Even the most accurate landfill gas and biogas analysers may need field calibration bet-ween manufacturer service and calibration. The reasons for this include user need for the most accurate results possible; complying with site operator in-house requirements; and for audit, inspection and consultant reports.
For this, it is essential field calibration is done correctly, especially as incorrect calibration may decrease gas analyser accuracy. Even if accurate, the validity of a set of readings can be enhanced with pre-analysis field calibration. The most commonly user-calibrated landfill gases are methane (CH4), carbon dioxide (CO2) and oxygen (O2).
The two simple, but key, steps are:
- zero – at this point the gas analyser is calibrated when there is none of the target gas present. For CH4, CO2, and hydrogen sulphide (H2S), operators can use clean ambient air to purge the unit and use the zero function to set the unit to zero. For CH4, operators should do this at the start of a series of readings – well away from sampling areas to avoid low-level gas contamination. For O2, a calibration bottle is required containing no O2
- span – next the gas analyser is to be calibrated with a known quantity of the target gas present. For best analyser accuracy, users should select calibration gases of similar concentrations to those of the landfill gases expected at the site. Once the correct gas has been selected, the analyser should be checked to see that it is correctly reading the known level. If it is, the analyser requires no calibration. If not, users should set the analyser to display the known level of the calibration gas.
There are variations of this for different applications, but the principles and steps remain the same. While the advice is usually for portable gas analysers, the same principles apply to static or fixed-gas analysers where the steps are usually automated.
To meet anticipated landfill requirements, a gas analyser unit will need not only a secure audit-trailed data log of sampling point readings, but also of field calibrations.
Today’s gas analysers have to meet these criteria, as must some fixed applications.
This is especially true for clean development mechanism carbon credits. It is also on the cards for water treatment sludge waste to meet integrated pollution prevention and control (IPPC) regulations and Environment Agency compliance where continuous analysis, monitoring and data logging with audit trail and five years’ data storage may be required. In these applications, automatic calibrations are essential for valid data.
While not all users carry out field calibration of portable units, those who do should use manufacturer-supplied calibration gas bottles fitted with factory-set regulators. Users should look to manufacturers for on-site training, online manuals and specifications, consumables ordering and down-the-line phone support. Most important of all, the oldest advice around always applies – check the instruction manual.
Geotechnical Instruments www.geotech.co.ukvenues