Topics in..
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Chemical Instrumentation Edited by
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S. Z. LEWIN, New York University, New York 3, N . Y
T t w e articles, most of whzch ore to be conlributed by guest authors, are zntended lo serve the readers of this JOURNAL bv calling altentwn lo new developments i n the theory, design, or availability of chen~icallaboratory instrumenlation, or b y presenting nsqful insights and ezplanations of topics that are o j practical importance lo those who ?tap, or teach the use of, modern instrumentation and instrumental techniques.
II. Temperature Measurement S. T. Zenchelsky, School of Chemistry, Rufgers University a ronscquence of the temperat,ure-irtdnced In principle, temperature may hc pressure changes within the t n h ~ . This uisilsured by measuring any property oi :I system which is tcn~~eratnre-dependent, is the Hourdon-gauge principle. provided that tho relationship hetween Electrical S e n s i n g Elements the two is known. Some properties which have heen employed in special Elrctricd sensing elements commonly r:ascs include the fallowing: dielectric used are tho thermocauplr and the reeoustant, magnetic susceptibility, piezosistance thermometer. The iormer ronmleetricity, viscosity, and reaction velocity. sists of a pair of junctions hetxeen two Those temperature-sensing elements which dissimilar metals, where an emf is genhnvr heen most commonly used may be prated when a temperature differential ~ l ~ ~ m i f iine done of t,he iollowing caterxists hetween the junctions (Seeheck xories: (1) mechxniral, ( 8 ) rlertriaal, rffect). This voltage is small, and does : ~ n d(3) optiezl. The first depends on not vary linearly with temperature. physical displacements which result from I t is usually measured by means of a temperature chsnges: the second uses a sensitive millivoltmeter or potentiometer, variation in soma ekrtrical property and the temperature is read from con(resistance, olertramot,ive force) with version tables. For coarse measurement, temperature; while t h r t,hird employs a temperature may be read directly on a change in the spset,rd distribution of m&er with an empirically graduated dial cmitted radiation v i t h temperature. O p of non-uniform spacing. Thc resistance t i e d methods are tho least frequently thermomet,er is generally a metal with wad; they are generally restricted to the ~nrwsurementof very high temperatures or t,o measurement in inarrensihle locat,ionli.
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5. 1. Zenchelrky i. Associate Profewor of Chemistry at Rvtgert Univerrity. He war educoted at New York University 1B.S.. 1941; M.S., 1947; Ph.D., 19521. He teoches and does research in onolyticoi and physical chemistry.
high temperature coefficient of resistanee (nickel, tungsten, platinum) or, more recently, a semiconductor (thermistor) with R temperature coefficient, of resistance about one order of magnitude greater. A comparison of resist ance thermometer ehamcteristics is shown in Figure 1. Empirical equations for the variation of resistance wit,h temperature are indicated for the bhermistor and the
M e c h a n i c a l S e n s i n g Elements Mechanical sensing elements are t h r ~ u s familiar t type encountered in t h r laboratory. Thus the himetdlic strip is found in many t,hermostntic switches, especially those used for drying-oven 1,wnperature control. I t s usefulness stems from the fact that t,he strip, which consists of two different metals bonded together, bends because of bhe unequal thcrmal expansion of the met,als; and this displacement is used t,o make or Ixeak m deetrical cinwit. The samr strip, of greater length and hence increased sensitivit,y, usually coiled into :L spiral, is used as the means for displaeing a pointer in t,he d i d thermometer. T h r fluid-filled type of inerhanical sensing &,vice is very well known as the ordinary merrury-in-glaas thennometer; hut other fluids, including gases, are employed, 11sua1Iyin a spiral wound tube. In such cases the mrclrxnieal displacement resalts from the unwinding of t h spiral ~ as
I
%
PLATINUM
t
TEMPERATURE ? 190
("C.) 2po
Figure I . Comparison of metal and semiconductor choracterirticr. The resistance, Rr, at temperature, T, is given in terms of the resistmce, Ro, mearvred at reference temperature. Tn. The conrtonts. A and B, are empiricdly evolumted.
Volume 39, Number 9, September 1962
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Chemical lnstrumentation platinum resistance thermomrter, respectively, I t is clearly seen that thc coefficient is positive for platinum (and other metds, as well) while i t is negative for the thermistor (and semironductors, in general). I n either case the resistance is measurcd hy conventional methods to bo described below. Until comparatively recently, the mechanical devices wcrr the only ones commercially available for general laboratory use, with the electrical sensing elemonts hdng reserved for special research applications or restricted to temperatures beyond the range of the mechanical type. Tho reason for this was the lack of self-contained instruments (independent of a vihration sensitive gdvsnometer) which would he direct reading in t,emperature, as was the case for tho mechanical type. Tho eleet,rieal deviem, on the other hand, have the following advantages: ( 1 ) the sensing dements can he small, hence possess lowor heat capacity and iaster response; (8)they lend thcmselves rcxdily to remote indicnt,ion and aut,omatie recording; (3) they have g r r ~ t e range; r (4) they may he used for differential temperature measurements; and (6) they permit the use of proportional control without tho intervent,ion of displacement transducers.
Resislance Thermometers Currently available laboratory instruments for temperat,ure measurement make
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Journal o f Chemical Education
RESISTANCE
8 . 2
OHMMETER TYPE
BRIDGE TYPE NULL
DEFLECTION
Figure 2.
R3.
Temperature-rendng rerirtonce, 1
Resistors Itemperoture-insensitive). R,
Ri,
R?,
Galvanometer, G.
usc of the resistance thcrmornrtc,r prinriple, employing either u mctnl winding or a themmistor as the srnsing clommt. Whilc i t is true that srlf-cont,ained thcnnocouple potentiometw~srr nvsil:~hla, they are unsatisfaet,ory, either 1,eraosn t,hey are not direct reading in t,emperatnr~or hecause bhe.y lark the rcrluireil sensitivity for gcnernl laborntory mrasurements. The thermistor is nhout t,en times as sensitivc as the platinum rrsistnnee
thermometrr (for cqnal resistanw n l room trmprmturr), and is pl~pically much sinnllor ( h m w possesses I o w r heat eaprrrity and faster respanw). On thc ot,hcr h:md, the metals (csprrially nahlo metals) are usnhlc avrr n murh wider temperatwe range 511d ~ O S S ~ ~ S B more stable rhnr;trtrrist,irs. I n fs
