How to resource Gas and controlling the supply for testing Gas Sensor?

Thread Starter

oridroo

Joined Feb 5, 2012
27
Hi,

I am trying to develop a Gas detector module to detect Methane (CH4) gas.
I am using the 'ezPyro CH4 Gas sensor' (SMD).

Now, the difficult part is resourcing the required Methane Gas. I believe I need small amount of Methane gas for lab testing. But I coudn't find any such supplier who sells such small amount in a cylinder.

Another issue is, how would I be able to control the amount of gas I would be supplying to my testing chamber in terms of PPM? I want to know exactly what amount of PPM is getting supplied to the gas chamber thus I can form a calculation from the sensor's output to reflect the amount of gas present in the air.
This sensor's output is in current. So, I'm planning to calculate the PPM of the given gas in terms of the current I will be getting at the output.

So please suggest me how can I perform the testing successfully.

Regards
 

Papabravo

Joined Feb 24, 2006
13,560
How familiar are with the chemistry and processes for producing your own methane? In theory you can try to create a chamber with methane and nothing else, but I don't recommend that for safety reasons. Alternatively you can try to create a gas mixture with a little bit of methane and quite a bit of other stuff. I don't know if it will be possible to compute the partial pressures of each component, but I think that is what you must try to do.

Check out this page
https://www.engineeringtoolbox.com/partial-pressure-ideal-gas-law-total-mixture-blending-d_1968.html


Molecular weight of methane is 16.043 grams/mole, while natural gas is a bit heavier at 19.00 grams/mole. This suggests that natural gas is not pure methane - surprise, surprise.
 
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Thread Starter

oridroo

Joined Feb 5, 2012
27
How familiar are with the chemistry and processes for producing your own methane? In theory you can try to create a chamber with methane and nothing else, but I don't recommend that for safety reasons. Alternatively you can try to create a gas mixture with a little bit of methane and quite a bit of other stuff. I don't know if it will be possible to compute the partial pressures of each component, but I think that is what you must try to do.

Check out this page
https://www.engineeringtoolbox.com/partial-pressure-ideal-gas-law-total-mixture-blending-d_1968.html


Molecular weight of methane is 16.043 grams/mole, while natural gas is a bit heavier at 19.00 grams/mole. This suggests that natural gas is not pure methane - surprise, surprise.
Hi,
Thanks for your suggestion. Yes, you are right. I can't get the pure Methane. And I don't have that much of Chemistry expertise, so I would have to buy the gas from outside.
I found one supplier who supplies calibrated gas. They would be able to supply 100L, 5% CH4 balanced with N2 and the price would be AUD 370. I am not sure whether I gonna need this much of gas for lab testing or not.
Also, how would I be able to know the approximate PPM of the gas if I purchase this one? Any such gas regulator which gives reading in PPM?
Thanks
 

Papabravo

Joined Feb 24, 2006
13,560
Well if you know the volume (100L) and you know the percentage by weight 5% CH4 and 95% N2 and you know the total weight you can compute the ppm. Again you have to go back to the process of mixing your gas mix with dry air then working out the partial pressures of each component to get the ppm of the Methane. Don't know anything about gas regulators, so you're on your own there. On my air compressor, the regulator only allows the establishment of a differential pressure between the line and the tank. There is nothing about volume, weight, flow that would help in measuring the amount of anything.
 
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Thread Starter

oridroo

Joined Feb 5, 2012
27
Well if you know the volume (100L) and you know the percentage by weight 5% CH4 and 95% N2 and you know the total weight you can compute the ppm. Again you have to go back to the process of mixing your gas mix with dry air then working out the partial pressures of each component to get the ppm of the Methane. Don't know anything about gas regulators, so you're on your own there. On my air compressor, the regulator only allows the establishment of a differential pressure between the line and the tank. There is nothing about volume, weight, flow that would help in measuring the amount of anything.
I tried to get some ideas about the calculation from internet. I have got one equation in a website to convert the % to PPM which implies:

PPM =%×10000
According to them, % is : "This unit expresses concentration in parts per hundred (percentage) of a substance in 100mL of a medium such as air. "

And PPM is : "This unit that expresses concentration in parts per million is measured as the volume (denoted in litres [L]) of a substance found in 1L of a medium such as air. "
I guess they multiplied the part per hundred by 10^4 to make it parts per million. But I couldn't understand why they didn't scale up mL to L?

