In the Laboratory
An Integrated-Circuit Temperature Sensor for Calorimetry and Differential Temperature Measurement
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Mark A. Muyskens Department of Chemistry, Calvin College, Grand Rapids, MI 49546 Precise temperature measurement is an important experimental technique in a variety of chemical analyses. Techniques like calorimetry that involve monitoring the time progression of a temperature change clearly benefit from automating the data collection, allowing the analyst to focus on results rather than tedious measurement. Automation requires that the temperature sensor produce a signal, ultimately a voltage, that is converted to a digital value and stored, usually in a computer. The conventional sensors chosen for digital thermometry are thermocouples, thermistors, and platinum resistance thermometers; however, use of these devices involves some experimental challenges (1). Thermocouples require a very stable reference temperature and a highprecision voltmeter, while the other two sensors require nonlinear temperature calibration as well as careful resistance measurement to avoid skewing the results. This article describes our application of a fourth type of sensor, an integrated-circuit (IC) chip, which provides an easy-to-use, inexpensive, rugged, computer-interfaceable temperature sensor with good precision. Another ICbased sensor using a clinical digital thermometer has been described by Hon in this Journal (2). While Hon’s sensor has comparable precision, it suffers the disadvantages of covering only a 10-degree range, recording only rising temperatures, and having an output that depends on lead length and battery voltage. In addition, use of the clinical thermometer requires modifications to the original unit that are more complicated than the circuitry described here. The digital temperature sensor we have been using for the past five years in courses ranging from general chemistry to physical chemistry is based on the National Semiconductor LM35 precision integratedcircuit temperature sensor. This IC chip produces a 10mV signal for every degree Celsius. The reading from a voltmeter simply multiplied by 100 gives the temperature reading in °C. A 3-1/2 digit voltmeter can read temperatures below 20 °C to the nearest 0.01 °C; similar precision above 20 °C requires a 4-1/2 digit voltmeter. The LM35 series of integrated circuits offers a variety of accuracy and linearity tolerances. This article reports on the LM35CA chip used in the typical temperature range of calorimetric measurements near room temperature, which is just a small portion of its full range of {50 to +150 °C. We have also used the LM35D chip, which has lower cost, a more limited temperature range (0 to 100 °C), and somewhat worse linearity tolerances, although we have found its precision to be suitable for calorimetric measurements. Simplicity and ruggedness guided our design for this integrated circuit application. A description of our current design follows, and suggestions for improvement appear later in the article. For resistance to acids and bases, the IC plastic package is placed in the bottom of a 10-cm × 8-mm o.d. glass tube filled with thermally con-
850
ductive paste. A 3-foot, three-conductor cable terminated at one end in a 1/8-in. miniature stereo plug provides power and output lines to the chip, and heat-shrink tubing seals the open end of the glass tube to the cable. The chip has three leads: supply voltage, ground, and output voltage. Our power supply unit, which will work with all ICs in the LM35 series, consists of a small plastic enclosure containing a +9 V battery,1 two diodes, and a resistor as shown in Figure 1a. The diodes and resistor are needed only to allow the circuit to give negative readings; for strictly positive readings the circuit can be even simpler (see legend for Fig. 1).
(a)
LM35 sensor
power supply enclosure
+9 V
+ –
D
R
stereo plug connection
(b)
Vout
banana plug connections
detail of IC temperature sensor chip +Vs Vout
GND
Figure 1. (a) Circuit diagram for the LM35 temperature sensor with +9 V battery providing the supply voltage, +VS. The resistor, R (18 kΩ), and the two diodes, D (1N914), allow negative temperature readings. (b) Detail of the chip with pin assignments. For strictly positive temperatures, the circuit can be as simple as connecting the 9 V battery to + VS and ground (GND) and connecting the output Vout and GND to a voltmeter.
In our design, banana-plugs are used for connection to the digital voltmeter, and the miniature stereo jack for the temperature probe is wired so that, when the temperature probe is unplugged from the supply, the battery is disconnected from the circuit. The temperature reading is not affected by small drops in battery voltage; for example, one sensor gave identical readings when used with a fresh battery (>9.0 V) and a somewhatspent battery (8.1 V). Figure 2 shows the equipment required for digital temperature measurement, comprising the probe, power supply, two banana-plug cables, and a voltmeter. Table
Journal of Chemical Education • Vol. 74 No. 7 July 1997
In the Laboratory Table 1. Parts List for Integrated Circuit Temperature Sensora Description
Vendor
IC temperature sensor,
Digital Voltmeter
LM35CAZb
Stock Number
HamiltonHallmarkc
Glass tubing, 8-mm o.d.
