Repairing hygrometer transmitter

I have one of those sort-of cheap Oregon Scientific weather stations with t he wireless outdoor temperature/humidity sensors. It's maybe about 6 years old, so normally a person would simply buy a new one but I'm not like that . I like fixing stuff rather than tossing it.

The outdoor unit senses the temperature just fine, but not humidity. When it's not displaying 2% or 98% for RH, it's displaying another number that i s more sane but still wrong. I took it apart and the sensor looks exactly like this:

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Any pointers on how these work or what to look for? I've done nearly every thing else but humidity. It appears these aren't driven with DC, which mak es things more difficult. How probable is it that the sensor simply needs replacing?

Reply to
hondgm
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The outdoor unit senses the temperature just fine, but not humidity. When it's not displaying 2% or 98% for RH, it's displaying another number that is more sane but still wrong. I took it apart and the sensor looks exactly like this:

formatting link

Any pointers on how these work or what to look for? I've done nearly everything else but humidity. It appears these aren't driven with DC, which makes things more difficult. How probable is it that the sensor simply needs replacing?

+++++++++++++++++++++++++++

Try cleaning the sensor with isopropyl alcohol, 99% that you can get special order from the pharmacy.

Reply to
Tom Miller

DENATURED Alcohol at most hardware stores. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

So is this not the same as standard rubbing alcohol? Incidentally I tried this, but no go.

Reply to
hondgm

Standard rubbing alcohol is 30% water. You can get 91% alcohol OTS. Best for sensitive electronics is 99+% but you will need to order it.

But this seems moot now.

Reply to
Tom Miller

Does the wx station and sensor have model numbers? Such information is amazingly useful and will tend to produce specific answers instead generalized guesswork.

You can buy 99% alcohol at most drug stores and online. For example:

Now that you may have trashed the sensor, try cleaning the PCB (both sides) around the sensor. Greasy and sooty films tend to cause leakage problems.

However, I think you're wrong about the humidity sensor. The photo you supplied is what's used for measuring condensation or dew point.

Most cheap weather stations have humidity sensors made from polymer film capacitors:

There are also solid state sensors: However, I haven't seen these appearing in commodity wx stations. Davis and Adafruit use them (from some other vendor):

Cleaning a capacitive sensor with alcohol is probably ok, but the recommended solvent is distilled water. If you've touched the exposed sensor with your fingers, you might as well use alcohol to clean off the skin oils.

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

I briefly considered providing the model # but figured this wasn't all that important since it's unlikely anyone here has the same, and if they do, probably hasn't tried fixing it. But anyway the outdoor sensor is THGR122N.

I was careful not to touch it, and used rubbing alcohol with a cotton swab to clean it. It initially seemed to fix it but then began acting up again. Here's a pic of the sensor in my device:

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It appears to match the sensor in the link I originally provided, but I of course I can't say for certain.

I will be trying distilled water since I have that on hand. Would replacing the sensor with one that looks similar likely work? Again, I know nothing about humidity sensors. I know what the answer would be regarding a temperature sensor.

Reply to
hondgm

The hardware stores and BORGs here don't carry denatured ethyl, at least in their paint departments, rather they carry IPA.

Reply to
krw

That would be a THGR122NX: I have a THR268. Very similar electronics inside, but no humidity sensor. There's a humidity sensor in the BAR888RA base unit, but I don't want to tear it apart.

It's amazing what one can find with a model number: (see photo)

Much better. Looks like your original photo is an exact match: "HPR-MQ is a humidity device formed by distributing a humidity sensitive macro-molecule onto an electrode substrate. The resistance of the device varies exponentially with variation on Relative Humidity." That's new to me as I'm used to such sensors being used for condensation and dew point, not humidity. Something like this: I couldn't find any cleaning recommendations, applications info, real specs, or test circuits.

Interesting that the Oregon Scientific data sheets offers a humidity range of 5% to 95%, while the sensor data sheet says 20% to 95%. Hmmmm...

It's a match. Good find.

After distilled water, try a little heat or hot air to evaporate the water. My guess(tm) is that the ceramic substrate is porous and you'll need to evorate any absorbed water before it will work again.

Looks like no PCB to clean. However, follow the wires from the humidity sensor to the PCB and clean up with alcohol and water around the pads on the PCB. Leakage between the leads is another possible culprit. However, if there's some kind of wax coating on the PCB, forget about cleaning it. That's a seal to prevent the PCB from acting like a sponge and soaking up water. Looking at the photo, there is a stringy blob of the stuff covering the thin wires going to the PCB from either the humidity sensor or temperature sensing thermistor. Leave that alone.

Replacing the sensor might be an option if nothing else works. Looks like a few sources on Alibaba at $3.00/ea, but nothing from the usual US distributors. Nothing on eBay or Amazon.

Good luck.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

That sensor is fairly wild with resistance varying over four decades throug h the RH measurement range in addition to several thousand ppm change per o C at fixed RH. Manufacturer reference design is simple ac- drive into therm istor compensated voltage divider actively peak detected by OA. If the read ing is bouncing around all over the place, I would suspect the peak detecto r hold capacitor.

