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A Comparison of Dilution Methods: (continued)

or, How to know when the Model 1010 is Better Than Other Methods

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This section is quite comprehensive and covers:



Permeation Tubes

A permeation tube is a short length of tubing, often made of Teflon, filled with a gas or liquid. The substance effectively dissolves in the Teflon and diffuses through the tube walls into the outside air. The diffusion rate is dependent on temperature. Permeation tubes are useful for making low concentrations of certain gases.

To use this method, you need a permeation tube oven, which maintains the tube at a preset temperature. The oven will also have a thermometer as well as a flowmeter for the purge gas. You also need a set of permeation tubes, at least one for each gas. Each perm tube needs a logbook to store weight and time data on each tube, for calibration. You also need access to a five-place balance. The ticket for entry here is about $3000-$5000.

Start by weighing the perm tube to five places and logging the weight.

Inside the permeation oven is a glass container. Open it, and insert the perm tube (or tubes). Close up the oven and turn on the heat. The time should be noted in the calibration log. Let the oven stabilize for several hours before use, with a small volume of gas flowing and discharging into an absorber or up a fume hood (rules permitting). The table that is supplied with each perm tube will allow you to set the flow rate for the concentrations desired.

After use, remove the perm tube, weigh it again, and log the time. Concentration is calculated from the loss of weight of the perm tube and the total volume of purge gas that has flowed over the tube during the time it was heated.

Permeation Tubes: Pros

    * Constant concentrations for long periods of time.
    * Several tubes can be used at once to obtain a mixture.
    * A single tube may last for years (if a stable compound).
    * Concentrations can be changed by changing temperature and purge flow rate.
    * Reliable source of very low concentrations.
    * Cheap source of standard gas, after initial costs.

Permeation Tubes: Cons

    * The flow rate is fixed for a given concentration.
    * Not suitable for high concentrations or certain compounds.
    * Long warm-up and stabilization time.
    * Some compounds decompose or are oxidized in the perm tube (a sample of ethyl mercaptan
            showed four peaks on the GC after a year).
    * You need a five-place balance to check and log your calibration.
    * Permeation tubes can be dangerous if cut or broken, since the contents are often under pressure.
    * High initial cost.
    * Fragile, bulky, and nonportable apparatus.


Gas Syringes

Gas syringes are used to draw a sample of gas at high concentration and discharge it into a sample bag, which is then filled with dilution gas using a flowmeter or large gas syringe.

The syringe method is fundamentally a batch method. It is not practical when flows of gas are needed for long time periods.

The diluent gas requires either a rotameter and needle valve or a large gas syringe. This will be the most expensive component.

Gas Syringes: Pros

    * Syringes can be inexpensive (but not necessarily).
    * The method is intuitively understandable and calculations are simple.
    * A wide range of concentrations can be made without more equipment.

Gas Syringes: Con

    * The method is not suitable for large volumes or continuous flow.
    * Syringes must be kept scrupulously clean. Use no lubricants for the plunger seal.
            (Disposable polyethylene may sometimes be used.)

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