Humidity in bloom

Measuring humidity is a well-entrenched technology. Sensors based on chilled mirrors, which detect dew point by observing the temperature at which the mirror is obscured by condensing water, have been around for many years. But technology marches on. New developments in humidity sensors are based on advanced techniques in micromachining - the manufacture of tiny, near-microscopic devices from silicon. But their inspiration some-times comes from unlikely sources.

For example, one type of micromachined sensor borrows its mechanism from an unlikely source - the seed-pods of geraniums. These are sensitive to moisture, and at certain humidities, they split, roll up and throw the seeds away from the plant.

Hygrometrix, a sensors company based in Alpine, California, harvests the humidity-sensitive material from geranium flowers and incorporates it into its Xeriton range of humidity sensors. According to chairman Ralph Fenner, who developed the technology, the material consists of tiny cellulose crystals to which water molecules can adsorb by hydrogen bonding. This forces the crystals to bend, in proportion to the amount of water it has adsorbed. Measuring the strain in the crystal as it adsorbs water therefore gives a measurement of the amount of water in the air.

Fenner also found that replacing the crystals with a thermosetting polymer with similar properties produced faster, more repeatable responses than the natural crystals. Combining this with micromachined silicon devices produced a new type of sensor, dubbed the Hygrotron, which works by binding the polymer films to a four-beamed strain gauge similar to those found in loadcells. As the polymer films bends under the influence of moisture, the strain gauges detect the forces and translate this into an electrical signal. Hygrotron sensors measure 0-100 per cent relative humidity, with full deflection in five seconds, the company says. Newer developments are taking the moisture sensors into less well-charted territory. The new sensors use porous silicon, a material whose properties include changes in capacitance and conductivity when molecules adsorb on to its surface. Because of this, it can be used in chemical sensors.

Extremely sensitive

In a recent report on analytical sensors and instruments, market analysts Frost & Sullivan explain that porous silicon-based humidity sensors consist of a parallel-plate capacitor with a porous silicon dielectric. The ambient humidity causes water vapour to condense inside the silicon, changing its electrical permittivity, and hence the capacitance of the system. The devices are extremely sensitive, the report says, especially when they are fitted with a membrane structure which both improves the quality of capacitor and allows the system to be heated. This means that readings can be taken at a constant temperature, further improving the accuracy. Sensors like this could soon be on the market, according to Eamon Connolly, who is developing the technology at the Delft University of Technology. 'My guess is between six months and two years, because of the easy availability of silicon processing facilities,' he says.

Connolly is investigating how doping the silicon - inserting atoms with different numbers of free electrons into the material - affects its dielectric behaviour. Currently, he is working with doped porous polysilicon, whose resistance changes very little with temperature. 'If this can be incorporated into humidity sensors, we should be able to reduce the influence of temperature on relative humidity measurements,' he says.

The sensors are likely to find their way into process plants and climate control applications, says Connolly. 'We are developing the sensors to have the possibility of on-board read-out and signal processing electronics, with re-set, self-test and self-calibration facilities,' he says. 'We expect that the requirements of the sensors will be application-specific - so for humidity sensing in a chemical plant, in situations where it might be difficult to reach individual sensors for calibration, it might make more sense to use slightly more expensive sensors with full on-board electronics.'