An Inexpensive Thermistor Thermometer for Beginning Chemistry Laboratories Shyam B. Srivastava and Clifton E. Meloan Kansas State University, Manhattan, KS 66506
Heats-of-reaction measurements in beginning chemistry classes require temperature measurements to O.l°C over a short temperature range, usually only 10-15°C. Experience with glass hulh thermometers has not been satisfactory. Many get broken, particularly if magnetic stirring bars are used, and then the replacement costs become excessive. The introduction of a thermistor thermometer offers an opportunity to have students become familiar with new principles and techniques, as well as providing a rugged thermometer. By properly selecting the temperature range it is possible to ohtain the components necessary for less than $25.00. The component parts for the thermistor thermometer shown in Figure 1 cost slightly less than a O.l°C glass hulh thermometer, yet this thermometer can he badly mistreated and still continue to function. Depending upon the extent of the explanation accompanying the experiment, students can become exposed to the principles of thermistors, the Wheatstone bridge, and the concept of calibration.
Figure 1. A phdograph of lhe hrmistor h m t e r .
Instrument Deslgn The circuit diagram is shown in Figure 2. Rather than measure a small voltage that would require an expensive meter, a constant voltage is applied to the thermistor and the current changes are measured as the resistance changes ( E = I R).The circuit is a hasic Wheatstone bridge. The two diodes ( 1 ~ 6 0 are ) to protect the meter. ~ u r r e n r f r o mthe battery flows to point A. The current from side A-B-C is compared to side A-D-C. Initially the current at point B is adjusted (resistor 3) to match the current at point D and the meter will then read 0. When the resistance of the thermistor changes because the temperature changes then the current at point B will he dif-
Ftgule 2 A wiring diagram fa hh r m s t o r lhermometer Camponen1 Darts: (1)Thermistor. Fenwall CiA41PS. FenwalEkbonic. K i i & Co. nc.. 63 F o m i n St.. Framic#am. MS 01701 (2) 2.5 K reslna. (3) 15 I< variaole r e a i m (41 2.15 K resistor; (5) IN60 diodes: (6) 11.5 K resistor; (7) 9 V transista banery; (8) Micromarnmeter. Micronta 270-1751 Radioshack: (9) 11.5 K resistor.
Volume 61 Number 11 November 1964
1027
Table 1.
Materlals Used For Heat Capacny Measurements
Material
Source
Leather snaps Sprinkler system alloy Cooking skewers DBnture plastic Bubble gum Beach sand
Table 2.
Amount
Shw store Plumbing store Grocery store Dentist (old plates) GTDcery store Anywhere but Kansas
6 cm3 5 releases 2 skewers 112 plate Ipiece 20 g
Materlals for Acld-Base Heat-Of-Reactlon Studles
Commercial or Natural System
Milliliters of 1 Macid
Milliliters of 1 MKOH
Formic acid Ant and Bee venom Phosphoric acid Rust-proofing metals ktydrochloricacid Stomach reactions Salicylic acid Wart remover Nicotinic acid &in vitamin Sulfuric acid Component of acid rain
50
50
50
50
50
50
50
50
50
50
50
50
ferent from point D and the meter will measure the difference. Resistor 2 is used to change the scale expansion. The meter is calibrated by placing it in warm water. Several thermistors can be calibrated a t one time, so only a few glass bulb thermometers are needed for each class. If time is short, the instructor may just post a calibration curve or assume that the linearity is sufficient. Figure 3 is a plot for one such thermistor. The meter can be read to about 0.3 divisions. With 2.4 divisionsldegree this thermometer is accurate to about 0.15°C. By the proper selection of the amounts of reactants the temperature change of the heat of reaction or specific heat measurements can be adjusted to he within the 25-50°C working range of this thermometer. If linearity over the entire range is desired, then the $4.00 GA41PS thermistor can be replaced by either one of the following depending upon the range desired: 1) Fenwel Electronics type 44201 linear thermistor circuit no 29F218 covering the 0°-100°C range, &0.2'C. Cost $18.10.
2) Type 44212 covering the range -50'-5O0C, $34.05.
1028
Table 3.
Journal of Chemical Education
&0.15°C. Cost
Materlals For Oxidatlon-ReductionHeats-Of-Reaction Measurements
System Examined
Oxidant and Amount
Bombadier beetle defense Liquid rocket fuel
50 mL of 1 MH202In elham1 50 mL of 3 MHNOSIn water
Hot shaving l a m
50 mL of 1 MH202in water 0.5 g Ai powder in 50 mL of water 0.5 g Ai powder in 50 mL of water
Chemical heating pad Drain cleaner
Reductant and Amount 50mLoflM hydroqulnm In elhand 50mLof3M unsymmetrical dimethyl hydrarine in water 50 mL of 0.25 Mthiourea in water 50 mL of 0.3 MCuCi.
Figure 3. A calibration curve for lhe termistor h r m t e r .
Tables 1,2, and 3 list several commercial products that have been tested and the recommended amounts of each compo-
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This apparatus has been tested with a group of 30 students, and it worked well. The students were fascinated by it, and this helped during the explanation of how it operated. The biology and medical students were quite interested when it was oointed out how small the thermistor could be made.Thev thought up several experiments that would involve measuring the temperature in various places in insects or animals.
