The colorimetric methods are able to identify, in many cases in a quantitative way, the presence of a gas. It is possible to use patches or colorimetric tubes or badges.
The badges are a one-use piece of cardboard with a small surface coated with a chemical that changes its colour when exposed to a certain gas. Both the time exposure and the intensity of the colour change are important. The badges show an integrated averaged value not very precise but useful when dealing with personal protection exposure.
These devices might be hanged anywhere, or be carried on the worker's cloths or they might be combined with small fans for different applications. Hydrogen sulphide badges are well used in farming, for example to limit H2S exposure in workers.
There are different types of colorimetric tubes to measure a wide range of gases. The Colorimetric tubes are glass tubes with both ends closed. To perform a measurement with a colorimetric gas tube, both ends of the tube must be broken. Then the tube is connected to a pump (manual or electronic). The pump makes the gas to circulate through the tube with a known volume of odorous gas. The media inside the tube reacts with the gas and change its colour according with the type of gas sampled. A scale is used to measure the amount of gas that reacts with the media. This scale indicates the concentration of the gas detected.
The colorimetric tubes usually have limited scales and the precision is around 10-20% of the full reading scale of the tube. The colorimetric tubes offer almost instant readings with detection limits down to 0,2 ppm in the case of Hydrogen sulphide (H2S). The cost of a manual pump stay between 300-500 €. The diffusion tubes that measure a mean concentration are also available for certain gases.
It is possible to find colorimetric tubes that work by means of a mechanism of diffusion, without the need of a pump. To use these passive samplers, one of the ends of the tube is opened and then the tube is placed in the site to be monitored. After a known period of time (normally 6 to 8 hours), a reading is taken by the colour change observed in the tube. This parameter and the exposition time are used to calculate average concentration for the time period sampled. This tubes have a price of around 6-8 € each.
You can check the different colorimetric tube makers in the directory of this site, or just clicking here.