Dear Jonathan . 

Include some method of eliminating humidity, or at least controlling the sample .The most effective approach is to make multiple calibration checks using the same .The problem then becomes the rangeability of the flow control devices.Please go with attachment , may be it will help to you 

thank you 

http://kin-tek.com/article/ADDING-HUMIDITY-TO-TRACE-CONCENTRATION-GAS-STANDARDS.pdf

http://www.npl.co.uk/upload/pdf/Beginner's%20guide%20to%20humidity%20measurement%20(draft%20for%20comment).pdf

You might consider using a cold trap like http://www.walz.com/products/gas_exchange/kf/introduction.html

Dear Jonathan, 

Typically, you can humidify your gas within the range 3-50 vol.% by bubbling your gas through a water bath with a specific temperature. In order to better control humidity, you should use dual bubbler system. In the first one you set higher temp than required, to "oversaturate" the gas. In the second, you set a desired temperature and condense additional steam. In this way you can get better control of the steam content.

When bubbling, you should use some "sponge" to create small bubbles, that will reach equilibrium with water faster. This can be conveniently done with some aquarium stones, cheap and efficient.

Regarding the control of humidity, I am not so familiar with these methods, but I would use the pO2 sensor to check the real pO2 sensor and back-calculate the amount of water. Or, you can simply observe how much water evaporates from your bubbler.

Best regards,

Sebastian

Thank you all for your responses.

Sebastian, I am currently using a single heated water bath although I like the idea of having a second one to condense some water. I'm sure that would give better control.

For now though I will continue to use the single water bath system unless I can't achieve the accuracy I am looking for. The problem I am having at the moment is getting an accurate humidity reading. When I bypass the humidifier my humidity sensor reads 0% rh. But when I switch the flow through the humidifer at only 40degC I am getting >100%rh. So I am wondering how I can get a humidity level somewhere in between...

I believe the sensor is accurate enough since when the gas bypasses the humidifier it gives a 0% reading, and when I expose it to ambient conditions it gives a value similar to that available from the weather forecast.

If you have any ideas how I can achieve a humidity level somewhere in between I would love to hear them.

Thanks.

Dear Jonathan, 

What kind of sensor are you using?

You have a gas with 100% RH (relative to RT) at temperatures of water higher than ~25degC. 

So if the reading is 100RH then it is a right result for a simple device.

Best regards,

Sebastian

I am using a simple handheld sensor.

Perhaps it's my understanding of humidity that's in question. I think I see the problem - The relative humidity is 100% because the partial pressure of water taken up by the gas is equal to the equilibrium pressure. So actually I don't want to vary the relative humidity at all. I want to vary the absolute quantity of steam for a given flow rate. So I should just be able to set the temperature of the water bath and calculate, right?

Whenever you bubble a gas through water, you should in theory, for given temperature get 100% RH. It is relatively easy to control humidity below 100%RH, you can just bubble through cooled water.

As you wrote, if you want higher steam content, you need to have higher temp of the water and additionally, you have to heat all the gas lines after the bubbler, so that no condensation will occur. In principle, gas line temperature should be at least equal to the water temp, preferably higher.

One way to measure steam content of your oversaturated gas, might be to dilute it, so that the range will be measurable by your sensor.

Best regards,

Sebastian

I'm sorry I didn't see that you had replied to my previous message.

My gas lines are very short and I have insulated them so I think condensation shouldn't be an issue.

Would you mind explaining a little more about the diluting of the gas for RH measurement?

Thanks.

About calculations on the relative humidity and the humidity ratio of a humidified gas stream (other than air), you may possibly want to check the following discussion, which specifically addresses the humidified hydrogen case:  https://www.researchgate.net/post/In_a_100_humidified_hydrogen_what_would_be_the_mass_of_water_molecules_per_unit_mass_of_hydrogen

I would recommend using a Texas instruments humidity sensor, or use the already integrated sensor tag. It's accuracy is within +- 2% and detects even 1% change in RH levels.

If your gas flow is low, your will for sure reach 100 %RH in your 'bubble bath', but you have a high risk of at least partial condensation even with short insulated lines between bubbler and your experiment. If your gas flow is high, you cannot be sure that you will reach 100 %RH in your bubbler, but you will have much less condensation (in relation to the flow of water) on your insulated lines. If you work at elevated pressures, remember that you will need to consider the pressure when calculating the vol%.

If you have the budget, a commercial humidifier will give you peace of mind. With the Cellkraft P-Series humidifier with heated tube option you will can directly set desired vol% (or RH or dewpoint) and be sure you have it in your fuel cell. I am affiliated with Cellkraft.

https://cellkraft.se/humidifiers/flow-through-humidifiers/

I know its a late reply, but maybe it is an idea which can be used further.

Through my reaserch in designing and testing a humidity compensated CO2 sensor, I humidified my gas stream through two humidifying columns.

One column for humidifying the synthetic air. The column was half-filled or half-packed (bottom half) with wetted clay balls (one can also use Glass wool) which have a huge surface area for gas and vapour exchange (this factor depends on the flowrate for the gas streamed through). Since the column was always flushed from bottom to top, half-packing enables the gas to loose the entrained water droplets through its way by gravitational influence. Humidified Snthtic air was then mixed with CO2. The relative humidity decreases because of the change in total pressure. This mixture was saturated again in a smaller column by same method.

I measured the rH % in my test column where the sensor was exposed to the gas stream using a HYT series from IST, Switzerland. Its a small sensor easily mountable. The results were reproducible and humidity was maintained. Ofcourse the temperature of the humidifying columns was regulated at Lab ambient temperature through lamp heating.

Thank you for your contribution. It's never too late for a good answer as I'm sure others reading this in the future will find it useful.

I am also trying to control humidity with a bubbler flow mixed with dry N2, both flows controlled with high precision needle valves. My issue is that I need to get to ~100%RH, but the current bubbler, which is just a 1/16" ID tube hanging to the bottom of a corked Erlenmeyer flask, can only manage 80-90%RH. I was going to rebuild the bubbler with a beer aeration stone with a 500nm pore size. My question is whether anyone has tried this and can you actually reach saturation conditions at RT? I'm pretty confident putting the bubbler with the aeration stone onto a hot plate will get me there if the N2 isn't saturated at room temperature, but I'd prefer not to do that since I don't want oversaturated air condensing downstream and entraining liquid into the TEM environmental cell I am sending the gas to.

I also wanted to add a useful (for me anyway) contribution for anyone trying to build a bubbler. The Honeywell hih-4000 is an excellent and tiny hygrometer sensor that can be read with any DC power supply and a standard high precision multimeter. It's tiny size means it can be inserted into the downstream gas flow very easily, I just drilled a hole in some tubing, stuck the sensor in and sealed the gap with JB Weld.