Gas analysers are far from fit-and-forget devices and require regular calibration and service to ensure they continue producing accurate results. This applies to all common gas analyser types, of which there are several: nitrogen oxide (NOx) analysers measure chemiluminescence using light created by mixing ozone with NOx; FID (flame ionisation detection) analysers use a flame to measure ions within hydrocarbons; paramagnetic analysers measure oxygen (O2) by placing the measured gas in a magnetic field; and NDIR (non-dispersive infrared) analysers use infrared technology to measure absorbing gases such as carbon monoxide (CO) and carbon dioxide (CO2). Applications are wide and varied, and include the testing of VOCs, vehicle/engine emissions, impurities in special gases, power station emissions and boiler/burner combustion emissions.
According to Roy Kinslow, service manager at gas analyser manufacturer Signal Group, much of the requirement for calibration is due to compliance. He says: “The regulator of a coal-fired or gas-fired power station, for example, will insist that the company has to measure levels of certain gases, mainly NOx, CO, CO2, O2 and sulphur dioxide [SO2],” he says. “The period of calibration varies in line with usage levels. NOx analysers in the automotive industry, for instance, will sometimes require monthly calibration.”
Among the consequences of failing to undertake regular calibration is false reporting. “A power station is supposed to check the drift of its analyser’s calibration over time,” explains Kinslow. “Doing so allows them to apply compensation factors to their readings accordingly. If you don’t check, you could be recording rubbish, basically. That’s why we recommend daily calibration checks by plant personnel. After all, if a fault occurs with the analyser you could have a whole 24-hour period of incorrect gas readings.”
DAILY CHECKS
Gas analyser users undertake daily calibration using a gas bottle containing a certified quantity of gas. They put the gas into the analyser and check the results accordingly.
“In simple terms there are two datum points: 0 and 100ppm, for example, so the daily check is calibrating the analyser to those two points, just like you would a temperature gauge or voltmeter,” says Kinslow. “However, you don’t know if it’s reading correctly between those points, which is why we come in annually or biannually and perform a series of other checks, such as interference [with other gases], noise and linearity tests.”
Linearity tests require the input of gas in different concentrations, allowing Signal Group service engineers to move up and down the scale of the analyser, checking it reads correctly. Calibration usually takes place at the same time as the service and could take a couple of days to complete. This activity also serves to confirm that Signal Group has completed the service correctly. As a point of note, the regulatory authority normally permits a certain amount of analyser downtime for maintenance. A small number of sites, where gas analysis is highly critical, will have a back-up system.
WHO TO USE?
“Of course, the option exists to use third-party service providers rather than analyser OEMs for calibration, but if there’s an issue they won’t know how to put it right, which means the equipment is out of action,” states Kinslow, who adds that Signal Group trains customer personnel in basic maintenance.
“Alongside daily calibration checks, they can look after diaphragms, seals, probes and filters, for example,” he says. “Filter checks might take place weekly or fortnightly. Trained customer personnel can also perform a number of important visual checks: are the gas bottles are full; are the gas services connected correctly and leak-tight; is there power; are the analysers functioning?”
“Some sites only need one analyser,” says Kinslow. “For instance, if someone has complained about the smell from a company producing wallpaper, the local authority might insist on a gas analyser to measure the hydrocarbon levels against a pre-determined emissions limit. The analyser will run 24/7 and we’ll go in every six or 12 months to perform service and calibration.”
KEEP ON RUNNING
Signal Group has some gas analyser contracts that have been running for 25 years or more. “As long as we have access to third-party components, such as CPU chips, we can keep our gas analysers going for many years,” says Kinslow. “The only time we lose a contract is if the business ceases trading or no longer has a requirement for that particular measurement.”
He concludes: “We find that reliability drives repeat business. An unreliable gas analyser only leads to trouble with the local authorities or the Environment Agency. If the authority has any questions about the integrity of the results, it could lead to a fine or even a plant shutdown situation. For this reason, companies must be proactive and ensure regular calibration of their analysers so they can provide the necessary reports - containing the correct readings.”
BOX: WHY CALIBRATE FLUE GAS ANALYSERS?
Flue gas analysers (FGAs) are vital to test the safety, performance and emissions of any fossil fuel boiler, fire, cooker or heating system. Thousands of heating engineers in the UK uses these devices on a daily basis. To meet strict compliance rules and keep customers safe, it is necessary to calibrate and recertify FGAs annually.
FGAs have moving parts and sensors that wear over time. Checking, repairing or replacing is required to stay within the performance requirements of BS7967 and TB143. Failure means that the FGA may give incorrect readings, causing unsafe situations for heating engineers and customers. The work will also fail inspection and assessment, risking disqualification or worse.
FGA manufacturer Kane has service centres in Atherton, Greater Manchester, and Welwyn Garden City, Hertfordshire. When it receives an FGA from a customer, after sending a quotation, engineers perform tests to confirm what to repair or replace, including sensors, probe, charger, carry bag and printer – any items causing concern – along with software and hardware upgrades. Once complete, Kane calibrates and recertifies the instrument to national standards for the gases, temperatures and pressures that the FGA measures.
The company then performs a final check and clean, replacing batteries with new rechargeable batteries, before returning the device. Kane FGAs show customers when calibration and recertification is due every time they switch on their FGA, on their FGA print-outs and calibration certificate.
Meanwhile, Northern Ireland-based Hamilton Gas Products, which calibrates Testo and Anton FGAs, says heating engineers should seriously consider an annual service and calibration for their analyser because it contains delicate sensors that require replacement at regular intervals for the instrument to work correctly. Any report that heating engineers create with an analyser which does not have an up to date calibration will also be invalid and expose them to all sorts of risks, the company says.