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Saturated salts issue - Humidity Chamber
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Saturated salts issue - Humidity Chamber
Saturated salts issue - Humidity Chamber
Saturated salts issue - Humidity Chamber
Saturated salts issue - Humidity Chamber
Saturated salts issue - Humidity Chamber
Saturated salts issue - Humidity Chamber
Saturated salts issue - Humidity Chamber
Saturated salts issue - Humidity Chamber
Saturated salts issue - Humidity Chamber
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  Post Number #1  
Old 12th March 2018, 11:53 AM
MetroBioCen

 
 
Total Posts: 33
Please Help! Saturated salts issue - Humidity Chamber

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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Saturated salts issue - Humidity Chamber-dsc00023-jpg   Saturated salts issue - Humidity Chamber-dsc00026-jpg  

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  Post Number #2  
Old 12th March 2018, 05:02 PM
Ninja's Avatar
Ninja

 
 
Total Posts: 1,018
Re: Saturated salts issue - Humidity Chamber

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?
  Post Number #3  
Old 12th March 2018, 05:04 PM
Ninja's Avatar
Ninja

 
 
Total Posts: 1,018
Re: Saturated salts issue - Humidity Chamber

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.
  Post Number #4  
Old 13th March 2018, 10:32 AM
MetroBioCen

 
 
Total Posts: 33
Re: Saturated salts issue - Humidity Chamber

Quote:
In Reply to Parent Post by Ninja

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

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

Quote:
In Reply to Parent Post by Ninja

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?

Quote:
In Reply to Parent Post by Ninja

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

Quote:
In Reply to Parent Post by Ninja

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

Quote:
In Reply to Parent Post by Ninja

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

Quote:
In Reply to Parent Post by Ninja

Is the reference sealed?
I don't know

Quote:
In Reply to Parent Post by Ninja

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

Quote:
In Reply to Parent Post by Ninja

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

Quote:
In Reply to Parent Post by Ninja

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

Quote:
In Reply to Parent Post by Ninja

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.
  Post Number #5  
Old 13th March 2018, 02:09 PM
Ninja's Avatar
Ninja

 
 
Total Posts: 1,018
Re: Saturated salts issue - Humidity Chamber

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
  Post Number #6  
Old 17th May 2018, 03:12 PM
greif

 
 
Total Posts: 50
Re: Saturated salts issue - Humidity Chamber

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
Thanks to greif for your informative Post and/or Attachment!
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