The Hygrotron humidity and temperature sensor from Hygrometrix Inc., Alpine, Calif. (, is smaller than a dime but can determine relative humidity (RH) between 0 and 100% with 1.5% accuracy after being frozen, totally immersed in water, or exposed to high levels of air pollution and diesel fumes. It works by using a piezoresistive strain gage with a built-in temperature sensor. Water in the atmosphere attaches itself to a thin polymer film, creating a strain in the film. The gage measures this strain and derives an RH reading based on it. The MEMS device needs only 1.25 V. It operates in pulsed or dc modes with an average response time of less than 5 sec.