The use of Photo Ionisation Detectors or PID’s to delineate hydrocarbon, or solvent, plumes is common practice these days. Since its introduction to the UK in the early 90’s the technique has won common acceptance and many equipment hire companies have offerings from various manufacturers.

Many practitioners recognise the need for achieving adequate calibration of the instrument relative to the material being tracked. The commonly recognised difference is that of the response factors between petrol mixtures and diesel fuels. The later material responds far less strongly even to high energy lamp instruments and the results need to be interpreted accordingly. Most manufacturers publish correction factors that can be applied to the instruments response relative to other solvents such as those historically used as dry cleaning or degreasing fluids.

For general purpose use there may not be a major risk in terms of business loss prevention; provided plume delineation can be achieved, professional judgements can be applied in the normal way in terms of the area of ground affected. However, there is one operational risk that may be problematic, and poorly understood by a lot of users, this is the impact of humidity.

RAE systems, the makers of a commonly used instrument, assert that at 60% humidity only 80% of the true atmospheric response may be achieved. Clearly this is problematic if the instrument is being used to provide safety critical data during drilling, other ground works, or if the solvent/substance being studied is a ‘low responder’.

There are, of course, other factors which need to be taken into consideration in this respect, such as: extension tube length; and the creation of critical alarm set points for mixtures of solvents.

RAE Systems publish a host of free literature which enhances the safety critical and other uses of data obtained from PID instruments in general; as well as their own products. This information can be found at:

http://www.raesystems.com/AppTech_Notes/TN .

Loss Prevention Measures

The following are some loss prevention measure which should be considered:

1. Obtain, and study, the manufacturer’s supplementary product literature.

2. Do not rely entirely on what the hire company has provided; they will only supply what is needed to ensure the equipment operates as intended. The instrument output is for user interpretation.

3. Ensure that members of staff using the equipment have read, and understand, the instrument manual.

4. Ensure that staff members using the equipment are qualified to deal with the information in the instrument manual and supplementary information (this can often be chemical in nature).

5. Where staff are not adequately qualified provide adequate supervision to ensure that safety critical work is properly carried out;

6. Ensure that the calibrations performed, and any correction factors adopted, are relevant, and adequate, for the task being performed; and the end use of the data.

7. Ensure that the mission critical nature of the results, and the potential for inaccuracies, have been recognised in your safety risk assessments.

8. Contact the AGS Loss Prevention – Chemical Safety Hot Line for further information, if required.

TC White Marquis & Lord