Custom Sensor Solutions, Inc.

A Comparison of Dilution Methods: or,

When and Why the Model 1010 is Better Than Other Methods

Compare the pros and cons of the most common methods of preparing working concentrations of gases:

Twin Rotameter Method

Diluters are most often constructed of items easily obtained from laboratory catalogs. The most common form of diluter is made from two rotameters with needle valves, and a tee fitting. (A rotameter is the typical scaled vertical tube with the floating ball.) The bottom input connectors of the rotameters are connected to the sources of standard and diluent gas, usually the regulators on cylinders. The top outputs are connected at a tee fitting.

To use the twin-rotameter diluter, you must first know the flow demand of your experiment. Suppose it is 1000 cc/min. Then you must calculate how much flow is needed through each rotameter to give the required dilution. Knowing the flows, you can read the required scale settings from the table supplied by the manufacturer.

Now, turn on the cylinder regulators, and set the rotameters to the required flows. Start with the diluent first, so that your experiment is not hit with a pulse of high concentration. Be sure to check the flow rates regularly, at least every 15 minutes for the first two hours.

If you must change the flowrate to your experiment, the flow rates must be separately recalculated and set.

Rotameters: Pros

Wide dilution ranges are possible with a small selection of rotameters.
They are intuitively comprehensible to set up and to use.

Rotameters: Cons

Rotameters are expensive and fragile.
You need to keep flow tables or cal curves at hand -- and they are forever getting lost.
Needle valves wear out quickly and become unstable.
Changes in dilution or flow rates are awkward to make, and time consuming.
Flows will change with supply pressure, such as cycling of a pressure regulator.
Correction factors are needed for different gases, e.g., helium vs. air.
Needle valves and flowtubes get dirty and oily after a while (just look carefully inside six-month-old rotameter). This spells doom for VOC and oxidant measurements.
Certain gases come out of a cylinder in a chemical form that you might not expect (e.g., NO₂ comes out as N₂O₄). Time is needed to allow the gas to come to equilibrium at normal temperature and pressure.
Rotameters may be made of materials incompatible with your gases (though Teflon is available).

Permeation Tubes

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.
Syringes should be kept scrupulously clean. Use no lubricants for the plunger seal. (Disposable polyethylene may sometimes be used.)

The Model 1010 Precision Gas Diluter

Using the Model 1010 requires no flow meters. Setting up and using the Model 1010 is straightforward and quick. Sample bags of standard gas and diluent are connected to the inputs. The dilution ratio dial is set and the instrument started. Depending on the model (pumped or not pumped), the required concentration will issue from the GAS OUT port or can be drawn at will from the port.

Model 1010: Pros

Very simple to use.
Wide flowrate range.
Output concentration independent of flowrate.
Can be used for serial dilutions.
No or little carryover between gases or from high to low concentrations.
Dilution settings stable indefinitely.
Dilution range of 0 to 50-fold adjustable by front panel control dial alone.
No consumables or operating costs (except electricity).
Audible and visible confirmation of operation.

Model 1010: Cons

Standard and diluent gases must be at atmospheric pressure, i.e., in sample bags or from a zero filter.
Serial dilutions needed for dilutions greater than 50-fold.
Cannot be used for hydrogen halides or some other corrosive gases.

Current March 22, 1998

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