The Energy Monitoring Company Ltd

Tracer gas tests

Tracer gas tests provide a measure of how the air within a building moves under normal operating conditions. A small quantity of a harmless tracer gas is released into the building, and its concentration at a series of points is charted. In this way the rate of introduction of fresh air can be deduced, or the transfer of contaminants or odours around the building determined. The technique also gives useful information on the proportion of building working heat loss which is due to infiltration and ventilation.

The gas detection system used by EMC consists of a highly sensitive chromatograph which detects Sulphur Hexaflouride, a common choice of tracer gas. This allows us to work at tracer concentrations below 50 parts per billion, minimising the amount of gas which has to be released even in large buildings.

Tracer gas tests are conventionally carried out using one of three methods:

  • decay measurements, in which the gas is released and its subsequent decay charted,

  • constant release measurements, in which gas is released at a constant rate and the resulting concentration used to calculate air movement rates, and

  • constant concentration measurements, in which the concentration of gas is held constant and the air transfer rates deduced from the amount of gas that has to be released to achieve that.

    Recently the Energy Monitoring Company has been involved in the development of a fourth technique, in which gas is released in a carefully randomised sequence. This provides significant advantages over conventional measurements:

  • it can give a measure of the degree of mixing in a single large zone. Poor mixing is a problem which often makes the results of decay rate measurements difficult to interpret,

  • it allows multi-zone tests to be carried out using a single tracer gas. Normally such tests have to use a different gas for each zone, which makes the detection requirements even more onerous. Using the new technique the same gas is released in each zones but with different, mutually uncorrelated relase schedules. The source of gas arriving in any other zone is determined from the statistical structure of its concentration, rather than its chemical structure,

  • it may allow more commonly available gases, for example Carbon Dioxide, to carry out longer term tests. Very low cost detectors are available for this gas, and the technique may provide a way of carrying out tests at much lower cost than with other techniques which require gases to be used which do not occur in the background.

    The basis of the method and some preliminary assessments using numerical modelling have recently been published:

    A new tracer-gas testing technique: Theory and numerical simulations J D Balcomb, C J Martin and J Littler
    Building Services Engineering Research and Technology 1996 Vol 17 Number 3

    At the moment the technique is still under development, but preliminary results look very encouraging.

    In any building which has infiltration the results of a tracer gas test will depend on the climatic conditions at the time of the test. The test thus gives a measure of the working conditions in the building, not a measure of how well sealed it is. For that purpose a leakage test, which gives a result independent of climate, is more appropriate.

    A CASE STUDY: Satisfying COSHH requirements in a laboratory complex

    The advent of COSHH legislation meant that a large research company had to ensure that their staff were not at risk from the combustion products produced in tests at their London laboratory complex. A complex series of over 40 tracer gas tests established the ventilation rate in each laboratory, and the transfer of contaminants between adjacent laboratories. This information allowed the company to be confident that they were complying with their statutory obligations.

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