Topics in..
.
Chemical Instrumentation Edited by GALEN W. EWING, Seton Hall University, So. O r a n g e , N.
feature
J. 07079
These articles, most of which are invited contributions by guest authors, are i n t a d e d to serve the readers of THIS JOURNAL by calling attention to new deuelopments in the theory, design, or availability of chemical laboratory instrumentation, or by presenting useful insights and explanations of topics that are of practical importance to those who use, or teach the use oj, modern instrumentation and instrumental techniques.
port also discusses the relat,ive merits of Japanese and American bathrooms as constant-humidity chambers.) Another classical method for atmospheric moisture is the psychrometer, consisting of paired wet- and dry-bulb
XXXVII. Moisture Measurement The determinat,ian of moisture is of such great importance in so many fields, that i t is not surprising to find ra wide variety of tools for t,he purpose (5, 11, 14, 19). I have found more than 25 distinct andytical methods, and more than 25 manufacturers. I will make no attempt to include all manufacturers in this review.
MOISTURE IN GASES The quantity of water vapor in a gas can be reported as partial pressure, as vapor densitg, as reldive humiditu, a? specific humidity, as mole ,fraction, or in terms of the dewpoint. Partial pressure can be specified in m y pressure units, as bow, psi, or millibars; vapor density may be quoted in terms of grams of water per liter of gas. Relative humidity is t,heratio of the partial pressure of water to the pressure which would exist in equilibrium wibh liquid water a t the same temperature. Specific humidity is the weight of water vapor per unit weight of dry gas. T h e dewpoint is the tempemture a t which moisture starts to condense on a surface t h a t is being cooled; this can perhaps better hestrtted as the temperature of a plane water surface which would be in equilibt.iuh with the water vapor pmsent in the gas. Hnndbook tables give dntn lor lhc intet.conversion of dewpoint and relalive humidity in air.
Figure 1 . chrometer.
Atkinr
Portable
Thermistor
Ply-
Classical M e t h o d s T h e conventional methodof determining the wst,er vapor content of a gas is gravimetric, via absorptiun in a preweighed container of granular magnesium perchlorate or other desiccant. This procedure has long h e m m essential feature of combustiun analysis, though recently gas-chromatographic techniques have been upp planting gravimetric ahsarption. Environmental humidity measurements, can be made with a hygrometer depending
Figure 2. Cambridge Systems Model 992 Thermoelectric Dewpoint Hygrometer.
on the reversible elongation of a human hair with increased humidity (10). A strand of heir (or many strands in parallel) is mounted under slight tension in such a way that a change in length willproduce motion of an indicator needle over a scale. This instrument is direct-reading in relative humidity, butnotvery necurate.* Even less pvecise humidity indications depend upon the change of color of cobalt. chloride; this seems especially to fire the imnginstion of ingenious advertisers, but, can be a useful laboratory indicator as well. A semi-quantitative prncedure has heen published (80)for the quick determinatiou of water vapor in oxygen intended for breathing in high altitude sviatirin, which makes use of the CoCL color. (This re-
* A Honeywell bulletin mentions that, IhVinci, in the fifteenth century, placed a ball of wool on one end of a. balance arm; t,he wool changed its weight with the humidity, giving an indicntian on the balance.
temperahlre and of the dillereme between the two indications. Many models, suited for various purposes, are available through supply houses. An electronic version of the psychrnmeter, utilizing wet- and dry-thermistors, is made by Atkins (Fig. 1);a meter reads tempemture directly first from one, then the other, thermistor. I t gives relative humidity within O.5yOof the true value over most of its range. Dewpoint
.
"
,
solid 60%, or even liquidVnitrogen, until condensation is visible. This occurs a t s. sharply defined temperature, which is the dewpoint or frostpoint. A modern photoelect,ric example is the dewpoint hygrometer of Cambridge Systems (Figs. 2 and 3). A lamp and two photocells are so oriented that speculsrly reflected light will strike one of the cells, while only scattered light will activate the second. This is a. particularly favorable arrangement, because deposits of abvorbing foreign m a t h which build u p slowly an the mirror will diminish t,he direct reflection, but not produce appreciable scattering. As a m d t , cleaning of the mirror is only infrequently required. T h e mirror consists of a thin sheet of stainless steel in contact with a silver block which acts as a heat transfer medium between the mirror and a thermoelectric (Pelt,ier) cooling unit. A thermocouple or therrnislor, imbedded in the silver block, measures the dewpoint,. Another dewp.int sensor, by Vip-Air, ut,ilises the electrical anduetivity of the condensed moisture rather than its optical properties. T h e sensor consisl,~ of a double grid of noble metal, flush with a smoot,h insulating surface of glass cloth impregnated with an epoxy resin. The unit is cooled t,hermoelectrically and its t e m p e r a t u r ~abscrved by a small thermocouple. An automatic regulating circuit alluws the surface conductance to contml the a~oling,so that the temperature adjusts itself to the dewpoint., which can be recorded continuously on a strip-chart recorder. Panrtmetrics offers a dewpoint hygromet,er (Fig. 4 ) which depends far its action on the change of impedance of a. porous (Continued on page A378)
Volume 45, Number 5, M a y 1968
/
A377
Chemical Instrumentation layel. uf aluminum oxide wit,h moisture content (11). The senan. is made by anodizing an aluminum strip, then coating the anodized sul.inee with a very thin pe:~meaI~lefilmof gold. The resultingsandwich nmonnt.; to RLI eleclrdylic capnei1,or with varying diclectrie constant, which a d s as n parallel combination of n re& t a m e and capacitance, both of which vary with mois1u1.e. The impedance changes from s e v e r ~ lmegohms in dry air to perhaps 200 ohms a t high humidity. I t ir
~i~~~~4. panametricr grometer.Made1 1000.
~l~minum oxide ~ y -
PRINCIPLE OF OPERATION
I Power Supply
Thermoelectric Cooler Figure 3.
A378
/
Journal of Chemical Educofion
I
-
Data
OU
