need  Hygrometer that can measure very dry air. - Page 1
Forum section: electronics / testgear

author: Leland_Gaunt
author_rank: Contributor
timestamp: May 18, 2025, 07:11:16 pm
content: Hello gentlemen,anybody knows something about hygrometers? I tested some drying agents in my desiccator and was surprised the humidity never really went below 20%, when it should have been pretty much 0%. All my hygrometers are small, digitals ones, because they need to be small enough that i can drop them into the closed desiccator. A little research on these devices showed me that they have in general a problem with readings below 20% humidity.But for sure there must be a way to get an accurate reading inside a small container somehow?

author: Fungus
author_rank: Super Contributor
timestamp: May 18, 2025, 07:58:03 pm
content: Cobalt chloride and a color sensor?https://bartovation.com/product/moisture-leak-detection/4-pad-cobalt-chloride-humidity-test-strip-20-80-humidity-vial-of-50-strips/

author: Leland_Gaunt
author_rank: Contributor
timestamp: May 18, 2025, 09:08:47 pm
content: dude just by looking at the link i can see its only 20-80% range.

author: DaJMasta
author_rank: Super Contributor
timestamp: May 18, 2025, 10:24:01 pm
content: Look for a "dew point hygrometer" and you can do better with handheld and reasonably priced versions, I see some good down to 10% and some rated full range with less accuracy at the margins even using capacitive sensors. You want the premium measure-anywhere version, look at chilled mirror hygrometers.

author: Simmed
author_rank: Frequent Contributor
timestamp: May 18, 2025, 10:41:51 pm
content: https://www.mouser.com/ProductDetail/Sensirion/SHT4x-SmartGadget?qs=TuK3vfAjtkVlOF8U1qFtVA%3D%3Dthe EVB for SHT40

author: Hydron
author_rank: Super Contributor
timestamp: May 19, 2025, 01:01:01 pm
content: I just threw a cheap (something south of 3GBP) Aliexpress Zigbee temp/humidity sensor into a sealed container full of freshly dried silica gel dessicant and it's been reading 0% for 3 weeks now (I unpotted a GR standard capacitor and I want to be real sure it's internal dessicant is dry before potting again).Sadly I seem to have lost the initial data showing it dropping from ambient humidity, but I know the sensor is working because I can see 3 weeks of temperature swings. Got a pic of the sensor itself somewhere but I can't find it right now, I think it was an AHT10/AHT20 type possibly. Certainly nothing fancy given the price, but I did confirm in another test (with saturated NaCL salt/water in a sealed container - gives a stable buffered humidity reference) that it wasn't wildly inaccurate.

author: Sorama
author_rank: Frequent Contributor
timestamp: May 19, 2025, 01:05:00 pm
content: Quote from: Leland_Gaunt on May 18, 2025, 07:11:16 pmwe have, among others, these:https://www.testo.com/en/testo-174-h-bt/p/0572-1743-02#tab-technicalData

author: vindoline
author_rank: Unknown
timestamp: May 20, 2025, 09:55:48 pm
content: I use these all the time at work. I have one in my desiccator cabinet and it reads “0”. They're very reliable.https://www.coleparmer.com/i/traceable-calibrated-temperature-bluetooth-enabled-data-logger-compatible-with-traceablego-app-ambient-sensor/1800412I've also had a good experience with this:https://www.amazon.com/dp/B084X1DB38

author: Leland_Gaunt
author_rank: Contributor
timestamp: May 28, 2025, 09:25:27 pm
content: thx for your help guys,that SHT40 sensor that Simmed linked me looks very good, from the datasheet perspective. I think ill try that one. Or is it called SHT4x SmartGadget? I hope this one works on its own, or do i need anything else for it to work? Or do i need this SHT4x gadget AND a SHT40-Sensor?I dont know how you guys can achieve 0% on your cheap digital hygrometers. I have several very cheap ones from Aliexpress and ebay. recently bought some Thermopro T49, alleg. some slightly better cheapos..doesnt make a difference, they dont show very dry air. With the Thermopro TP49 , it shows at least 12%, the others stop anywhere from 20 to 24%. weird. Both with freshly dried silicalgel(should be just a little over 0%, maybe 2-4% at max) and also with fresh Calciumchloride.well im eager to see if i finally get a reading with the SHT40...

