Saturated salts issue - Humidity Chamber

MetroBioCen

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#1
Hello everybody

I'm trying to make a fixed humidity chamber to test the stability in time of several items, including instrument-related parts. I'm trying to get a 75% fixed humidity environment using saturated NaCl solution as I have read on several papers. The problem is that it is not functionning as expected and I don't know why. I prepared an over-saturated NaCl solution and put it in a beaker inside a desiccator along with a calibrated thermohydrometer. The fact is that it never read 75% and instead it is reading a humidity very close to that of the environment outside the desiccator. See attached images for more details.

Any help most welcomed

MetroBioCen
 

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Ninja

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#2
Very basic starter suggestions (in the form of questions):

1. are you using seal grease for the desiccator closure to prevent leakage?

2. Are you prepping the solution with hot water to speed evaporation? (or heating the solution before entry into the desiccator?

3. Do you trust your humidity gage? Why?

4. How does your gage work? What does it use as a reference electrode? Is the reference sealed? Are you bypassing its functionality in your current setup?
 

Ninja

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#3
One more since you're at 69% already, trying to get to 75%...

Can your gage actually tell the difference between 69% and 75%?
Packaged humidity gages are notoriously wide in error band.

When I used them, we would buy new calibrated ones every year rather than to bother checking if they were still good...they never were so we stopped trying to vet used ones.
 

MetroBioCen

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#4
Ninja said:
Very basic starter suggestions (in the form of questions):

1. are you using seal grease for the desiccator closure to prevent leakage?
Yes

Ninja said:
2. Are you prepping the solution with hot water to speed evaporation? (or heating the solution before entry into the desiccator?
No, but I have waited for two days, so IMO evaporation has had time to occur, hasn't it?

Ninja said:
3. Do you trust your humidity gage? Why?
Yes, because I calibrated it before put it inside de desiccator. I calibrated it against a psychrometric couple of high accuracy RTDs

Ninja said:
4. How does your gage work?
Capacitive, I guess

Ninja said:
What does it use as a reference electrode?
I don't know

Ninja said:
Is the reference sealed?
I don't know

Ninja said:
Are you bypassing its functionality in your current setup?
No idea

Ninja said:
Can your gage actually tell the difference between 69% and 75%?
Yes, according to the previous calibration. It is a selected one.

Ninja said:
Packaged humidity gages are notoriously wide in error band.
I know, I've been calibrating these gages for 5 years.

Ninja said:
we would buy new calibrated ones every year rather than to bother checking if they were still good
We are a third world country so, we can not afford that.
 

Ninja

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#5
Davis Instruments...$35.00 USD

Looks like you've got everything covered except for not knowing if the gage can take full enclosure (it "should" be able to...out in a room is full immersion in atmosphere after all).

I don't have an answer for you then, but I would try to vary the salt solution to see if the reading then moves accordingly. Not tight to the theoretical salt mix, but sometimes trial and error is what gets it done.

HTH
 

greif

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#6
FYI- I have been replacing my various thermo-hygrometers with new digital units that cost all of around $15. They are Acu-rite model 01083M.
They have no push buttons that users can set wrongly (like displaying MAX all the time). Resolution is 1% RH and 1 degree F. Buttons on the back let you set correction factors to really calibrate if needed. I have calibrated 9 units using saturated salt baths at 3 different RH points and a NIST traceable calibrated thermo -hygrom for comparison, and all 9 Acu-rite units were right on and needed no correction factor.

Amazon link:
Amazon.com: AcuRite 01083 Pro Accuracy Indoor Temperature and Humidity Monitor: Home & Kitchen
 
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