And is this method would be reliable to convert % to ppm?

Thanks
 

jpanhalt

Joined Jan 18, 2008
9,129

Thread Starter

oridroo

Joined Feb 5, 2012
27
Methane calibrator gas can be bought: http://www.calgasdirect.com/methane-calibration-gas-ch4-35-ppm-balance-air-in-a-17-liter-steel-disposable-cylinder/?gclid=EAIaIQobChMIzKzKmPqn6AIVksVkCh2CyAhMEAQYASABEgIwx_D_BwE

Even Amazon has it; although, I didn't search for lowest price: https://www.amazon.com/Methane-Calibration-Balance-Liter-Cylinder/dp/B011T9NLVU

Home heating gas is a mixture of methane and other low-boiling hydrocarbons. It works and is not worth the cost to further purify.
This is great! Thanks.

They mention the PPM as well. From their website, I saw that they also provide 99.999% Vol. Do you think it's necessary to have it for the testing purpose?

Also, can you please suggest any regulators as it seems a bit pricey for this type of cylinders.

Thanks
 

jpanhalt

Joined Jan 18, 2008
9,129
Those cylinders look like standard hardware-store type cylinder as is used for MAPP, propane, etc. The first contains 17_L at atmospheric pressure. If the cylinder volume is 1L, that would mean about 200 psi gauge ( v*p = v'*p' ; 1 atm = 12.7 psi, so 17 * 12.7 = 216 psi absolute). I suspect a simple needle valve regulator will work. You would need to be sure the threads matched.

We used to have a variety of research gasses in the lab. They all used a simple needle valve. If you want an accurate volume, you can use water displacement with an upside down graduated cylinder. Alternatively, fill a syringe directly and let it equilibrate to atmospheric pressure.. Methane is fairly soluble in water, but you probably don't need to correct for that.

The Amazon cylinder with 34L would be about twice that pressure.
 

Thread Starter

oridroo

Joined Feb 5, 2012
27
Those cylinders look like standard hardware-store type cylinder as is used for MAPP, propane, etc. The first contains 17_L at atmospheric pressure. If the cylinder volume is 1L, that would mean about 200 psi gauge ( v*p = v'*p' ; 1 atm = 12.7 psi, so 17 * 12.7 = 216 psi absolute). I suspect a simple needle valve regulator will work. You would need to be sure the threads matched.

We used to have a variety of research gasses in the lab. They all used a simple needle valve. If you want an accurate volume, you can use water displacement with an upside down graduated cylinder. Alternatively, fill a syringe directly and let it equilibrate to atmospheric pressure.. Methane is fairly soluble in water, but you probably don't need to correct for that.

The Amazon cylinder with 34L would be about twice that pressure.
Another novice question. I am a bit confused about the relationship between ppm and volume.
That cylinder says, 35 ppm. Does this mean 35 parts(molecules of CH4) per million of Air molecules per Litre? Or it is the total concentration of the gas available in the cylinder i.e. 35/17=2.06 ppm/L ?

Thanks
 

jpanhalt

Joined Jan 18, 2008
9,129
ppm is just a way of expressing percentage without a bunch of zeros. That is 0.001% = 0.00001 fraction = 10 ppm . As with all fractions, in theory it can be any ratio. Weight/weight and volume/volume or combination s of weight and volume are common. In the case of gases at atmospheric pressure it is probably volume/volume which relates directly to number of molecules / total number of molecules for ideal gases. Ideal gas = 22.7 L/mole at stp (https://en.wikipedia.org/wiki/Ideal_gas).

Another complication is the difference between a mixing ratio, say 3 parts A with 7 parts B, which is sometimes shown as 3:7. If the "parts" is mass (g), then 3 g A + 7g B = 3g A in 10 g total or 30% "by weight." Edit: When you mix liquids by volume, there can be a change in volume, so % by volume is not so easy to calculate. Gases can do the same as liquids, but that is "non-ideal" behavior.

In general, ppm will not mean a mixing ratio as you state (i.e., 35 parts methane per million parts air) but rather 35 parts methane per million parts total. With dilute mixtures, like 35 ppm, the difference between 35 parts of methane per million parts air and 35 parts of methane per million parts total is very small (https://en.wikipedia.org/wiki/Parts-per_notation) and probably insignificant.

