The Hygrotron humidity and temperature sensor from Hygrometrix Inc., Alpine, Calif. (hygrometrix.com), 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 and 400 to 500 A of current. It operates in pulsed or dc modes with an average response time of less than 5 sec.