Hygrometers electrical

Electric hygrometers measure the electrical resistance of a film of moisture-absorbing materials exposed to the gas. A wide variety of sensing elements have been used. 

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. 

Smith, PR. 1971. The determination of equilibrium relative humidity or water activity in foods A literature review. The British Food Manufacturing Industries Research Association, U.K. Stekelenburg, F.K. and Labots, H. 1991. Measurement of water activity with an electric hygrometer. Ini. J. Food Sci. Technol. 26 111-116. Stoloff, L. 1978. Calibration of water activity measuring instruments and devices Collaborative study. J. AOAC 61 1166-1178. 

Electrolytic (coukxnetric) hygrometers The quantity of electricity required to carry out a chemical reaction is measured. The principle is based upon Faraday s law of electrolysis. Water is absorbed on to a thin film of dessicant (e.g. P2O5) and electrolysed. The current required for the electrolysis varies according to the amount of water vapour absorbed. The current depends also upon the flowrate. Capable of high precision. Used in the range 1000 to 3000 ppm of water by volume. Somewhat complicated procedure. Recombination of products to water is necessary after electrolysis. Density, pressure and flowrates have to be maintained precisely. Contamination can poison the cell. It is ideal for binary mixtures but is of limited range. Suitable for on-line operation. 

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)... 

Hashimoto-Kumasaka, K., Takahashi, K., and Tagami, H. Electrical measurements of the water content of the stratum comeum in vivo and in vitro under various conditions comparison between the skin surface hygrometer and Comeometer in evaluation of the skin surface hydration state. Acfa Derm. Venereol. (Stockh). 1993 73 335. 

The impedance hygrometer measures the water content of a sample by means of a probe whose electrical impedance is a function of the vapor pressure of moisture in the fluid. The probe consists of an aluminum strip that is anodized to form a porous layer of aluminum oxide. A thin coat of gold is applied over the aluminum oxide. Water vapor penetrates the gold layer and equilibrates on the aluminum oxide. Leads from the gold and aluminum electrodes of the probes connect the sensing element to the measuring circuitry. The moisture content of solids can also be measured indirectly by detecting the moisture in the atmosphere above or near the process solids because the atmosphere near the solids is in equilibrium with the moisture content of the process materials. 

Many standard lab activities use simple measuring devices such as a thermometer, a clock (or wristwatch, chronometer or stopwatch), a ruler (or micrometer or tape measure), or an electrical meter (e.g. voltmeter, ammeter). Humidity is measured with a hygrometer. 

Figure 31.13 Experimental rotating jet spouted bed with central and peripheral air jets 1—air distributor, 2—screen, 3—distributor cover, 4—ball bearings, 5—v-belt, 6—pulley, 7—drive, 8—motor controller, 9—glass vessel, 10—bir blower, 11— electric heater, 12—PID controller, 13—Pitot tube, 14—gate valve, 15—pressure transducers, 16—pressure taps, 17—thermocouples, 18—multiplexer, 19—IR hygrometer, 20—dew-point sensor, 21—dew-point hygrometer, 22—humidity meter, 23—computer, 24—terminal panel, 25—filter. (From Jumah, 1995.)...

Electric hygrometer - utilizing a substrate, such as a thin film of polymer or aluminum oxide. Water from a sample is absorbed by the film that causes changes in the electrical properties of the film, such as capacitance or resistance. The change of the electrical quantity is related to the amount of water vapor contained in the gas sample and the amount of water is calculated by use of a calibration curve derived from standards. 

Dunmore, F. W. 1939. An Improved Electric Hygrometer, Research Paper RP1265, Journal of Research of the National Bureau of Standards, 23 701-714. 

John Frederic Daniell (1790-1845). English chemist and meteorologist. In addition to invention of the Daniell cell, an improvement over Volta s electric pile, Daniell made contributions to instrumental meteorology through his invention of a dew-point hygrometer that became a standard instrument for measuring relative humidity. 

Electrical hygrometers typically measure the electrical resistance of a substance, such as hthium chloride or of a semiconductor device which varies with the humidity. The change in resistance is indicated by the calibrated instrument. The response of such an instrument can be slow because the previously absorbed moisture should be completely replaced by the flesh incoming moist air. Also, regeneration of the saturated material may become necessary for fresh readings. 

Donovan discovered in mountain-ash berries what he called sorbic (malic) acid, which he found in rhubarb stem he afterwards admitted the identity of sorbic and malic acids. He described the preparation of pure potash from carbonate and slaked lime, investigated whether alcohol is a product of fermentation or distillation, preparation of pure silver and of phosphorus, and denied the identity of ordinary (frictional) electricity, voltaic electricity, magneto-electricity, and thermo-electricity. He wrote on alchemical and chemical physicians and the use of hyoscyamus — both in titles only, and on smelting lead, as well as on pharmaceutical and medical subjects in the Dublin Journal of Medicine and Chemistry (1839-46). Donovan invented a gas lamp, a gas table-lamp, a hygrometer, and an apparatus for filtration out of contact with air. Donovan s solution is a i per cent solution of equal weights of arsenious and mercuric iodides used in the treatment of skin diseases. 

The main types of instruments measuring humidity in gases are various types of hygrometers (gravimetric, mechanical, condensation, infrared absorbance detector, electric sensor, thermal conductivity, Al203/sili-con, P2O5) and psychrometers. 

Corrosion tests were performed in the CORALLINE facility, a specific device equipped with two test sections at ambient pressure. The exposures ran at 750°C and 950°C for up to about 800 h. Each test section is composed of a horizontal quartz tube heated by an electrical furnace and can house only four specimens at the same time, in order to avoid interference between coupons. Furthermore, in order to limit the depletion of the impurities, each specimen was placed in its own gas flow line. The impure helium is supplied from pre-mixed cylinders. At the test-section inlet and outlet, a mirror-type hygrometer monitors the helium moisture and a gas chromatograph (He-ionisation detector) analyses the permanent gases. We verified that no significant depletion of the impurities occurred during the tests. Table 26.3 summarises the experimental conditions. 

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