thin film thermopile sensor open circuit resistance? - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: October 06, 2022, 08:06:25 pm
content: If you want to check a thin film thermopile sensor, can you measure the resistance of the string with a DMM to get a nominal resistance ?I would assume it should be measurable as some value of ohms. I know it will be different per sensor , but what are reasonable ranges to consider if testing them quickly? Ohms, kiloohms, megaohms...For a wire I would consider ohms, but for a thin film.. I am not sure how high it can go.

author: Fraser
author_rank: Super Contributor
timestamp: October 06, 2022, 09:10:17 pm
content: This datasheet may help. It states 100 junctions total 50 KOhmshttps://www.willow.co.uk/TS118-5eng.pdfFraser

author: coppercone2
author_rank: Super Contributor
timestamp: October 06, 2022, 09:53:47 pm
content: so my 88 meg ohm sensor that reads in only one direction is likely toast.I don't know why its measuring like a diode, but oddly enough when connected to a DMM the resistance drops from 88 to 20 mega ohm when I shine a flashlight on it, a HP radiant flux sensor, that looks like it has a cigar burn on the plastic in front (laser?). On the flux meter it reads very low sensitivity.What kind of damage am I looking at for this sensor?What life span do these sensors have?why am I able to measure light with a 34401A in ohms mode when the sensor is connected with the correct polarity? I am waiting on the manual still to get an idea.I wanted to open up the sensor but I am still not 100% sure its not just weird.. hopefully the HP Flux Meter manual tells me something when it arrives.

author: Fraser
author_rank: Super Contributor
timestamp: October 06, 2022, 10:13:22 pm
content: This, may be of interest as it describes the head construction…..https://www.radioexperimenter.us/1971-07/this-new-lowcost-radiant-flux-meter-system-gives-direct-radiometric-measurements-in-the-infrared-visible-and-ultraviolet-regions-of-the-spectrum-it-zeros-and-calibrates-itself-too.htmlFraser

author: Fraser
author_rank: Super Contributor
timestamp: October 06, 2022, 10:32:28 pm
content: Be warned, connecting a thermocouple string to a multimeter that is in resistance mode will likely create unusual readings as the thermocouples are generating a voltage and this interferes with the multimeters circuits. It is better to test the using a sensitive Millivoltmeter.You could try using the diode test mode on the head to see if there is a genuine, or false diode at work in series with the thermocouples.Note that if dissimilar metals are joined with a corroded/oxidised layer you can get a false diode effect. This effect breaks down if current is passed through the ‘junction’. If your thermopile has suffered an open circuit, plus corrosion, you might just get a false diode forming.Fraser

author: coppercone2
author_rank: Super Contributor
timestamp: October 06, 2022, 10:34:52 pm
content: hmm yeah I was thinking about that being parasitic diodes, can you clean them with a current source or something by any chance? I figure its worth a shot at least.I tried a LCR meter and it reads nothing also BTW.I read that article before but it gives me no clue about the parameters or failure modes of this thing. I am wondering if I should buy another one, or is that vintage totally dead because of age.If so, is there another thermopile I can interface the meter with that is modern, (maybe modifying some resistor network) or is this just useless at this point?

author: coppercone2
author_rank: Super Contributor
timestamp: October 06, 2022, 10:39:03 pm
content: I would like to take apart and restore this meter, but if those detectors are all borked its a waste of time.

author: Fraser
author_rank: Super Contributor
timestamp: October 06, 2022, 10:42:22 pm
content: Sadly I have no great experience of such radiance heads beyond basic theory. All thin film sensors of significant age are liable to degradation as they were never expected to still be in service when designed and made. As such, these sensors are a real gamble for the used equipment buyer. At ~50 years old I personally would not pursue such technology for use in any meaningful tests. You would be better served to investigate later technology that is still affordable. More modern Thin film laser power meter heads do come up on eBay relatively often.Fraser

author: coppercone2
author_rank: Super Contributor
timestamp: October 06, 2022, 11:06:02 pm
content: Do you think they would interface with the meter signal wise?I don't know the schematics to see how easy it would be to modify it to accommodate a slightly different sensor.What is a thermopile sensor manufacturer to look at for a similar performance detector head, or sensor?

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2022, 01:24:49 am
content: Well the manual should come in next week, I wonder how big a job that would be to modify for a new sensor.I have a feeling at most I would have to attenuate a little, for some reason I don't think with modern technology you will have to boost gain.Do you think there is a chance its just the bond wires? I can take the sensor package apart. Maybe its just a broken connection so you can splice it with some silver epoxy or something.I think precision LCR measurement to chassis might tell me where the failure is or if its distributed. If capacitance is equal, it means its a distributed resistance problem, if its lopsided, that means it might be a interface.

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2022, 01:36:01 am
content: pin 1 to tube earth : 37.54pF , 0.45 kOhmsPin 2 to tube earth : 200.9pF, 8.43kOhms@100KHzinterestingpin 1 to pin 2 at 100Khz Cs - 31.59pF, ESR 8.88 kOhmsWhat do you think about these numbers? have I got a shot? that kind of smells like a failed junction of some kind. its not a TDR but still maybe it means something, taken with DE5000 meter. That 8Kohm sounds like a ballpark correct number. It kind of sounds like it might be the third TC junction that failed though, so at best I could try to bridge something with silver paste to get 1 cell unit reduction of span. 8.88/64 = 134, 134*3 = 416. I am sure having a blob of silver paste is going to do something bad but maybe you can still get within 10% if your careful? I have a feeling its probobly going to have some bastard problem that makes it difficult/impossible to do this but who knows.I wonder if its filled with argon. When I am lookin through the window it looks symetrical, I don't think there should be a 5x capacitnace difference.Could it be a cracked bond wire? at 400 ohms it makes me doubt it, unless of course the resistance of the junction is adding to the total resistance and its not 3 cells. I wonder how to desolder the hemisphere. I want a copper ring to put on a hot plate to put it on so I can lift the dome off. I am thinking to take some brass or copper rod into a circle of the correct diameter, file a flat on one side, for good themral contact area, then get it to temp and put the sensor on top. Or I will bend a bus bar into a circle then its flat on both sides to make a faux hollow thick walled core, it should be a proper heated ring.

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2022, 02:42:35 am
content: ah I was mistaken about that silver sheen, it looks like it was just glue on, came off easily with a catalytic butane air pencil heater.

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2022, 02:48:09 am
content: well thats over, it looks like the wire bond junctions are in tact. Its actually on a film, like a piece of film suspended in the air (makes sense from a thermal mass prospective), there is no continuity to the top side and I managed to damage the film probing it, wow that is super duper fragile.I can see how it could have broken from flexing since its literarly like on a piece of kapton or some thing that is uber thin. The second I made that foil deflect like 0.001 inch, it read open circuit from 20 megaohm in light. I guess you need micropositioners to probe this kind of thing, its like a spider web, even if you got through what i assume is a solder resist coating or somethingI am just waiting for someone to tell me "oh 80 mega ohm is within spec and the meter is broken"So its kind of built like a speaker really, or you might say trampoline.So now the main question is there a replacement sensor that could fit into the original HP chassis and also interface with the meter.Oh right, and is a perfectly circular burn mark a sign of laser damage? It looks like if you dropped a fat cigar on the plastic, its very round. Kinda wonder if I would want to restore that chassis, assuming I can get a replacement sensor, or leave it because its rare laser damage.