A Clinical Digital Thermometer for Calorimetry Ping-Kay Hon The Chinese University of Hong Kong, Shatin, N.T., Hong Kong In calorimetric measurements, the change in temperature is normally 2-3 T.The differential thermometer of the Reckman type is usually employed for the desired accuracy and precision. The mercury-in-glass thermometer is quite expensive and easily broken hy students in running the experiments. In addition, the low contrast of the mercury capillary and the fine graduations of the scale make it difficult to Lake readings. Digital rhermometers with 0.01 "C resolution are too expensi\.e for the undergraduate teaching lahoratorv. Thermistors have been used satisfactorilv for this purpose. The resistance of a thermistor in the logarithmic scale is in good linear relationship with temperature. A good digital multimeter should be used (a 411%DMM is preferred) tomeasure the resistance to the desired accuracy. The recent commercially available clinical digital thermometers are of low cost (about $10 apiece) and have the resolution of 0.01 OC. They should be ideal for calorimetric measurements. However, the clinical thermometers are usually designed in the fixed temperature range of 32-42 "C, consistent with body-temperature limits (37 5 OC). For calorimetric measurements the temoerature ranee 20-30 OC is more desirable. The followingpresents a st;dy of the characteristics of a clinical dieital thermometer and a method to change its temperature-measuring range. The Philips clinical digital thermometer, model HP 5311M (Fig. I),has been investigated. I t is made of just one integrated circuit and the LCD. There is not any adjustable component that can he modified. Fortunately, it has been discovered that when the sensor tip was separated from the main unit by an extension lead (this not only provides convenience but is necessary) the temperature range was shifted to the lower values. The longer the lead was, the lower the temperature that could be measured (Fig. 2). (The readings werestill from 32.00 to 42.00 OC. but the actual temperatures could be from 0.00 t o 10.00 OC.) The temoerature sensor is possihlv a transistor. I t has input and a third three leads-postive and negative for t The resistance between the output and for the o u t ~ usienal. the ground has fhe chararteristics of a thermistor. Its logarithmicvnluechanples linearly with temperature.'l'he linearity can he a ~ c u r a t ~*0.03 t o % in any 10-degree range (Fig. 3). The shift in temperature detection was found to be from the slight variations in capacitances between the positive output and the negative output when the length of the lead ~ a ~ c h a n e eThetwin-core d. screened cable was used in the test with:he screen being the ground. Calibration against a Beckman thermometer for a 150-cm lead length is shown in the table. The equation for the linearleast-squares fitting is
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Figure 1. The original clinical digital thermometer (a) did not give a reading at rwm temperature (no numerical indication for temperature below 32 "C).The modified unit (b) with the sensor probe separated by an extension cabie had a reading higher than lhe actual value. A stabilized voltage circuil has been added to the unit and 4-6 V dc either from batteries or from an ac adapter wuid be used.
Beckman scale reading = -33.25 + 0.9987X digital reading correlation coefficient = 0.99997 The slope is very close to 1.000. A simple substraction of two readings will he good enough for differential temperature measurement. and the relative error due to the scaling factor is about 0.14, which is much smaller than the normalIv.exoected error.. sav. . - . an absolute error of 0.02 degree in a 2degree range. The average deviation was *0.01 'C in the 6deeree ranee of the Beckman reading. This precision and accuracy are very satisfactory for calorimetric measure~~
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~
0
0
50
100
150
200
Lead Length lcml
Figure 2. Measurable temperature range at different lead lenglh of the extension cabie tor the detector. Volume 66 Number 8
August 1989
695
Figure 4. At a given ternperamre the reading varies with
Figure 3. me plm of l w resinance of me sen=
banw voltage.
vs. me temperahlre
Callbratlon ol the Clinical DlgHal Thermomater agalnsl a Beckman Thermometer Figure 5. Circun dlagam of a .stabilized dc source. A single IC LM317 is usad.
Beckman
Digmi Reading
Cab. (Beckman)
0.60 1.00 1.50 2.00 2.60 3.00 3.60 4.00 4.50 5.00 5.50 6.00
33.90 34.31 34.81 35.29 35.68 36.30 36.89 37.28 37.79 38.32 38.80 39.32
0.80 1.01 1.51 1.99 2.58 3.00 3.59 3.98 4.49 5.02 5.50 6.02
DBviation
0.00 -0.01 -0.01 0.01 0.02 0.00 0.01 0.02 0.01 -0.02 0.00 -0.02 Average *0.01
ments. Therefore the clinical digital thermometer should be a good substitute for the Beckman-type differential thermometer. I t is of low cost. is virtuallv unbreakable.. and eives no reading errors. The unit is oowered bv a small silver oxide button cell of 1.588 V. The power con&mption is very little, about 40 FA. A fresh battery should last for many hours of operation. However, it was found that for a given temperature the
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Journal of Chemical Education
reading varied with the voltage of the battery (Fig. 4). There is no readine for batterv voltaee below 1.48 V. And the maximum pe'rmissible voltage ii2.00 V. If the hartery voltaee isdecreased hv0.100 V Le.. from 1.600 V to 1.500 V) the reading will be inkeased b; 0.18 O C for the same temperature. Fortunately, the cell voltage varies less than 0.005 V in several hours of operation. For continuous use one may consider a stabilized power supply as illustrated in Figure 5. This simple circuit will provide a voltage stable to 0.01% and over 100 h of operation when it is powered by three AA-size dry cells (stabilized output is about 1.56 V for a source voltage of 3.6-6.0 V). I t is also noted that the clinical digital thermometer can show the current value of the rising temperature. I t will update the reading every second. However, it will not give the current reading if the temperature is falling (the tbermometer is the maximum-temoerature tvve). -. In order to measure a decreasing temperature one has to turn the unit off and on. It aivesa readina inahout 3safter it is ush he don. This may be small drawback.
a
