Humidity measurement for engine testing and development

The measurement of humidity plays an important role in measuring the efficiency, power output and emissions of internal combustion engines. As an environmental requirement in the design and production of any combustion engine, including petrol and diesel motor vehicles, trucks, diesel locomotive engines, aircraft engines and gas turbines, a requirement exists to comply with the Clean Air Act. Testing is regulated by the Environmental Protection Agency (EPA) and the Californian Air Resources Board (CARB) in the USA and by various EC legislative standards in Europe.

Typical emissions which are monitored and regulated by various government agencies include NO, NO2, N2O4, SO2, SO3, CO, CO2, Pb and residual hydrocarbons. The quality and relative concentrations of these emissions are affected by various factors, including ambient temperature, pressure, humidity and the type of engine, fuel and conditions of use. It follows therefore that in order to minimise variability of test results all influencing factors need to be controlled or measured wherever possible.

Studies carried out in the USA by the EPA (C D Paulsell, Moisture and Humidity, 1985: pp 735-744, Instrument Society of America) illustrate that the most successful method of determining the ambient humidity for the correction of the emissions concentrations is to use a cooled mirror dewpoint hygrometer. Up until that time it was common for ambient humidity to be measured with a psychrometer. Paulsell's research showed that such instruments are extremely prone to errors in measurement due to a wide variety of factors and, even if used under carefully controlled conditions, have no significant advantages over the use of a cooled mirror hygrometer. The user should also be aware of the dangers of using capacitive type % rh probes that can be prone to drift and large measurement errors in such applications.

Michell Instrument's Optidew Dewpointmeter is the ideal product for this application. The Optidew offers accurate and continuous measurement (within as little as 0.15 °C dew point) of the ambient dew point in an engine test cell and perhaps more importantly, the Optidew range is capable of extended service with full confidence and without measurement drift. Every instrument is delivered fully calibrated and traceable to international standards (NPL, UK and NIST, USA) through Michell's UKAS accredited Humidity Calibration Laboratory. This satisfies the most stringent of quality system requirements and gives ultimate confidence in the measurement made. The Optidew has a measurement range to -40 °C dew point at normal room ambient temperature - more than enough to cope with any ambient humidity conditions.