In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
Low-Cost Thermocouple Signal-Conditioning Module Marcelo K. Lenzi,* Fabricio M. Silva, Enrique L. Lima, and José Carlos Pinto Programa de Engenharia Química/COPPE, Universidade Federal do Rio de Janeiro, Cidade Universitária, Rio de Janeiro, 21945-970, RJ, Brasil; *
[email protected] Michael F. Cunningham Department of Chemical Engineering, Queen’s University, 19 Division Street, K7L 3N6, Kingston, Ontario, Canada
heat dissipater to the voltage regulator with higher voltage power supplies. A switch and an LED diode were also added between the power supply and the voltage regulator to address safety issues. The most important components are listed in Table 1. Wire, solder, and tools are not listed. The actual circuits assembled over prototyping boards, before closing the enclosure box, are shown in Figure 2. Experimental To lower the operational costs and to enable uninterrupted use of the module, a +6 V dc power supply (AC adapter, Radio Shack, part number: 273-1761) was used. It should be noted that commercial +9 V dc batteries could also be used, but the operational costs may increase. To check the linear behavior of the sensor a hotplate–magnetic stirrer (Fischer Scientific, Inc., ISOTEMP), equipped with tempera-
power supply +
–
+
–
1
14
switch
2
13
3
12
4
LED 5
+ – +
11 10
6
9
7
8
–
+
voltage regulator
Module The module is easy and straightforward to build. A detailed circuit diagram is presented in Figure 1. The AD594C chip needs a +5 V dc power supply, but to increase the robustness, a LM7805 (Fairchild Semiconductors, Inc.) voltage regulator was added so that power supplies up to +12 V dc could be used. However, it is recommended to connect a 122
thermocouple input signal
AD594C
It is well known that reaction rates and physical properties of any substance depend on the temperature. Therefore, an accurate temperature measurement is a key factor for successful activities both in chemical laboratories and industrial plants. The most common sensors for temperature measurements are integrated-circuit temperature sensors, thermistors, platinum-resistance thermometers, and thermocouples (1). Each sensor has its own features, including temperature range, accuracy, cost, and durability, among others. More details can be found in (2). Many applications of temperature sensors can be found in the literature, for example, refs 3 and 4. Thermocouples are widely used in industrial and laboratory activities. They are formed by two different metals that are typically welded together. The junction between the two metals generates a voltage that is temperature dependent; thus the voltage can be used to determine the temperature. Some concerns about thermocouple use must be taken into account. The voltage produced by the thermocouple is usually low, which may require special voltmeters. The reference temperature in the thermocouple circuit may be another problem since an ice bath is usually used, which is impractical in an industrial environment. Finally, the relationship between temperature and voltage output is normally nonlinear (5). However the low cost, robustness, accuracy, and operational temperature ranges of the thermocouple cause it to be widely used. In this article we describe a low-cost thermocouple signal-conditioning module, which was built using the AD594C chip from Analog Devices, Inc. This chip compensates for the drawbacks of the thermocouple because it amplifies the signal, provides the cold-junction temperature compensation, and also produces an output of 10 mV兾⬚C directly from the thermocouple input signal (6). It should be noted that commercially available thermocouple sensors are expensive, that is, over $300 (U.S.), while the module we present can be easily built for $50. In addition, the module can be easily connected to any analog-to-digital converter to store the data or be used in any kind of control application.
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module output 10 mV / °C Figure 1. Detailed circuit diagram of the thermocouple signal-conditioning module.
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In the Laboratory
ture controller and digital indicator, was used. Distilled water, 300 mL, was heated, starting at 30 ⬚C, increasing the hotplate temperature set-point by 10 ⬚C, up to 80 ⬚C. Every time the set-point was reached we waited 15 minutes to ensure that the temperature reached steady-state. The measurement using the module was then performed. The temperature indicated by the hotplate, a Hg thermometer, and the module were compared. In a first set of experiments, the module was connected to a plug-in data acquisition card (Advantech, PCI-1711), and in a second set of experiments the module was connected to an inexpensive voltmeter (Kosmos RE830B multimeter). Similar readouts were obtained and thus the latter could be successfully used as an indicator, with the advantage of low cost. It should be noted that the hotplate and the Hg thermometer indicated temperature values with differences less than 0.1 ⬚C. The procedure was repeated four times leading to the same readouts. The module output versus temperature is plotted in Figure 3. Linear behavior is found and the value of R2 is equal to 0.9994. The slope of the line is 10.011 mV兾⬚C, which is close to the literature value of 10 mV兾⬚C of the AD594C chip. The difference can be attributed to noise, which may be present in the measurements and wire connections. The module has been used continuously in polymerization reactors in our labs, giving good results and robustness.
Table 1. Parts Used to Assemble the Module Quantity
Part Number
Supplier
Description
1
91F6222
Newark Electronics
LED diode
3
34C9478
Newark Electronics
Terminal block connector
6
50N122
Newark Electronics
Grommet
1
AD594C
Analog Devices
Thermocouple signal conditioner
1
LM7805
Fairchild Semiconductor
Voltage regulator
1
276-148
Radio Shack
Prototyping board
4
64-3011
Radio Shack
Screw 4/40
8
64-3018
Radio Shack
Hex nut 4/40
1
270-183
Radio Shack
Enclosure box
1
275-664
Radio Shack
Switch
Conclusions We describe a signal-conditioning module, based on the AD594C chip that can be easily built at low cost and overcomes the drawbacks of a thermocouple when used alone. The results show good linearity and reproducibility of the module. After testing, the module has been used to monitor the temperature of polymerization reactors in our labs. It should be stressed that the module could also be used in industrial plants.
Figure 2. Thermocouple signal-conditioning module.
Acknowledgments
Module Output / mV
800
The authors would like to thank CAPES (Brazilian Agency) and Queen’s University for funding and resources for this research.
y = 10.011x − 1.2952 R 2 = 0.9994
700
600
Literature Cited 500
400
300 30
40
50
60
70
80
Temperature / °C Figure 3. Comparison of the module output voltage and the temperature from a Hg thermometer.
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1. Skoog, D. A.; Holler, F. J.; Nieman, T. A. Principles of Instrumental Analysis, 5th ed.; Saunders College Publishing: Chicago, 1998. 2. Montague, Jim. Control Eng (On-line Extra) 2003, 50, 30. 3. Muyskens, Mark A. J. Chem. Educ. 1997, 74, 850–852. 4. Li, Chia-yu; Zhuang, Qun-meng. J. Chem. Educ. 1988, 65, 344. 5. National Instruments. Measuring Temperature with Thermocouples—A Tutorial. Application Note 043, 2001. 6. Analog Devices, AD594/AD595, 1999.
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