© 2009
Custom Sensor
Solutions, Inc.
Making Accurate Vapor Dilutions for Calibrating Gas Analyzing Instruments
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Making Dilutions to Working Concentrations

We're not going to be shy about it. We think that our Model 1010 Precision Gas Diluter is the best all-around way to make dilutions of standard gases. But to be fair, we're going to cover all the major methods. You can make up your own mind.

From an accurate stock concentration, a series of dilutions can be rapidly made for preparing calibration curves. The term stock gas refers to a precisely-prepared higher concentration, and diluting gas or diluent refers to the gas used to dilute it to working concentrations. Typical diluting gases may be air or nitrogen.

Stopwatch and Flowmeter Method

For volumes of gas of five liters or more, the stopwatch and flowmeter method is convenient. It requires more attention, because it depends on careful timing, and on maintaining a constant flow rate over the time it takes to fill a sample bag.

The stock gas source is connected to a flowmeter. The gas flow is started and adjusted to a suitable value, say 1.00 liter per minute. The flow is then connected to the sample bag and the stopwatch is started. The flow rate is remeasured and the total time needed to pump the required volume of air is then calculated. At the end of this time, the bag is disconnected. The diluting gas may come from a cylinder of air or nitrogen or from an air pump. If laboratory air is used, it is a good idea to pass it through a charcoal or Purafil filter, although this does not guarantee purity. When using typical floating-ball flowmeters with needle valves, the flowrate may have to be adjusted from time to time during the filling of the sample bag.

Gas Syringe Method

For smaller volumes of gas, syringes may be used in some cases. A typical strategy used in our lab is to make a 10,000 PPM stock sample in a 4-liter bag. Some chemicals might be stable for as long as a week. Working dilutions of 10 PPM would be made by withdrawing 40 cc of stock, injecting it into a sample bag, and filling the bag with 40 liters of air.

Gastight syringes can be purchased at fabulous cost, but plain disposable polyethylene syringes can often be used instead. These syringes are available in volumes up to 60 cc. To avoid loss of standard gas from the open syringe by diffusion, a 12" length of thin tubing is connected to the Luer fitting on the syringe, and the end of the tubing is closed with a valve or a plug closure.

The large-volume syringes such as the Tracor-Atlas 722K four- liter model, are useful for measuring diluting gases, but they should not be used for measuring standard gases. These syringes use a thin layer of silicone oil inside the barrel to aid plunger movement. Silicone oil will efficiently absorb many chemicals from the vapor phase, reducing the final concentration.

Multiple Flowmeter Method

The apparatus for the multiple flowmeter method consists of two flowmeters, preferably equipped with needle valves. One flowmeter is connected to the standard gas, and the other to the diluting gas. The outlets of both flowmeters are mixed together at a tee connection. The flow rates of both gases should be set with the needle valves and allowed to stabilize for a few minutes before the sample bag is connected to the third arm of the tee. Be sure to check the flowmeters frequently during the filling operation.

Time Proportioned Flow Switching (That's us.)

Gas mixtures can be conveniently made by drawing the sample air through a three-port solenoid valve whose inlets are connected to the standard and diluting gases, respectively. The solenoid valve is switched back and forth between the two gas sources at intervals of about one second. The pulses of gas are combined in a small-volume mixing chamber before being conducted to the instrument under calibration. The percentage of time that the solenoid is drawing from the standard gas is a measure of the dilution ratio. Dilution ratios of 20:1 are easily obtained, and dilutions of 1000:1 or more can be reliably carried out using two or more stages of dilution.

The cycling time and duty cycle of the solenoid valve must be controlled by highly-accurate electronics, and precautions must be taken to assure that the flow rates through the two gas paths are the same.

Custom Sensor Solutions, Inc., sells the Model 1010 Precision Gas Diluter for diluting gases by dynamic flow switching.

Mass Flow Controllers

Mass flow controllers are the "Cadillacs" of gas mixing devices. When supplied with a gas sample at low pressure, they will provide an accurately-controlled flow rate at the outlet. They must be provided with electrical power, since they use a delicate flow sensor and control valve to maintain a constant flow. Many can be operated under computer control.

The downside of mass flow controllers is that they are not compatible with all gases. Also, they seem to spend a great deal of time in the repair shop. Mass flow controllers are now available from many sources, but the cost begins at about $800.


Examples and Special Situations

Ozone

As mentioned before, ozone is a special case among important gases because it is so reactive. Any apparatus that is used with ozone should be first "scrubbed" with ozone for several hours before use. If you are using the ozone to calibrate a measuring instrument, you can determine when scrubbing is finished by taking periodic measurements of the ozone concentration at the outlet, without changing the span adjustment on the instrument. When the output signal no longer increases, the apparatus is ready to be used.

For information on calibrating ozone instruments, call us at 520-544-7523 or contact us at info@customsensorsolutions.com

Nitrogen Dioxide

Nitrogen dioxide calibration is often done with standard gas from a cylinder. Under the pressures existing in the cylinder, NO2 is actually present as the dimer, N2O4, even at concentrations of 50 PPM and below. Most instruments will respond differently to nitrogen tetroxide than to nitrogen dioxide; electrochemical analyzers, for example, do not seem to respond to N2O4 at all. After making dilutions of nitrogen dioxide at atmospheric pressure, let the gas sample stand for ten to fifteen minutes to allow the nitrogen tetroxide to dissociate.

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