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Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/06/2016 11:19 PM

Hi all,

I'd like to seal off my DIY fumehood and turn it into a sealed glovebox with nitrogen (or argon) atmosphere.

I would also like to be able to measure and control the relative humidity of a 100% nitrogen atmosphere, ideally with a capacitance probe (which I think are what cheap arduino/raspberry Pi temp/humidity probes use) so I can automate the control of humidification via software.

I know that measurement of nitrogen can be done with the wet bulb method, but it seems like this quickly goes over my head, and, not to mention, seems to involve one-time calculation using manual tables, instead of a rolling sample: https://books.google.com/books?id=1xj_CAAAQBAJ&pg=PA19&lpg=PA19&dq=nitrogen+psychrometric+chart&source=bl&ots=UneF5qWtzP&sig=9JKldsZ5ZjOKZQNSXZ1lewYXdqM&hl=en&sa=X&ved=0ahUKEwi-nI-C9pTNAhUD8mMKHVyKAcQQ6AEIZDAN#v=onepage&q=nitrogen%20psychrometric%20chart&f=false

There are no shortage of psychrometers, but they all seem to be handheld and none of them have I/Os! (just search "digital psychrometer" on google shopping)

Does anyone know if small capacitance probes can be used, or can be calibrated, to measure humidity in a pure nitrogen environment? Or, if there are hygrometers specifically for measuring RH (or some other dissolved vapor:gas ratio unit) in inert gas chambers?

As far as I know, compressed gas from a cylinder is completely desiccated, and in fact it seems changing the ratio of ambient air to controlled release of pressurized inert gasses is how some gloveboxes control humidity: http://www.laboratory-supply.net/gloveboxes/shop_humidity_control_glove_boxes.html

I've seen nitrogen humidified prior to injection through industrial or biomedical spargers at the bottom of tanks of solutions, to prevent the accumulation of solid salt/solid crystals nucleating out on the sparger mesh. So I know the humidification part isn't hard. It could go through a bubbler first, or, there could be an atomizer, or even a household humidifier in the chamber on a relay-but that all assumes the measurement of the % dissolved H20 in nitrogen is accurate and known to a computer system.

And there are solutions out there, like this: http://www.environics.com/medical-gas-mixing. But these are custom, very precise rigs that probably cost four or five figures.

Is there a way that I can measure nitrogen humidity by modifying an inexpensive atmospheric hygrometer, or make one using OTS components?

Thank you

Sylvester

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#1

Re: Simplified measurement of humidification of inert gas controlled atmospheres?

06/06/2016 11:43 PM

You should be able to find something here that will measure the humidity and hook it up to control the humidity with whatever method you choose to add or subtract moisture with...

http://www.omega.com/subsection/humidity-temperature-transmitters.html

..an air pump through a desiccant drier to reduce moisture...

https://www.zoro.com/wilkerson-dryer-desiccant-x03-02-000/i/G0780071/?gclid=Cj0KEQjwvtS6BRC8pcKn8OXIg_wBEiQAqtpiz9Dfyz0Wf7lH-C1YC9R4hAdaz_poIIoQikwfyULXdFoaApb38P8HAQ&gclsrc=aw.ds

....small humidifier to raise moisture level....

http://www.amazon.com/Portable-Bottle-Humidifier-Cable-Office/dp/B00GEAE32Q

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#5
In reply to #1

Re: Simplified measurement of humidification of inert gas controlled atmospheres?

06/07/2016 5:00 AM
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Re: Simplified measurement of humidification of inert gas controlled atmospheres?

06/07/2016 1:49 AM

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#3

Re: Simplified measurement of humidification of inert gas controlled atmospheres?

06/07/2016 3:03 AM

The post is nonsense: <...the relative humidity of a 100% nitrogen atmosphere...> is zero, otherwise it wouldn't be a <...100% nitrogen atmosphere...>. Alternatively, if it were a <...100% nitrogen atmosphere...>, then there wouldn't be any need to measure the humidity.

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#4

Re: Simplified measurement of humidification of inert gas controlled atmospheres?

06/07/2016 4:16 AM

I would imagine that a hygrometer designed to work in air (~80% N2) would have been fairly accurate in N2.

At the worst you would need a lookup table to convert the readings.

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#6

Re: Simplified measurement of humidification of inert gas controlled atmospheres?

06/07/2016 6:28 AM

If this is a truly sealed glovebox, you can do a progressive purge with dry nitrogen, making sure you introduce the N2 from the top with a flat plate of open pore foam to diffuse the N2 and allow laminar downwards displacement. First, place a tray of baked water adsorbent (dessicating agent, Silica gel, CaCl2, 100% H2SO4 or whatever.

After converting to a 100% Nitrogen atmosphere, seal case and turn off the input N2.

The dessicant will remove residual water.

You will need a purgable gas lock to use to introduce samples, which are placed therein and it is purged with N2 before opening to the interior space

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#7

Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/07/2016 9:57 AM

Hey dude, if it is 100% nitrogen, humidity is like nowhere, man. If you are purging down to that point, introduce the nitrogen at the bottom, not the top, since water vapor is lighter than nitrogen gas. Oxygen and water vapor both diffuse rather rapidly, so this is not as important as purging until you reached zero moisture.

