Measuring humidity has been accomplished using dry and wet bulb hygrometers, dew point hygrometers, hair hygrometers and electronic hygrometers. Recent years have brought increasing demands for humidity measurement in the field of electronics, including measuring or controlling instruments. In response to this demand, polymeric humidity-sensing film-type hygrometers, which are easy to use with output in electric signals, are finding ever-increasing application.
Structure and principle of electronic-type polymeric humidity sensors
Polymeric humidity sensors can be broadly divided into two types: an elastic type and an electronic type. The electronic type has two subtypes: a resistance change type and a capacitance type. We will examine the differences of the two electronic types in structure and detection principle as well as their advantages and disadvantages.
Resistance change-type humidity sensors pick up changes in the resistance value of the sensor element in response to changes in humidity, to send them as electric signals. The basic structure is shown in the model below.
A thick film conductor made of precious metals such as gold or ruthenium oxide is printed and calcinated in the shape of a comb to form an electrode. Next, polymeric material is applied to the electrode to make a humidity-sensing film.
The existence of movable ions, moving around freely with water molecules adsorbed, explains how the polymeric film senses humidity. It is believe that it treats changes in impedance that arise from changes in the number of movable ions as electric conduction.
On the other hand, capacitance change-type humidity sensors pick up changes in the capacitance of the sensing element arising from humidity changes as electric signals.
As shown in the model below, a lower electrode of gold, platinum or other material is made on the alumina platform. Next, a polymer layer of several micrometers in thickness, such as PVA, is evenly applied onto the electrode as a polymeric humidity-sensing film. On top of this, an upper electrode is created using thin gold film. This means that the capacitance-type sensor is a condenser with the polymeric film as its dielectrics.
Features of the capacitance change type and the resistance change type
Humidity sensor elements for the capacitance change type so excel in linearity that measurement of 0%RH in relative humidity is possible.
But they have a capacitance that runs into the hundreds of pico-farads at 0%RH, and the variable range of the capacitance at 0 to 100%RH is as low as double-digit pico-farads. To expand the capacitance range and at the same time to cancel out the huge zero offsets, the circuit must be so complex that high cost is involved. It also needs regular calibration.
Humidity sensor elements for the resistance change type have difficulty in measuring low humidity ranges (5%RH and below). With the exponential properties of the variable range of impedance hitting four of five figures, it is difficult to secure the dynamic range of the circuit system. And the change in properties based on temperature is substantial (refer to the characteristic graph below).
Overall superiority of the resistance change type
Nevertheless, improvements in circuit techniques can solve the problems with impedance variation and temperature characteristics in the resistance change type, enabling us to take full advantage of it low cost, ease of use and other features. The question of non-sensitivity in low humidity range can be turned to good practical effect if we abandon low humidity measurement at 5%RH or less, freeing the system from offset adjustment, which in turn eliminates the secular variation arising from offset variation and, therefore, regular corrections as well.
As a result of this study, we have decided to introduce the resistance change type to our humidity sensors. We have developed a unit to make maximum use of the advantages of the resistance change type described above, using our original polymer materials and circuit design techniques. This unit is widely used in consumer appliances, office appliances, measuring instruments and other applications.