As for your question about relationship with volume, if a gas is 35 ppm, the volume doesn't matter. That is, compressed to 1 L or at atmospheric pressure of 17 L, it is still 35 ppm. With methane in relatively dry air at low concentrations, you can assume ideal behavior.
 
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Thread Starter

oridroo

Joined Feb 5, 2012
27
ppm is just a way of expressing percentage without a bunch of zeros. That is 0.001% = 0.00001 fraction = 10 ppm . As with all fractions, in theory it can be any ratio. Weight/weight and volume/volume or combination s of weight and volume are common. In the case of gases at atmospheric pressure it is probably volume/volume which relates directly to number of molecules / total number of molecules for ideal gases. Ideal gas = 22.7 L/mole at stp (https://en.wikipedia.org/wiki/Ideal_gas).

Another complication is the difference between a mixing ratio, say 3 parts A with 7 parts B, which is sometimes shown as 3:7. If the "parts" is mass (g), then 3 g A + 7g B = 3g A in 10 g total or 30% "by weight." Edit: When you mix liquids by volume, there can be a change in volume, so % by volume is not so easy to calculate. Gases can do the same as liquids, but that is "non-ideal" behavior.

In general, ppm will not mean a mixing ratio as you state (i.e., 35 parts methane per million parts air) but rather 35 parts methane per million parts total. With dilute mixtures, like 35 ppm, the difference between 35 parts of methane per million parts air and 35 parts of methane per million parts total is very small (https://en.wikipedia.org/wiki/Parts-per_notation) and probably insignificant.

As for your question about relationship with volume, if a gas is 35 ppm, the volume doesn't matter. That is, compressed to 1 L or at atmospheric pressure of 17 L, it is still 35 ppm. With methane in relatively dry air at low concentrations, you can assume ideal behavior.
Thanks for such detail explanation.
What I have understood is, the concentration of gas would be similar irrespective of the quantity (Please correct me if I'm wrong).

So, for a 35 ppm CH4 gas cylinder, the concentration would be same (35 ppm) both for 17L and 1L or any other amount of gas?
 

Thread Starter

oridroo

Joined Feb 5, 2012
27
ppm is just a way of expressing percentage without a bunch of zeros. That is 0.001% = 0.00001 fraction = 10 ppm . As with all fractions, in theory it can be any ratio. Weight/weight and volume/volume or combination s of weight and volume are common. In the case of gases at atmospheric pressure it is probably volume/volume which relates directly to number of molecules / total number of molecules for ideal gases. Ideal gas = 22.7 L/mole at stp (https://en.wikipedia.org/wiki/Ideal_gas).

Another complication is the difference between a mixing ratio, say 3 parts A with 7 parts B, which is sometimes shown as 3:7. If the "parts" is mass (g), then 3 g A + 7g B = 3g A in 10 g total or 30% "by weight." Edit: When you mix liquids by volume, there can be a change in volume, so % by volume is not so easy to calculate. Gases can do the same as liquids, but that is "non-ideal" behavior.

In general, ppm will not mean a mixing ratio as you state (i.e., 35 parts methane per million parts air) but rather 35 parts methane per million parts total. With dilute mixtures, like 35 ppm, the difference between 35 parts of methane per million parts air and 35 parts of methane per million parts total is very small (https://en.wikipedia.org/wiki/Parts-per_notation) and probably insignificant.

As for your question about relationship with volume, if a gas is 35 ppm, the volume doesn't matter. That is, compressed to 1 L or at atmospheric pressure of 17 L, it is still 35 ppm. With methane in relatively dry air at low concentrations, you can assume ideal behavior.
Thanks for such detail explanation.
What I have understood is, the concentration of gas would be similar irrespective of the quantity (Please correct me if I'm wrong).

So, for a 35 ppm CH4 gas cylinder, the concentration would be same (35 ppm) both for 17L and 1L or any other amount of gas?
 

jpanhalt

Joined Jan 18, 2008
9,129
Thanks for such detail explanation.
What I have understood is, the concentration of gas would be similar irrespective of the quantity (Please correct me if I'm wrong).

So, for a 35 ppm CH4 gas cylinder, the concentration would be same (35 ppm) both for 17L and 1L or any other amount of gas?
That is correct.

However there is another aspect of gases to consider that may or may not apply to your sensor. It is calibrated at some standard pressure. So long as you test it at that pressure close to it, there will be no problem.

