The Filtrometer(tm) air filter monitor fromPrecision Air Technology(Kirkland, WA) measures both constant and variable airflows. Features include a local LED, a dry contact relay for remote monitoring, operational temperature ranges for the control unit from 32 degrees F to 150 degrees F, and operational temperature ranges for the sensing probe from -40 degrees to 150 degrees. The units are fully compatible with standard control systems and they operate on 24 vac or vdc.