scale. Therefore, calibration/zero-set  at the low end of the scale is important for those who will push 1300-1500 for a couple of seconds where the instrument is at the higher end of its range.

These comments are from hot rodding a 2.5 ton 6x6 Diesel (hypercycle) military truck having 135HP stock and maybe 160-170HP after turning the fuel up, but it applies generally to any diesel.

Vehicle exhaust TC-type  pyrometers do not use heaters but it is easy to see that the main TC (not the connections to the leads) will always be hottest because it is in the probe tip which is in the exhaust stream.

The application to electronic equipment may have a similar or even greater ambient temperature variations but they are required to respond accurately to much smaller temperature variations than found in an exhaust stream.

[WB4AIO]

I have several Weston RF ammeters with external thermocouple.  The thermocouples are all marked Type "B" Heating Element.  Some have four terminals: Two marked "METER", + and −, two others marked "HEATER", H and H.

But others have five terminals on the thermocouple heating element.  Like the others, they have the two terminals marked "HEATER", but there are three terminals on the METER side: +, − and a third one between the plus and minus, marked "H".

My question is what does the "H" terminal on the METER side connect to?

I don't know why they used the letter "H" but I just disassembled my Weston 500 mA RF ammeter (for which I'd really like to get the external thermocouple some day) and that terminal is connected directly to the metal armature structure.

All the best,


Kevin, WB4AIO.