There are entire weather kits for Arduino, you need to go back and search better.

Look on Amazon, or Ebay for cheapy ones.

You could just stick a couple probelets that conduct into some caustic soda flakes in a plastic bottle, any humidity will result in conduction between the electrodes.

Why are you turning the fume hood into a glove box, are you making an air sensitive explosive this time Ahmed? Is this for your garage nuclear fusion reactor? What?

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#8
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Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/07/2016 10:35 AM

Water evaporated = gas, nitrogen = gas. They mix, but there is a carrying capacity for gaseous water in air or nitrogen. A reference to the table will show how much water will 'dissolve' in air or nitrogen, above which you can not go for any particular constant temperature.

The water will diffuse throughout, and will not be subject to settling, so air is displaced downwards by nitrogen by their slight difference in density (N=28, O=32, so air =28.4 grams per 22.4 liters at STP) the traces of water and assorted noble gasses can be neglected.

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#9
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Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/07/2016 10:39 AM

How do you know the air and nitrogen are isothermal with respect to each other? I think you assume more than is needed. He can just stick a dang tube in there and get the job done (eventually). Have a nice day, kitty.

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#10

Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/07/2016 10:52 AM

Is the operating temperature around ambient?

What is the target RH value? 0.2% 2%? 20%?

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#11
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Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/07/2016 11:14 AM

12%.

Thanks for replies everyone, especially the helpful ones.

I think everyone is assuming I want a "traditional" "purged" inert gas glovebox, which would, yes, be completely dry b/c the gas coming out of the cylinder would be dry.

However I want a humidified nitrogen atmosphere. Appx 12% RH. "12%" meaning 12% of the total water vapor carrying capacity of nitrogen gas, which I assume is different than the RH regular earth atmosphere can carry.

(don't know how to format a quote, sorry)

"The post is nonsense: <...the relative humidity of a 100% nitrogen atmosphere...> is zero, otherwise it wouldn't be a <...100% nitrogen atmosphere...>. Alternatively, if it were a <...100% nitrogen atmosphere...>, then there wouldn't be any need to measure the humidity."

So, what I'm saying here, is, in an atmosphere that is composed of only nitrogen, i.e. 100% nitrogen, how do I measure the humidity in that atmosphere?

I do no want a completely dry nitrogen box, as you would get normally.

Thanks.

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#12
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Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/07/2016 11:28 AM

Choose the correct dessicator and it will maintain the 12%, as long as new moisture does not enter

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#14
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Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/07/2016 4:32 PM

Then why don't you Google humidity standards and use them in nitrogen? They do exist at least for instrument calibration purposes.

You could try using something that will emit water vapor to that extent, or absorb from higher % to that.

OR, you could divide a dry nitrogen stream into two parts, one bubbles through water at the temperature required, one does not, the bubbly one is 12% of the total flow be design.

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#13

Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/07/2016 3:27 PM

A couple of suggestions:

If you want to actively control humidity, you can set up a small loop with a cooling cell, say a peltier junction, running at a constant temperature at the dew point of the general atmosphere in the glovebox at 12%. If the humidity goes over 12%, it will condense out in the cooling cell and pool. If the general humidity goes under 12%, the pool in the cooling cell will evaporate and bring the humidity in the glove box back up to 12%.

The key is the atmosphere inside the cooling cell always sits at 100% humidity at it's colder temperature. When it warms up to the glovebox temp it always goes to 12%.

If you want to go really old school, you could use a horsehair and measure the length. That's what the old hygrometers used. All you would need would be an encoder to transmit the length change to the controller.

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#15
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Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/07/2016 7:57 PM

Don't be fooled....

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#19
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Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/08/2016 11:55 AM

Whorse?

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#16

Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/07/2016 8:11 PM

Thanks for the help all.

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#17

Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/08/2016 8:33 AM

I'm currently working on a project that uses bottled gasses, and nitrogen is one of them. Bottled gasses are very dry and I too, needed humidification. I use an Ohmic HC 700 humidity sensor. Ohmic includes the formula for converting the output voltage to %RH in the data sheet. This device would work with an Arduino or Raspberry PI. I read the output voltage (0.8 to 3.9 VDC), with a NI-USB 6008, using an analog input channel. I run the input gas through a Permselect PDMSXA membrane filled with DI water. This method gets my calibration gasses into the high 90% range.

Ohmic HC700: http://www.ohmicinstruments.com/home/?page_id=16

NI-USB 6008: http://sine.ni.com/nips/cds/view/p/lang/en/nid/201986

Permselect silicone membrane hydration devices: https://www.permselect.com/products

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#18
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Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/08/2016 9:59 AM

That is a good way to get to the finish line of 12%, if OP chose to implement a flowing system with a split stream, good answer.

If OP wants a static system he will need an oxygen getter for oxygen infiltration through any polymer seals, and a carbon dioxide removal system, forget about argon, and he will need an internal fan to circulate all the air of the glove box past a 12% humidity standard.

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#20
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Re: Simplified Measurement of Humidification of Inert Gas Controlled Atmospheres?

06/08/2016 9:28 PM

Yes, thank you MickMack for technique and links!

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