Reply to
bloggs.fredbloggs.fred

Yes I found that, but there was no help in that forum.

No conformal coat on this PCB, which is a bit surprising considering it's for outdoors. But then again this is a cheap consumer grade device.

I actually found some on eBay that look the same, but without the big "3"; look for HR202L. $3 shipped for 2 from China. It's worth a shot. The specs appear very similar.

After cleaning, currently the reading is stable but the RH is too low (it's super humid here right now, A/C is a great invention). I'll update once I replace the sensor.

Reply to
hondgm

On Sunday, July 12, 2015 at 8:48:30 AM UTC-5, snipped-for-privacy@gmail.com wrot e:

ugh the >RH measurement range in addition to several thousand ppm change pe r oC at fixed >RH. Manufacturer reference design is simple ac- drive into t hermistor compensated >voltage divider actively peak detected by OA. If the reading is bouncing around >all over the place, I would suspect the peak d etector hold capacitor.

Hmmm, good to know. I never found a decent schematic for this type of sens or. I knew these were AC driven so I thought about hooking my DSO to it bu t then realized I don't know what I'm looking for. I'll see what happens w hen I replace the sensor.

Reply to
hondgm

I replaced the sensor with the model I mentioned earlier off eBay, shipped from China for a few bucks, and that fixed it! The RH seems to line up mor e or less with what is reported by NOAA, which is much better than the abno rmally low numbers it was displaying before. It might be off by up to 5% b ut I don't believe this thing was all the accurate to begin with.

A new unit would have cost about $30, which isn't horrible considering it's at least 6 years old. But repairing something instead of trashing it sure is nice (and at 1/20 the cost). Not much gets thrown out due to breakdown in my house.

Reply to
hondgm

Nicely done. Repair is always better than recycling or trashing. I'm also impressed that you actually found the part. I don't recall the specs, but 5% is probably better than the sensor accuracy specifications. Also RH varies radically with height above the ground (water vapor is heavier than dry air), so don't be surprised if it's not the same as NOAA.

If you want the official standards, it's probably buried somewhere in the docs here:

There's also quite a bit scattered around this web pile on sensor location and weather station construction:

I'm still having problems believing that the sensor in the picture is for humidity and not for dew point.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

Water vapour is much lighter than air. Air has a mean molecular weight of around 29 AMU (basically 80% N2 @ 28 and 20% O2 @ 32), whereas water is 18 AMU. But gases are fully miscible, so they don't separate meaningfully in the troposphere except by condensation. I suspect it's proximity to a large reservoir that makes the difference, but that the altitude effect is probably mostly temperature--I doubt that the dew point varies as much with height. But then I'm not an atmospheric physicist.

Cheers

Phil Hobbs

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Phil Hobbs

I can thank Google Images. I don't know how else I would have found it if I couldn't search for "humidity sensor" and look at pictures. Comes in han dy for looking for other parts too.

I'm likely stretching it by saying 5%. Without knowing the exact RH at my location I can't say for sure. But it's in the ballpark which is good enou gh in my situation.

There's a clear, slightly sticky layer over the interlocking fingers part o f the sensor that apparently absorbs moisture and affects its conductivity. I know it's sticky not because I touched it, but because the two sensors were stuck together in the bag they were shipped in. My guess is this laye r went bad on original sensor.

Reply to
hondgm

You can check the RH (at least at one point) using some salt and a sealed container. If you're interested just Google RH calibration salt and you should find all you need.

Reply to
Techman

the wireless outdoor temperature/humidity sensors. It's maybe about 6 yea rs old, so normally a person would simply buy a new one but I'm not like th at. I like fixing stuff rather than tossing it.

n it's not displaying 2% or 98% for RH, it's displaying another number that is more sane but still wrong. I took it apart and the sensor looks exactl y like this:

rything else but humidity. It appears these aren't driven with DC, which m akes things more difficult. How probable is it that the sensor simply need s replacing?

the wireless outdoor temperature/humidity sensors. It's maybe about 6 yea rs old, so normally a person would simply buy a new one but I'm not like th at. I like fixing stuff rather than tossing it.

n it's not displaying 2% or 98% for RH, it's displaying another number that is more sane but still wrong. I took it apart and the sensor looks exactl y like this:

rything else but humidity. It appears these aren't driven with DC, which m akes things more difficult. How probable is it that the sensor simply need s replacing?

the wireless outdoor temperature/humidity sensors. It's maybe about 6 yea rs old, so normally a person would simply buy a new one but I'm not like th at. I like fixing stuff rather than tossing it.

n it's not displaying 2% or 98% for RH, it's displaying another number that is more sane but still wrong. I took it apart and the sensor looks exactl y like this:

rything else but humidity. It appears these aren't driven with DC, which m akes things more difficult. How probable is it that the sensor simply need s replacing?

I solved this problem with a minimum of fuss.

I opened the external case and carefully opened the compartment that holds the RH sensor and thermistor by cutting through the hotmelt wit a scalpel b lade. >>>Be careful of the RH and temp sensor wires!

Reply to
bob.blarneystone

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