General > General Technical Chat
This is not a Pirani gauge Tube. What is it?
jpanhalt:
My Pirani gauge tube (Fredericks brand Televac) burned out last week, so I went to eBay. A kind seller had "two." One was used and the other had no visible signs of use. The price was good, and I bit. The used tube worked fine. The "new" tube didn't. In fact, when tested for pinout, it was completely different. A slight twist of the brass end unscrewed. I had never seen a Fredericks Pirani gauge tube that could be opened. There was an O-ring seal, a ceramic PCB, what looked like an IC, and one pin was soldered to a brass tube (see attached picture).
My guess is that someone at Sandia (Albuquerque) used a new Fredericks Pirani body to make something different, like an ion gauge. I could find no evidence of a filament, though. The quality of the machining is very nice, so I am assuming a professional instrument shop did it. The seller is in Santa Fe, NM, and I have asked him whether he got it there.
Any ideas what it is?
John
TimFox:
Could it be a home-brew thermistor (or possibly thermocouple) gauge, which measures thermal conduction to obtain pressure?
If from Santa Fe, it could also have been made at Los Alamos laboratory.
jpanhalt:
The brass tube has only a single wire to it. The flared end is attached to the ceramic PCB. The "IC" is on the opposite side. With a bright light down the tube, I cannot see any opening -- only the white of the ceramic. I am assuming that attachment is just a mechanical fixture. That single electrical connection is what me think of some sort of ion sensor. I see no evidence of any TC or heater wire, unless they are buried in the epoxy brick I am calling an IC. Pirani/TC gauge tubes from Fredericks are constructed much like an old incandescent bulb. Each of the 4 pins is connected to the end of a filament connected to an opposing pin. The two loops of filament are connected at about their midpoints.
Los Alamos is another option. I mention Sandia as we did some work with it years ago.
coppercone2:
oh, thats a picard gauge
jpanhalt:
UPDATE
After a little sleep, I examined the chip and was able to decipher LX1602A and a strange poorly defined logo. That part number led to Honeywell pressure transducers and this: https://www.datasheets.com/en/part-details/lx1602a-honeywell-34515327#datasheet
Sorry for that link to archive; I could not find the datasheet easily on the Honeywell site. A 1977 booklet by Honeywell does show the internals and that attached logo, which is consistent with the logo on the chip.
Working hypothesis is that someone took a lot of effort to gut an LX1602A and put it inside the brass body of a Fredericks gauge. I will be testing it later as the setup is quite simple -- only 3 leads (GND, V(excitation), and V(output)). The connection to the cylinder is probably a ground too. Notably, the actual sensor in the transducer is a ceramic disk, which might be what I saw. Will report when I have another update.
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