Hygrometers dewpoint

The dewpoint hygrometer detects the dewpoint temperature of air by cooling a surface in contact with the air to the dewpoint temperature. There are several ways to achieve cooling and to observe the formation of condensate on the surface. The early dewpoint hygirometers were cooled simply by applying the vaporization of ether or some other suitable liquid. Condensate formation on the surface was determined visually. Other cooling methods are to use a refrigerant flow in direct or indirect contact with the back of the surface, or to use electricity with a (thermoelectric) Peltier element. 

The dewpoint hygrometer is claimed to be the most accurate instrument tor measuring air humidity. When properly calibrated, the inaccuracy can be 0.5% RH.- On the whole, the dewpoint hygrometer is a reliable lundamen-tal instrument suitable for many ventilation applications, but is more expen sive than other humidity instruments. 

If the one-point calibration in ambient air is not sufficient, the next best approach is to use the calibration box method.- The air state is created in a closed box made of nonhygroscopic material, like metal or plastic. A controlled state of humidity is maintained by exposing the air in the box to a liquid surface of a saturated salt solution. In practice, a dish containing the saturated water solution of a salt is placed on supports at the bottom of the box. The air in the box is circulated by means of a small fan. The box should be airtight and positioned in a constant temperature environment. The calibrated instruments are placed in the box. A dewpoint hygrometer can be used as a reference. A wide range of humidity can be created by using solutions of different salts. Table 12.5 shows a few examples of equilibrium humidities achieved with different salt solutions. 

Piezoelectric hygrometers (Fig. 6.S6) These consist of a quartz crystal with a hygroscopic coating (Fig. 6.S60 and Section 6.3.3). Two crystals are usually employed and the wet gas (sample) and dry gas (reference) are passed over them alternately (normally every 30s—Fig. 6.566). The crystals absorb and desorb. The difference in angular frequency Am is proportional to the concentration of water vapour by volume. Capable of high precision. Can monitor moisture contents of the order of 1 to 3000 ppm by volume. Usually measures up to dewpoint of 310 K. Fast response but expensive. Sample must be dean to avoid contamination of the crystals. Complex sampling system. Suitable for on-line use. 

Panametrics hygrometer/thermistor probe No. M221RT for use with No. 2830 sample cells and model no. 2100-151 (F> 161 (B) multi-channel hygrometer (-60 to +60°C dewpoint and -30 to +70°C temperature range, electrical accuracy 1% of input, readability 0.5% of full scale)... 

Dewpoints can be measured manually with simple dewpoint apparatus but the use of an in-line dewpoint meter guarantees continuous monitoring of the moisture levels in air. The recommended instrument for in-line measurements is the Shaw hygrometer type HD. 

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