Gas Sensors

Colorimetric Tubes & Chip Systems

Chemistry in a glass tube: draw a measured sample through and read a color change. Cheap, no calibration, an enormous menu of chemicals — the technician's Swiss-army spot-check.

TECH chemical-specific spot checks, cross-check of gas-monitor alarms, confirming a suspected agent

How it works

A colorimetric detector tube is a sealed glass tube packed with a chemical reagent on a granular support, specific to one target chemical (or a family). You snap off both ends and insert it into a calibrated hand pump (bellows or piston). Each pump stroke draws a fixed, known volume of air through the tube. If the target chemical is present, it reacts with the reagent and produces a color stain that advances along the tube. The length of the stain (or, on some tubes, the intensity of the color) is read against a printed scale calibrated in ppm. More chemical → longer/darker stain → higher reading.

What it's good for

• Chemical-specific confirmation — unlike a broadband PID, a chlorine tube reacts to chlorine. Great for confirming or ruling out a specific suspected chemical and cross-checking a cross-sensitive electrochemical alarm.
• Enormous chemical menu — hundreds of tube types cover TICs, acid gases, solvents, and agents that no single electronic sensor addresses. If you can name the suspect, there's probably a tube.
• No calibration, no lamp, no battery, no warm-up — grab the right tube and pump and go. Ideal backup when electronics are unavailable or suspect.
• Cheap and expendable — a box of tubes costs a fraction of an electronic sensor.
• Detects things your monitor can't — HF, phosgene, many exotic TICs, and agent-related compounds.

What it CANNOT do / limitations

• Not continuous. A tube is a single spot measurement in time — it doesn't monitor a changing atmosphere. You can't clip it on and walk.
• You must guess the chemical first. You pick a tube for a specific target; a tube tells you nothing about a chemical you didn't choose to test for. It's characterization/confirmation, not blind identification.
• Modest accuracy — typically about ±25%. These are semi-quantitative. Read them as "roughly this much," not a precise number.
• Single use — one tube, one measurement.
• Slow-ish — several strokes over a minute or more per test; multiple suspects means multiple tubes and time.
• Temperature and humidity corrections often required (see the insert).

Cross-interference — read the insert

Reagent chemistry isn't perfectly specific. Other chemicals can produce the same color (false positive), a different color, or bleach/suppress the stain (false negative). Every tube ships with a paper insert that lists the calibrated range, the number and rate of strokes, temperature/humidity correction factors, shelf-life/lot, and — critically — the known cross-sensitivities and interfering substances.

Stroke counts & pump-technique errors

Accuracy depends entirely on drawing the correct number of strokes at the correct rate, because that sets the sampled volume the printed scale assumes. Common technique errors:

• Wrong stroke count — too few under-develops the stain (reads low); too many over-develops it (reads high). Follow the insert exactly.
• Not letting the bellows/piston fully re-expand between strokes — you draw less than a full volume, so you under-sample.
• Rushing strokes — the reagent needs contact time; pulling too fast can under-develop.
• Pump leaks — a cracked bellows or bad seal means you're sampling less air than you think. Leak-check the pump (per manual, e.g., an unopened tube inserted and a stroke that shouldn't fully collapse) before relying on results.
• Inserting the tube backward — tubes are directional; the arrow points toward the pump.

Shelf life & storage

• Tubes expire. The reagents degrade; every box has an expiration date. Expired tubes give unreliable results — check the date before use and rotate stock.
• Many tubes require refrigeration (or at least cool, dark storage) to reach their rated shelf life. Heat in an apparatus compartment shortens their life dramatically.
• Protect from light and temperature extremes; some reagents are light-sensitive.
• Keep the pump clean and leak-tight; store per manual.

Chip measurement systems & simultaneous-test kits

• Chip Measurement Systems (e.g., Dräger CMS): replace the glass tube with a multi-capillary chip and an electronic analyzer that controls the sampling and reads the result optically, giving a digital number. This removes stroke-count and stain-reading errors and improves repeatability, at higher cost and with its own consumable chips.
• Simultaneous / multi-tube kits (e.g., Dräger CDS — Civil Defense Simultest, or "Simultest"): a manifold runs several tubes at once from one sample, letting you screen for multiple hazard classes (e.g., a hazmat/agent screening set) in a single draw — a fast first characterization of an unknown vapor.

"Calibration" — there is none, but there's an expiration date

Common rookie mistakes

Representative instruments

The classic systems are Dräger tubes with the accuro/bellows pump, Kitagawa tubes with a piston pump, and Sensidyne/Gastec tubes and pumps. Digital chip systems include the Dräger CMS; simultaneous kits include the Dräger CDS / Simultest hazmat screening sets. Colorimetric tubes sit outside the RAE product line, so a RAE-fleet department still stocks these separately — note also that the UltraRAE's benzene separation tube (see the PID page) is a related tube-based idea bolted onto a PID. Brands are illustrative; the tube insert and your SOPs govern.

Next: detecting chemical warfare agents and TICs — Ion Mobility Spectrometry (IMS) →