Probe 1 cm
9V Battery Enclosure
Heat sink compound
Mouserd
524-8109-S
Heat shrinkable tubing, 3/8" i.d., blue
Mouser
5174-1382
Stereo plug, 3.5 mm, 72" cable
Mouser
172-2306
Stereo
Figure 2. Diagram of components required for digital temperature measurement using a probe based on the LM35 sensor. The digital voltmeter has a 4-1/2 digit display and shows a 25.00 °C reading.
1 gives a detailed parts list for the probe and power supply, including vendor and stock number information. The estimated cost for a single probe and power supply based on this list is around $25. For data acquisition by computer, each of our digital multimeters (Keithley, Model 175A/1753A) is equipped with a GPIB interface for communication with a Macintosh IIci (and higher) using a National Instruments NB-GPIB interface card and LabVIEW 3 software. Alternatively, the output voltage could be sent to an analog-to-digital computer interface card. Our approach to gathering digital temperature data has been used for acid–base reaction calorimetry (first-year chemistry lab) and bomb calorimetry (physical chemistry lab), as well as measuring freezing-point depression (first-year chemistry lab). The two parameters that are most important for gauging the performance of this sensor are the precision in differential measurement and the time constant for responding to temperature change. The precision evaluation below will first consider the uncertainty associated with using direct, uncalibrated readings and then discuss the improvement in precision based on calibration. The readings from six IC sensors (randomly chosen from our supply of about two dozen) were compared with results from a mercury calorimetry thermometer (Parr) that provides readings to the nearest 0.003 °C with a calibration correction chart traceable to NIST temperature standards. The readings were taken while the probes were immersed to a depth of about 5 cm in a large water bath with a temperature controller–circulator, and the data range from room temperature to about 10° above, covering the range normally used for calorimetry. Temperature difference measurements (∆T = T2 – T1) from each sensor were compared with the corresponding ∆T from the Parr thermometer for four different ∆T values ranging from 3.38 to 9.95 °C. Two sensors gave error results averaging slightly over 1%, while errors from the other sensors were well under 1% (see Table 2 for the best and worst case). The three sensors with the lowest errors gave differential measurements that were all within 0.02 °C of the calibrated thermometer measurement. Temperature difference measurements from a sensor blindly chosen out of the six tested and used without calibration over the tested range can certainly be considered good to about 1%, which is adequate for calorimetric measurements where the highest precision is not required. An example is so-called “coffee-cup” calorimetry where perhaps the sensor would be replacing a mercury thermometer with markings to the nearest tenth of a degree. On the other hand, choosing the best of the sensors allows using uncalibrated readings with an uncertainty of ± 0.02 °C in the difference of two readings. The emphasis in this report is on differential mea-
jack,e
3.5 mm
Enclosure, ABS plastic, 4.4"× 3.2"× 1.5"
Mouser
161-3501
Mouser
537-502-almond
Nonskid feet for enclosure (4)
Mouser
537-F2
9-V battery snap connector
Mouser
12BC006
9-V battery Banana jack, red
Mouser
530-108-0902-1
Banana jack, black
Mouser
530-108-0903-1
Diode, 1N914 (2)
Mouser
333-1N914
Resistor, 18 kΩ, 1/4 W, 5% tolerance
Mouser
30BJ250-18K
a
List does not include solder and assembly tools (wire stripper, soldering iron, screwdriver, and hypodermic needle for injecting heat sink compound into glass tube). b Z indicates a plastic package. c Hamilton-Hallmark, 800/332-8638; call for a local distributor. d Mouser Electronics, 2401 Hwy 287 N., Mansfield, TX 76063-4827; 800/ 346-6873. e We connect 9V to the tip conductor, signal output to the middle conductor, GND to the shielding.
