Assistance with a seemingly strange pirani gauge

I'm working on a vacuum system and so far things have gone unusually well for my projects and I really hope to keep doing well on it, but I'm hung up on the vacuum gauges. I've got a PR10-S Edwards Pirani gauge and the internal circuitry is strange. I can't find any literature on it anywhere or any schematics close enough to fudge. So I'm hoping I'blers might have a clue as to what is going on here.

Added a re-drawn circuit in an estimated configuration
After some googling, a 3-wire resistance thermometer seems close to what I have as thats essentially what the gauge is, a clever application of a resistance thermometer.

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seandogue6 years ago
It looks like a tadnard four wire sensor interface.

At a glance (and without too much thinking)... R3 and R4 are zero and sensitivity/linearity/gain/balance/something adjustments.

It looks like it's using a bridge (R1, R2, "calibrated resistor?", R4 (variable - sensitivity adj i think). I do not know what the value of "calibrated resistor" is, although I'm guessing it's a temperature stable resistor...or....! possibly....another pressure sensor that's sampling the ambient pressure as a reference to adjust the bridge and so adjust the ref voltage seen by one end of the vacuum sensor...

the combination of R3 and the bridge is a voltage divider from the sensor power supply (presumably a stable reference voltage source), which is why R3 looks to me to be the zero-adj section of the circuit.

1 gets V+ reference
2 sig -
3 sig +
4 V- reference

so you'll just need to find the sensor(s) to add, in order to duplicate the basic circuit. The vacuum (and possibly ambient) sensors are your challenge. You may be able to glean some basis for scoping a sensor by thoroughly examining their specifications for the completed unit and by adjusting R3 and R4 trimmers in situ and measuring the output variations as a result of those adjustments. Presumably R3 and R4 are 10-25 turn pots, although idk for sure, but you can do your measurements in "V vs turns" and figure the rest out later.
The Ideanator (author)  seandogue6 years ago
R3 is "set atm" and R4 is "set vac", they arent glued down so I'm guessing that they're meant to be adjusted on-site and the pirani and calibrated resistor are matched to provide a distinct temperature difference which would correlate with vacuum level.

As for values, I've added my in-circuit resistance measurements to the drawing
R3 is "set ATM" because that's it's "macro" purpose. It is actually just an offset adjustment. (hence my terming of "zero"). Don't be fooled by names.

(fwiw, and as an interesting side note...there are ~353 Pascals in 1 ATM. the ratio of R3:R1 just happens to be 351 from your measurements, well within 1% of an ATM to pascal conversion and better than the accuracy of most DMMs (Digital MultiMeters... My gut tells me that the original designer was thinking in Pascals when he/she designed the circuit)
The Ideanator (author)  seandogue6 years ago
After mulling it over, making a few more drawings, chatting it over with jayefuu and steveastro, drawing some more, and googling frequently, I believe That your suggestion is pretty damn close.
lemonie6 years ago

Why do you need to know?
The vacuum-bit connects with a cable and there's a mains supply for the gauge.
Is it broken in some way?

The Ideanator (author)  lemonie6 years ago
I would like to know because I would like to build a guage to measure my vacuum.
It is NOT a mains item, that kind of power would tear it to shreds. As for whether or not its broken, I cannot say since I don't have the gauge designed for it.
You've got the bit that plugs into the vacuum manifold, but not the actual gauge with the needle? (I had one of them once...)
Does it look clean or corroded inside?

The Ideanator (author)  lemonie6 years ago
I'd have to say its "like new". I have no doubt that if I had the proper other half this would work perfectly.

I dismantled mine for the ammeter, 100mA FSD I think (if that helps, which it won't...)

The Ideanator (author)  lemonie6 years ago
That may not help completely, but it does give me an idea of how much I can poke around with.

Non-linear scale on the meter face, inverse or logarithmic I'll guess. The "clockwork" is linear though.

What is your £ budget ?
eBay has a page of pirani gear.
I often wonder is the US gov does not allow you to see what i do because of
export rules, though it is made in the UK.

The Ideanator (author)  iceng6 years ago
I'm not sure how much Steve will let me spend on this, so far all but the neon indicators have been recycled, in stock, or already bought and set aside for this project.

STEEEEEEVE! Whats my budget on this?
Hi, Lemonie. That's not a mains supply. It's a four-pin connector to an Edwards control module. I suspect Ideanator's problem is that he doesn't have the control module in the surplus equipment locker, and Steve wants him to build his own :-)
He he he +1

Jayefuu hypothesised this afternoon that it was a wheatstone bridge config, but Idea's circuit doesn't look like it to me.

The Ideanator (author)  steveastrouk6 years ago
It could be a wheatsone with some circuit bending but that would defeat the factory setup which isn't what I want to do unless its absolutely necessary. That said, all I want is for it to do is for the controller to send data to a display and turn on some lights.
Speaking of which, could I mess with one of those 16x2 char displays for a data readout and a launchpad or arduino to run it?
I'm not sure what The Ideanator has got, but I agree about the connections and do suspect something to be missing.

kelseymh6 years ago
I'm going to guess that you already tried Google. According to Island Scientific's mostly useless page (and don't bother with the "data sheet", which is just a PDF version of the Web page :-/), the PR-10S needs a specific controller, "Edwards 10, 11 or 101 supplied separately."

Edwards' own Website doesn't even find the PR-10S in their own pages, so it must be a pretty old item.

Ah, I tried a different search, for "Edwards D021-24-000" (the latter is their internal model number). There are some hits for instruction manuals, but not for that model.
The Ideanator (author)  kelseymh6 years ago
Steve suggested that I ask you if you know anyone who might know what to do with it.
No luck there sorry.

The Ideanator (author)  kelseymh6 years ago
Thats what my research to date has resulted in.
iceng6 years ago
Why do you want to fudge something ?
The gauge is a Wheatstone bridge equivalent, using elevated temperature
to meld metal with gas particles changing the resistance.balance.
The better gauges re-balance the bridge and report the current needed
to do so. Your circuit diagram is not a W bridge.
Would you please answer Lemonie's questions. . . . A
Idea's trying to copy the circuit he sees in the existing gauge, it may or may not be right, he'll be reviewing it I suspect this morning- He's interning with me at the moment and building a vac pump system for us - the gauges came with the pump rig, and we'd like to use them, but we dont' have the meters.