A relative humidity sensor.

A new relative humidity sensor

Whether in the textile or semiconductor industry, uncontrolled humidity can dampen productivity. Unfortunately, many commercial humidity sensors--mainly polymers coated with sulfonic acid--suffer from hysteresis, in which the polymer becomes so saturated with water at high humidity levels that it is unable to perform well at lower ones.

Peter H. Huang at the National Bureau of Standards in Gaithersburg, Md., has received a patent for a humidity sensor that significantly reduces this problem. It should also last longer and operate at higher temperatures and humidities than conventional sensors. His device consists of a halogenated polymer (such as Teflon) coated by a mixture of a strong and a weak acid, typically sulfonic and carboxylic acid.

Because the strong sulfonic acid has a greater affinity for water molecules than does carboxylic acid, Huang can control the amount of water absorbed and released by the polymer-- ensuring that it doesn't get saturated--by fixing the ratio between the two acids. For example, relatively more carboxylic acid would be used for high-humidity applications and relatively less for detecting minute changes in low humidity levels. Huang says relative humidity is proportional to the polymer's water content, which can be measured electrically, optically or acoustically or by weight.