author: Hydron
author_rank: Super Contributor
timestamp: May 28, 2025, 11:38:48 pm
content: Are you sure the silica gel was dried enough to get all the way to 0%? Had to cook mine pretty aggressively to get it totally dry.

author: wraper
author_rank: Unknown
timestamp: May 28, 2025, 11:43:02 pm
content: Moisture indicator cards shipped with moisture sensitive components (and sold separately) by electronic component distributors go down to 5%.

author: Someone
author_rank: Super Contributor
timestamp: May 29, 2025, 12:16:22 am
content: Quote from: wraper on May 28, 2025, 11:43:02 pmAre they reversible?

author: wraper
author_rank: Unknown
timestamp: May 29, 2025, 12:20:08 am
content: Quote from: Someone on May 29, 2025, 12:16:22 amYes, text written on the bottom should give a clue (not all of them need be kept below 60% moisture).

author: Leland_Gaunt
author_rank: Contributor
timestamp: May 30, 2025, 12:53:59 pm
content: nahm i need a somewhat accurate device that measures 0-100% with a somewhat reasonable accuracy.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 30, 2025, 02:49:35 pm
content: You can build a DIY chilled mirror hygrometer more easily than you might think. A peltier cooler, a polished piece of copper for the mirror an LED and a photo diode so you can detect when it hazes over. Put the peltier in a feedback loop and add a couple cheap temperature measurement ICs and you've got it. I did it analog and just read it with a meter, but a smarter person would use some tiny micro and build in the calculations. The weak point of mine is I need to repolish the copper mirror periodically because I've never had it gold flashed.For a small system you could sample the air and blow it over the mirror, though I don't know how much air is available or how much you could blow before warming up the mirror.

author: vindoline
author_rank: Unknown
timestamp: May 30, 2025, 05:23:48 pm
content: Quote from: Leland_Gaunt on May 28, 2025, 09:25:27 pmI think you may be seriously overestimating the capacity of your desiccants.https://www.impakcorporation.com/desiccants/all-about-desiccantsIf you want to get down near “0”, try freshly activated 4A molecular sieves.

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: May 30, 2025, 06:39:57 pm
content: Check the specs on these and see if they'll suit your application.https://trutechtools.com/PRH2https://trutechtools.com/Testo-605i-Humidity-Smart-Probe-2nd-Gen

author: Poroit
author_rank: Frequent Contributor
timestamp: May 31, 2025, 08:11:22 am
content: G'day Leland_Gaunt,Are you wanting to measure Moisture Content or Humidity?

author: Someone
author_rank: Super Contributor
timestamp: May 31, 2025, 08:52:11 am
content: Quote from: wraper on May 29, 2025, 12:20:08 amThe applicable standards have examples of that exact text being used on both reversible and non reversible indicator cards. Without any clear confirmation that it is reversible I'd err on the side of caution.

author: wraper
author_rank: Unknown
timestamp: June 01, 2025, 09:36:19 am
content: Quote from: Someone on May 31, 2025, 08:52:11 amThink logically. If it's not reversible, how do you think it's supposed to work at all if 5/10% are already triggered when you put it in?

author: Someone
author_rank: Super Contributor
timestamp: June 01, 2025, 11:15:03 am
content: Quote from: wraper on June 01, 2025, 09:36:19 amThat exact text is on the cards explicitly marked non reversible.Quotefrom J-STD-033D

author: mzzj
author_rank: Super Contributor
timestamp: June 03, 2025, 11:30:31 pm
content: Vaisala drycap