surement because these thermometers have in some cases remarkably better precision than accuracy. For instance, one of the six sensors evaluated gave results that were consistently below the calibrated thermometer reading by 0.20 °C, while the ∆T comparisons were all within 0.01 °C of the calibrated thermometer results. This is an excellent illustration of how absolute measurements with poor accuracy but good precision can produce both good accuracy and precision when taken relative to each other. The manufacturer specifications state that a typical sensor is accurate to within ± 0.2 °C near room temperature (25 °C) with a maximum limit of ± 0.5 °C (at 25 °C). For complete specifications, I refer the reader to the manufacturer’s specification sheet (3). The linear relationship between IC and calibrated temperature readings allows a simple calibration to improve precision. Linear regression analysis of data from the six sensors reveals that the IC temperature readings are very close to linear, with a slope ranging from 0.9881 to 1.0019 (see Table 3). Ideal agreement would give a perfectly straight line with a slope of one and intercept of zero. The three sensors with slopes closest to unity also gave the best results in the ∆T comparison mentioned above. Estimating error based strictly on the slope of these linear calibration curves, the IC having the slope farthest from one is in error by 0.12 °C over a 10° change, while the IC with slope closest to one (0.9987) is in error by only 0.01 °C over 10°. For a sensor with a slope significantly different from one, a linear calibration against a good thermometer would allow improvement in both precision and accuracy. For instance, calibration of the sensor with slope farthest from one reduces the error over 10° from 0.12 to 0.04 °C (1σ). For the sensors with slopes very close to unity, calibration will not significantly improve precision. The response time of the sensor to a very rapid change in temperature can be characterized by a time
Vol. 74 No. 7 July 1997 • Journal of Chemical Education
851
In the Laboratory Table 2. Relative Error in Differential Temperature Measurements for Best and Worst IC Sensorsa Best (IC Sensor A) Measurement
∆T (° C)
Table 3. Linear Regression Parameters for Calibration of IC Sensorsa
Worst (IC Sensor F) ∆T (° C)
Parr
IC-A
error (%)
Parr
IC-F
error (%)
1
8.243
8.23
0.2
9.954
9.83
1.2
2
5.453
5.45
—
8.150
8.06
1.1
3
4.865
4.86
0.1
7.249
7.16
1.2
4
3.378
3.37
0.2
5.445
5.39
1.0
Sensor
Slope
| 1 – Slope |
y -Intercept (° C)
A
0.9991
0.0009
{0.1805
B
0.9989
0.0011
{0.0110
C
1.0020
0.0020
0.2838
D
0.9954
0.0046
0.2281
E
0.9894
0.0106
0.4396
F
0.9882
0.0118
0.1267
a
”Best” and “worst” are in terms of percent relative error. Comparison is between readings from a reference Parr calorimetry mercury thermometer and the IC sensor.
constant τ, which is the time required to rise to 63.2% (the 1/e point) of the maximum temperature, assuming the thermal leak rate is not very large. The time required to reach 99% of the maximum temperature is 5τ. For a 9° temperature excursion above room temperature the time constant τ is 6 s for the glass tube–encased IC described above. While this is certainly slower than other sensors including a mercury thermometer, it is faster than the thermal response time of a conventional bomb calorimeter and otherwise quite acceptable for calorimetric measurements where the time course is normally monitored for several minutes. The advantages of this simple device, especially its easily digitized and linear output, far outweigh the disadvantage of the response time. The LM35 specification sheet indicates that our IC with its plastic package immersed in a stirred liquid bath has a time constant of between 3 and 5 seconds, which suggests that the glass packaging contributes somewhat to the overall response time. The IC is also available in a metal package, which according to the manufacturer has a time constant of only one second. Thus, for applications where the sensor is used with noncorrosive liquids, design of a sensor probe that exposes a portion of the can to the liquid should give a greatly reduced response time. From our experience with using these sensors in a first-year chemistry lab to measure the freezing-point depression of xylene, we would make several recommendations for improving the design of the probe. We would use a longer tube, ca. 20 cm. The nonaqueous solvent was absorbed by the heat shrink tubing, which compromised the seal between the tube and cable. Consequently, we also recommend sealing the end of the tube with caulk or epoxy before applying the heat shrink tubing, which would then be used only for appearance. It is not necessary to fill the entire tube with thermally conductive paste; it is only necessary to provide good thermal contact between the chip and the end of the tube. Completely filling the tube is done primarily for appearance. Lastly,
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a Sensors are ranked according to increasing difference between unity and the slope. R2 for each linear fit is 1.000.
thin-walled glass tubing may give a better response time. Aside from these suggested changes, the above design has served us quite well. In summary, the sensor described above has a number of features that make it attractive for use in differential temperature measurement. The IC and its implementation is sturdy and inexpensive, provided that digital voltmeters are available. Once assembled, the components are easily used for digital temperature measurement, which provides a reasonable alternative to mercury-based thermometers. The circuit design is quite simple, primarily because much of the complexity is buried in the IC, and its linear output is very easy to interface to a computer. Differential temperature measurements over the temperature range normally used for calorimetry are good to 1% or less without calibration; and for selected sensors, good to within ± 0.02 °C. These features make the sensor appealing for use in first-year chemistry courses and throughout the curriculum, especially where digital results are desired. Acknowledgments I express my gratitude to Charles Holwerda and Richard Huisman for their roles in the design of the temperature sensor described in this article. Note 1. The supply voltage can be from 4 to 30 V dc.
Literature Cited
Journal of Chemical Education • Vol. 74 No. 7 July 1997
1. Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Experiments in Physical Chemistry, 5th ed.; McGraw-Hill: New York, 1989; pp 645–665. 2. Hon, P. J. Chem. Educ. 1989, 66, 695–696. 3. Data Acquisition Handbook, 1993, pp 5–12; National Semiconductor Corporation, 2900 Semiconductor Dr., P.O. Box 58090, Santa Clara, CA 95052-8090 (800/272-9959).
