Handbook of air conditioning and refrigeration / Shan K

2.16 CHAPTER TWO
2.8 HUMIDITY MEASUREMENTS
Mechanical Hygrometers
Electronic Hygrometers
Humidity sensors used in HVAC&R for direct humidity indication or operating controls are separated
into the following categories: mechanical hygrometers and electronic hygrometers.
Mechanical hygrometers operate on the principle that hygroscopic materials expand when they
absorb water vapor or moisture from the ambient air. They contract when they release moisture
to the surrounding air. Such hygroscopic materials include human and animal hairs, plastic polymers
like nylon ribbon, natural fibers, wood, etc. When these materials are linked to mechanical
linkages or electric transducers that sense the change in size and convert it into electric signals, the
results in these devices can be calibrated to yield direct relative-humidity measurements of the
ambient air.
There are three types of electronic hygrometers: Dunmore resistance hygrometers, ion-exchange
resistance hygrometers, and capacitance hygrometers.
Dunmore Resistance Hygrometer. In 1938, Dunmore of the National Bureau of Standards developed
the first lithium chloride resistance electric hygrometer in the United States. This instrument
depends on the change in resistance between two electrodes mounted on a hygroscopic material.
Figure. 2.6a shows a Dunmore resistance sensor. The electrodes could be, e.g., a double-threaded
winding of noble-metal wire mounted on a plastic cylinder coated with hygroscopic material. The
wires can also be in a grid-type arrangement with a thin film of hygroscopic material bridging the
gap between the electrodes.
At a specific temperature, electric resistance decreases with increasing humidity. Because the
response is significantly influenced by temperature, the results are often indicated by a series of
isothermal curves. Relative humidity is generally used as the humidity parameter, for it must
be controlled in the indoor environment. Also the electrical response is more nearly a function of
relative humidity than of the humidity ratio.
The time response to accomplish a 50 percent change in relative humidity varies directly according
to the air velocity flowing over the sensor and also is inversely proportional to the saturated
vapor pressure. If a sensor has a response time of 10 s at 70°F (21°C), it might need a response time
of 100 s at 10°F (�12°C).
Because of the steep variation of resistance corresponding to a change in relative humidity, each
of the Dunmore sensors only covers a certain range of relative-humidity measurements. A set of
several Dunmore sensors is usually needed to measure relative humidity between 1 percent and 100
percent.
A curve for output, in direct-current (dc) volts, versus relative humidity is shown in Fig. 2.6b
for a typical Dunmore sensor. It covers a measuring range of 10 to 80 percent, which is usually
sufficient for the indication of relative humidity for a comfort air conditioned space. This typical
Dunmore sensor has an accuracy of �3 percent when the relative humidity varies between 10 and
60 percent at a temperature of 70°F (21°C, see Fig. 2.6b). Its accuracy reduces to �4 percent when
the relative humidity is in a range between 60 and 80 percent at the same temperature.
In addition to lithium chloride, lithium bromide is sometimes used as the sensor.
Ion-Exchange Resistance Hygrometer. The sensor of a ion-exchange resistance electric hygrometer
is composed of electrodes mounted on a baseplate and a high-polymer resin film, used as a
humidity-sensing material, cross-linking the electrodes as shown in Fig. 2.7a and b. Humidity is