The dewpoint temperature signifies the temperature at which water condensation occurs when air cools to the dewpoint temperature. The dewpoint temperature in degree Centigrade is, related to the absolute humidity of the air, usually lower than the air temperature. Only at a relative air humidity of 100 % are both temperature values identical. For this reason, the gap between dewpoint temperature and air temperature, also known as dewpoint temperature gap, is of the highest significance as an early warning of the formation of condensate water. The absolute air humidity or true water vapour content of air is independent of temperature. For all production sequences where water precipitation from air has to be kept under observation, it is only this dewpoint temperature which is decisive. Before, dewpoint measurement could be effectively used for dryer control systems, a long and arduous path had to be trodden before reaching the present state of the art. There have been sensors for measuring air humidity for about 400 years. The value of relative humidity (RH), which they measured, was considered unproblematic for a long time, as measurements of highest accuracy were, required to a limited extent only. The mere concept of relative humidity already implies the dependence of this parameter on a second basic value subject to strong fluctuations, the air temperature. Dewpoint was previously established by means of a simple air humidity measuring instrument such as the air hygrometer. However, this instrument required continuous servicing and regeneration as, otherwise, reading errors of up to 20% became possible. Because of its strong dependence on temperature, the value indicated was valid only for the current temperature at the location of measurement. An air hygrometer, for instance, gives different readings depending on whether it is situated close to a source of heat or mounted on a cool wall, although the absolute air humidity in the room concerned is the same. It makes no physical sense in many cases to measure the relative air humidity as, from the point of view of process technology, the true water vapour content of the air, independent of the temperature at the time, has to be monitored.
With mounting quality requirements of industry, the RH measurements were increasingly seen as an inaccurate measurement method, associated with cumbersome installation and maintenance efforts. On top of this, there was the difficult re-calibration of the sensors which had to be carried out frequently. The development of the Lithium Chlorite (LiCL) measuring element paved the way for a more accurate determination of the true water vapour content of air and other gases. The measurement principle consists of warming a hygroscopic lithium solution on the measurement element for as long as, and until the heat exchange between the LiCl solution and the air surrounding the measuring element is balanced, so that the partial water pressure above the LiCl solution is also identical with that of the surrounding air. The temperature of the LiCl solution, which is reached upon achievement of this state of equilibrium, represents a direct measure of the absolute air humidity.
Figure 8.2.1
The LiCl measuring element (Fig. 8.1.1) consists of a thin glass tube (item 4) over which a specially prepared glass wool hose (item 3) has been slipped after being soaked in a special LiCl solution. Two precious metal wires (item 2), electrically insulated from each other, are wound around the glass wool hose. An alternating current of 24 Volt is applied to the terminals at the free ends, and this makes a current flow through the LiCl solution. The heat generated by the current evaporates the moisture and crystals are formed, so that conductivity and thus also the current, are reduced. The LiCl solution resumes picking up moisture from the air, renewed warming up takes place until a state of equilibrium is reached at a specific temperature. The temperature measured by means of a thermometer (item 1) determines the value of the absolute humidity. Neither the air hygrometer nor the LiCl measuring element provided a technically practicable solution which could have found utilisation for controlling adsorption dryers. Further endeavours were made to find a universal and accurately functioning humidity measuring system valid under harsh operating conditions. Engineers were looking for a suitable dewpoint sensor.
The decisive step forward occurred in 1968, when David Chleck an American Engineer applied for a world wide patent covering an aluminium oxide moisture sensor, gold coated by vapour deposition. For the first time, this measurement sensor, provided absolute measurement characteristics and eliminated the interfering temperature and hysteresis effects. Seen in general terms, humidity measurement is more difficult to carry out than the measurement of temperature, pressure, length or weight. Many interference factors are capable of influencing the exchange mechanism between the substances to be dried (compressed air) and the surface of a solid (sensor surface). The sensor technology known today has been tested and improved in the course of development. The characteristics of the aluminium oxide moisture sensors satisfy the extreme demands of industrial application. They resist harmful materials or pollution through dust, are insensitive towards flow, condensation, vibration or temperature shocks, and are stable for a long time. All this has contributed to the fact that process parameter humidity can now be measured accurately and without too much trouble as a universal quality indicator. For compressed air drying, humidity measuring technology will assume an ever more important role.
