© 2009
Custom Sensor
Solutions, Inc.
Testing Gas Sensors Over a Range of Relative Humidities
with applications to the generation of ternary gas mixtures.

For any sensor that will be used to monitor the environment, the effect of relative humidity (R.H.) must be measured. Many gas sensors are upset by changes in relative humidity (R.H.). Certain sensors, such as conductive polymers and solid-state (tin oxide) types, are notorious for their R.H. effects. Electrochemical sensors are sometimes touted as unresponsive to R.H., but this depends on both the sensor and the gas. In certain gold-electrode sensors, H2S may have a strong humidity dependence, and sulfur dioxide will not.

Measuring R.H. coefficients is a common example of a ternary gas-mixing problem: you want to mix the test gas, the carrier (usually air or nitrogen), and water vapor, so that all three come out to predetermined concentrations. Ternary gas mixtures are very tedious to make by the usual methods, and prone to error. With one or two Model 1010 Precision Gas Diluters and a hand calculator, you can make these mixtures in a short time, and depend on their accuracy.

Lesson One in making R.H.-controlled mixtures: Don't use bubblers to make your water-saturated gas. A simple bubbler is unlikely to achieve an R.H. above 70%. Even using fine spargers and deep water columns, it is hard to exceed 85% R.H. Furthermore, the bulk water will cool as evaporation proceeds, and the R.H. will gradually drift downward over time.

The conventional method of making water-saturated air is to boil water in a stream of air, and pass the steam through one or two condensers actively held at room temperature [Nelson, 1992]. This is a great pain if you only plan to make a few measurements. You can fill a large sample bag with 100% R.H. air by injecting a measured amount of liquid water into a bag filled with a known volume of dry air, and evaporating it with a heat gun. If you use a sample bag of a transparent material like TedlarTM, you can see the water drops. This simplifies the process considerably. After the humidified air has cooled to room temperature, it is ready for use. (Don't forget to start with dry air from a cylinder, or excess water will condense in the bag!)

Now to set up for our tests: We have a cylinder of dry air and a cylinder of standard ethane in air at 5,000 ppm. We want to test our sensor at 0, 100, 300, and 1000 PPM ethane, at 25%, 50%, and 75% R.H.

The schematic for the test system is shown in Figure 1. In this case, two Model 1010-P diluters are used, so that balancing adjustments can be made to keep one of the constituents constant while varying the other. Notice that a vented bottle is located between the two diluters. The function of the first diluter is to keep the vented bottle filled with air of controlled humidity; the second diluter will draw from this bottle as needed. Excess humidified air is vented from the top of the bottle.
From Figure 2, we see that water-saturated air at 20C contains 17.3 grams of water per cubic meter, or 0.52 grams in 30 liters. Partially fill a 24"x24" TedlarTM bag with exactly 30 L of  dry air. Next, inject 0.52 mL of water into the bag, and use a blow dryer to evaporate the water. Because TedlarTM is transparent, you can chase down the last droplets until they are evaporated. The bag should have been deliberately underinflated to avoid bursting during this process, but give it intervals to cool if necessary. When finished, let the bag cool to room temperature, usually no more than 5 minutes.
A table of diluter settings is best drawn up in advance of the tests. The setting of Diluter B is straightforward. To generate 100 PPM ethane from a 5,000 PPM standard requires a setting of 2.0% regardless of the R.H. of the diluting air. However, the setting of Diluter A must be corrected to allow for the additional dry air contributed by the ethane standard. The formula used to correct the Diluter A setting is shown in Figure 3.
Using the correction formula, a table of diluter settings can be drawn up (Table I). To carry out the experiment, it is only necessary to select the pair of R.H. and ethane concentrations and set the diluters accordingly. R.H. can be varied while holding ethane constant, or the reverse.

Table I.
Ethane
(ppm)
R.H.
(%)
Diluter B Setting
(%)
Diluter A Setting
(%)
0
25
0
25
100
25
2
25.5
300
25
6
26.6
1000
25
20
31.3
0
50
0
50
100
50
2
51.0
300
50
6
53.2
1000
50
20
62.5
0
75
0
75
100
75
2
76.5
300
75
6
79.8
1000
75
20
93.8









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