Intrumentation - Page 1 of 15

Module 1: Occupational Hygiene - Section 5: Instrumentation

OH5.1: Instrumentation - Direct-reading Instruments for Chemical Hazards

GENERAL OBJECTIVES

To familarise students with direct reading instruments that are used to evaluate workers' exposure to chemical substances and physical agents encountered in most workplaces.

SPECIFIC OBJECTIVES

After completing this Module students are expected to:

Know when direct reading instruments are appropriate for use in evaluating exposure;
Understand the basic operating principles of the various instruments;
Recognise the limitations of these instruments.

Intoduction:

The traditional method of evaluating workers' exposure to airborne contaminants involves drawing air, at a constant flow rate, over a specified period, through a collection medium (filter, charcoal or liquid reagent) which either traps or reacts with the contaminant of interest.

The collection medium is then sent to a laboratory where it is chemically analysed to determine the amount of contaminant present. The amount of contaminant present divided by the volume of air sampled gives a value for the concentration.

The major limitations of this sampling and analysis technique are:

the delay in obtaining results;
failure to identify specific tasks (that are averaged over the traditional sampling process) that contribute most to worker's exposure; and
high costs that are associated with sample collection and laboratory analysis.

To overcome these problems a wide range of instruments that give instantaneous results of the nature and amount of contaminant in air have been developed. These instruments use electronic, optical, chemical or a combination of principles to detect and quantify airborne contaminants. Each instrument is designed for monitoring a specific class of substances. Therefore, proper operation and calibration are essential for obtaining accurate information.

There are a number of instruments that are used by occupational hygienists which can give instantaneous readinga of the concentration of chemical contaminants present in the air. These instruments combine both sampling and analytical functions thus permit real-time or near real-time measurements. Commonly used direct-reading instruments include battery-powered gas chromatographs, organic vapour analyzers and infra-red spectrophotometers.

Direct reading instruments for gases and vapours are commonly used:

To detect the presence of contaminants;
For confined space entry;
In emergency response activities;
For 'hot work' where scources of ignition are present.

Lack of specificity, high initial capital outlay and the need for frequent calibration have worked against the use of direct reading instruments.

REFERENCES:

Barwise W. Sampling of Dust, Gases and Fumes. In: Occupational Hygiene. Edited by Schoeman JJ and Schroeder HHE. Published by Juta and Co., 1996.

Todd LA. Evaluation of the Work Environment. In: Encyclopaedia for Occupational Health and Safety. http://www.ilo.org/safework_bookshelf/english/.

General Introduction to Occupational Health: Occupational Hygiene, Epidemiology & Biostatistics by Prof Jonny Myers is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License.