Unlike liquids and solids, the density of a gas changes greatly with pressure. In an earlier response, I mentioned that at standard conditions of pressure and temperatures, 22.7 L of an ideal gas contain one gram molecular weight (mole). If you triple the pressure, there will be 3 times as many moles in the same volume and so forth. Many reactions depend on concentration not the fractional percentage. High pressure air, say 5 atmospheres pressure, will have the same concentration of oxygen in moles per liter as pure oxygen at standard pressure. There is another term, "partial pressure," that can be used to describe that. At 5 atmospheres of pressure, one would say that the partial pressure of oxygen in air is 1 atmosphere. (You can read more about partial pressure here: https://en.wikipedia.org/wiki/Partial_pressure.)

The impact of that on your sensor experiments is that you should be sure the gas you are measuring is at atmospheric pressure to get expected values in the datasheet and relative combustibility. If not at atmospheric pressure, this link will tell you more than you want to know about it: https://www.osti.gov/servlets/purl/7328370/ Aliphatic hydrocarbons start about page 20. :)
 

Janis59

Joined Aug 21, 2017
1,169
Few years ago I bought for 25$ some Chineese "combustible gas detector" I wanted to use for vacuum chamber leak detection. Just add the 5-10% Hydrogen to nitrogen so the kaboom never may happen, and most smallest defect in welding may become sniffable. So, the idea beyound is the current warmed niting which arise the temperature (means change the voltage on niting) if any comustible gas is touching this niting and burn.
 
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Janis59

Joined Aug 21, 2017
1,169
By the way: RE: ""I believe I need small amount of Methane gas. But I coudn't find any such supplier who sells such small amount in a cylinder.""
For my sake I would use a TransAmerican kitchen gas company. Kitchen devices use only two type of gas, if bottled (balloons) LPG what is propane-butane mixture what is not what You will, and if pipeline system then "Nature gas" or CNG or LNG both are 99% clean Methane. Except very very old mining places where probably somewhere at distant Africa still they use a CO gas coming off the carburisation plant of coal.
 

Janis59

Joined Aug 21, 2017
1,169
About gas titration. We in our institute often need to brew different gas concentrations for different aims. So we need a ultimate high purity we use the custom made quartz cylinders what our glass-blower master does. So, then we measure by means of water as much as possible exact volume of that cylinder. And then apply the add-in gas baloon with reductor to get out the very precise pressure. When it is done, we add the carrier gas up to calculated pressure. Technically, some 4-6 digit accuracy is everyday simple to get result. Only minus is that system may not work with gas pressure much over of 1 kg/cm2.
 

jpanhalt

Joined Jan 18, 2008
9,129
I believe the TS is using the ezPyro SMD derector, perhaps this one: https://pyreos.com/wp-content/uploads/2019/07/ezPyro-SMD-IR-Sensor-for-Gas-Sensing-Short-Form-Datasheet.pdf

That detector is based on infrared absorption at about 3.3 u (about 3000 cm-1). It has nothing to do with burning. This link will show you the spectrum: https://webbook.nist.gov/cgi/cbook.cgi?ID=C74828&Type=IR-SPEC&Index=1 You can display it in either wavelength (microns) or wavenumber ( cm-1).

@Janis59 Your posts #16-18 are interesting, as always. That detector does not ignite the methane, if I understand your comment, "So, the idea beyound is the current warmed niting which arise the temperature (means change the voltage on niting) if any comustible gas is touching this niting and burn" correctly.
 

Thread Starter

oridroo

Joined Feb 5, 2012
27
So, for a 35 ppm CH4 gas cylinder, the concentration would be same (35 ppm) both for 17L and 1L or any other amount of gas?
That is correct.
In that case, if I buy a 35 ppm, 17 L CH4 gas cylinder and then start passing the gas into the chamber, the volume will start reducing. So, if it becomes 12 L (considering 5L of CH4 being passed), the ppm will still be 35 ?

I think I am mixing up in between the 'Number of Molecules' and 'Parts Per Million'. I thought, number of PPM is representing the concentration of a gas indicating the number of molecules present in that gas. Now, if the ppm doesn't change with the volume of the gas, then what exactly is changing with the volume, the number of molecules present in that gas?
What is the way to determine the amount of gas (for this experiment it's CH4) present in air and how to determine whether this amount is increasing or decreasing?

Thanks
 
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