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Colorimetric methods

 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.

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.

 

Colorimetric tubes.

  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.

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Field olfactometry

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    The main idea of the Field inspections is to estimate the degree of annoyance in a certain problematic area, by means of the olfactory capacities of a group of people or panel members, especially trained and "calibrated" to this effect. The main difference between field inspection and dynamic olfactometry is that in the first case, the panellist are moved to the problematic area to measure the odours on site.

   There is a usual trend for technicians to try to measure odours at ambient air levels. This way of measuring odours has a few advantages:

1) Measuring odours when they produce annoyance, that is, in the receptors. 2) It is less complicated to measure at the fence line or in the receptors that in the stack.
3) In addition it could be carried on as an inspection for compliance procedures.
4) It is a similar approach to that taken for chemical compounds, that can be detected in many cases at ambient air levels.
5) You get instant values instead of averages or percentiles.

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Psychometry

 Psychometry is a very efficient tool in many cases to deal with the problem of odours in a community. The psychometry is the evaluation of the odour complaints distribution by mean of questionnaires given to the people of neighbouring areas. The statistical analysis of the answers given by the neighbours is an starting point and it can help to design a campaign to measure odours by any other method.

The psychometry can give us answers such as:

  • The time-intervals when there is a maximum of complaints.
  • The location of the most problematic areas.
  • The possible source of the odours.
  • The odours with a less accepted hedonic tone by the community..

  The psychometric evaluation is a very effective tool, widely used in many countries (R. Cervinka et al., 2004, E. Gallmann et al., 2004). In Germany there is a norm that specially deals with the psychometric assessment, this is the VDI 3883.

  The Psychometric analysis is quite convenient as a screening study when the purpose is to measure odours by any other method such as field inspections, field olfactometry or dynamic olfactometry + dispersion model, however, this step is often forgotten in many odour studies.

 

Notes:

In New Zealand there is a very interesting norm based on a Psychometric analysis. + info there. 

 

More Articles …

Subcategories

Instrumental Odour Monitoring (O-sensors)

Psychometry

Field Olfactometry

Field Inspections

Physico-Chemical analysis

Dynamic Olfactometry

Odour dispersion

Odour abatement

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