Gas Sampling Using Gresham Tubes

Information Sheets

Gresham tubes should not be used for taking gas samples from 19mm diameter boreholes for subsequent laboratory analysis.
The standard Gresham tube, when filled to 14 bar, will contain about 0.75 litre of sample gas.

A borehole of 19mm diameter will contain a volume of gas equal to:

Dry Depth of Borehole	Volume of Gas
1 metre	0.28 litre
2 metres	0.57 litre
3 metres	0.85 litre
4 metres	1.13 litres
5 metres	1.45 litres

Some of the borehole gas would have been consumed when using in-situ gas measuring equipment and also when purging the Gresham tube.

Hence air is bound to be pulled in to the borehole during filling, rendering the subsequent analytical data meaningless.

Corresponding figures for 600mm diameter boreholes are as follows:

Dry Depth of Borehole	Volume of Gas
1 metre	2.83 litres
2 metres	5.66 litres
3 metres	8.48 litres
4 metres	11.31 litres
5 metres	14.14 litres

Dilution by atmospheric air when taking a Gresham sample is hence far less significant.

For more information please contact:

Voelcker Consultants

Telephone: 020 8746 9550
Fax: 020 8746 9560
E-mail: info@voelckerconsultants.co.uk

Pharmaceutical Testing - Voelcker Consultants - Pesticide Residues - Restec - Asbestos Surveys