Colorimetric Tubes

How a Tube Actually Works

It’s worth understanding, because it explains both the accuracy and the limitations.

A Gastec detector tube is a thin glass tube containing a particulate matrix, such as silica gel or alumina, which holds a highly stable reagent chosen to be especially sensitive to the target substance. Both ends are hermetically sealed until use.

Break the ends, fit the tube to a Gastec pump, and draw a measured volume of air through it. The reagent reacts with the target gas and produces a distinct layer of colour change. The length of that layer shows the concentration, read directly from the calibration scale printed on the glass.

Precision comes from Gastec’s quality control. Each production lot is tested and calibrated independently, and each tube has its own quality control number printed on it, so a result can be traced back to its batch.

Five Hundred Gases

This is why tubes are still widely used, and it isn’t just about cost.

Gastec tubes are available for more than 500 applications, far more substances than any portable electronic instrument covers. For many substances, a tube is the only practical portable method, not just the cheaper one.

That covers a lot of ground in Australian industry: solvents and process chemicals in manufacturing, fumigants in grain handling and quarantine, refrigerants, specialty gases in laboratories, acids and alkalis, and anything where a risk assessment has identified a specific substance rather than a general flammable or toxic hazard.

Pick the Range, Not Just the Gas

Tubes are made in several ranges for many gases, and choosing the right range matters as much as choosing the right gas. The measuring range can also be extended by adjusting the sample volume.

The selection question is three part: the gas, the concentration you expect, and what the measurement is for. A tube ranged for a fumigated space won’t resolve a low level exposure measurement, and one ranged for exposure monitoring won’t read a fumigation concentration.

Our Technical Consultants can help pick the right tube and range.

One Tube, One Gas

Each tube is made for a single substance, and is single use. That means the substance has to be known before the measurement.

Where it isn’t, Gastec Polytec tubes have several reaction layers that respond to different groups of gases, to help identify what’s present.

Other gases in the atmosphere can still interfere with the reagent in a tube, and the instruction sheet supplied with each box sets out which ones. It’s worth reading rather than filing, particularly on a site with a complex atmosphere.

Storage and Shelf Life

This is the one thing that will spoil a measurement, and it happens before the tube is used.

Tubes are hermetically sealed and the reagents are selected for long term stability, with many tubes rated at a three year shelf life. The use by date is printed on the box.

Two rules follow. Store tubes in a cool place, and refrigerate them where the tube’s instructions recommend it. And don’t use expired tubes, because the reagent degrades and the reading becomes unreliable in a way you can’t see.

Check each tube’s operating temperature and humidity limits on its instruction sheet. On an Australian site in summer, that’s a real consideration, not a formality.

You Need a Pump

This is stated plainly because it catches people out.

A colorimetric tube can’t be used on its own. The measurement depends on drawing an exact sample volume through it, which is what the Gastec sampling pump does. Ordering tubes without a pump means ordering glass.

The GV-100 pump has an internal capacity of 100 mL and weighs 240 grams, and works with almost all Gastec short term detector tubes. It meets the leakage test of EN 1231, the standard for short term detector tube measurement systems, and has a non sparking design. A built in tip breaker opens tubes cleanly.

Where It’s Used

Occupational hygiene assessment of specific chemicals. Confined space entry where the hazard is an unusual solvent or process chemical. Grain and container fumigation. Quarantine clearance. Manufacturing and process plant. Laboratories. Refrigeration and air conditioning leak investigation. Emergency response. Compressed breathing air testing. Any measurement where the substance is known and no electronic sensor suits it.