Med. & Biol. Eng. & Comput., 1978, 16, 334-336

Technical note Water-bath temperature controller Keywords--Thermoelectric devices. Cooling, Servomechanism, Water bath

1 Introduction MANY experiments, such as those investigating single muscle fibres, require a temperature-controlled water bath of a specified size mounted in a particular way near other equipment. Sometimes such requirements make using standard commercial temperature controllers impractical. This communication describes an inexpensive and easily constructed water-bath temperature controller. The advantages of this system other than its low cost and custom-built dimensions are its artefactfree direct current operation and its temperature regulation precision. The control mechanism is proportional until the temperature error is 0-1C ~ After the error exceeds this amount the device is either completely on or off depending on the direction of the error.

from measurably affecting the solution temperature, and feedback minimises solution temperature changes caused by long-lasting room temperature changes. Experimentally, the bath temperature varies less than 0. 075C ~ for each degree change in room temperature. Controller component temperature or power supply variations do not affect the solution temperature. Less than 10 minutes after the thermostat setting is changed, the water-bath temperature will be within O" 1C ~ of its new specified temperature, and thereafter this temperature error halves about every eight minutes. 3 Mechanical construction Fig. 1 illustrates the physical arrangement of the water bath, the thermoelectric devices, and the associated heat removing apparatus. The water bath itself is milled from a 20 • 70 x 80 mm aluminium block and is fitted with a glass bottom. This block is cooled to the desired temperature by the controller and provides the large thermal capacity which stabilises the bath solution against rapid temperature fluctuations. A 3 m m layer of styrofoam (not shown) insulates the block.

2 Specifications The water bath holds 20 ml. Its controller thermostat fixes its solution temperature at any specified setting between 0~ and 20~ The large thermal capacity of the bath prevents momentary room-temperature fluctuations

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334

Medical & BiologicalEngineering& Computing

May1978

Two thermoelectric devices (t.e.d.s) (Cambion 8013959-01), sandwiched between the aluminium block and an 180 x 80 x 60 mm aluminium heat sink, pump heat from the block to the heat sink. Nylon screws threaded from the heat sink into the aluminium block hold these t.e.d.s in a pressure clamp. These screws must be thermal insulators. Water circulating through several feet of 1"5 m m internal diameter coiled copper tubing removes excess heat from the t.e.d, heat sink. A second aluminium plate d a m p s the copper tubing against the heat sink. The circulating cooling water, stored at room temperature in a 5 1 container suspended above the water bath; flows f r o m the upper container through the copper tubing and into a second lower container at a rate of 0" 5 1/h. The entire assembly should be mounted either from the heat sink or from the clamping plate. Mounting from the block itself may impair its thermal insulation.

4 Control circuit description Fig. 2 is a schematic of the cooling servomechanism. It has three general parts--the cooling section marked in Fig. 2 by the heavy line, the temperature-monitoring equipment, and the controlling circuit. 'Cooling occurs when current passes through the two Cambion 801-3959~1 t.e.d.s. The t.e.d, current determines the heat pumping rate and thus the aluminium block temperature. Since this current is the transistor T2 collector current, it can be controlled by adjusting 7"2 base-emitter voltage. T2 emitter current is monitored at the junction of T2 and the 0" 1 f2 resistor. The two t.e.d.s are connected electrically in series and thermally in parallel. Each has a resistance of about 0"25 ~, and pumps heat fastest when supplied with a 9A current. Since the control circuit regulates the output current, there is no need to regulate the t.e.d, power-supply voltage. It is important, however, to isolate the t.e.d. power supply from the + 15V controller power supply.



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All wires carrying this t.e.d, current should be at least 16 a.w.g, and all earths for this part of the circuit should be connected to a single lug physically separated from signal earth. Power transistor T2 is rated for 10 A. Both transistors 7"1 and 7"2 need substantial heat sinks. In this system both transistors are mounted on an aluminium rear cabinet panel approximately 370 x 80 ram. A thin mica layer electrically isolates the transistors from the cabinet panel. A type 8442 thermistor probe from the Cole-Parmer Instrument Company and a Yellow Springs Instrument (YSI) telethermometer model Y3TD transduce the waterbath temperature to an electrical voltage. The temperature-monitor voltage from this telethermometer is the voltage across two sides of a bridge. It decreases as the temperature increases. The monitor output must be connected to a circuit with a very high input impedance. The temperature controller consists of a low-pass filter, an input voltage follower, a temperature-scaling amplifier, a reference voltage potentiometer and amplifier, a current output regulator and phase advance circuit, and a current amplifier. All operational amplifiers are model 741. A + 15V regulated power supply capable of furnishing a 40 mA current powers the four operational amplifiers and two voltage dividers. The temperature monitor input feeds into a low pass filter with a 3 dB cut off of 3.5 Hz. Voltage follower A1 isolates the bridge in the YSI telethermometer from the rest of the circuit. The 100 ~ potentiometer cancels unwanted steady input voltages. Its wiper Voltage is set to equal the temperature monitor input voltage at 0~ At this setting A2 input voltage is zero when the waterbath temperature is 0~ Amplifier A2 is the temperaturescaling amplifier. It determines the desired voltagetemperature sensitivity. A2 output voltage, labelled V,e~p in Fig. 2, varies linearly with temperature. It should be connected to an external output jack, and is used to accurately monitor the water-bath temperature.

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Medical & Biological Engineering & Computing

May 1978

335

Capacitor 6'1 introduces a feedback-loop phase advance to compensate for the thermal resistance of the aluminium block. Such a thermal resistance may cause the water bath temperature to slowly oscillate. When changing the bath temperatures, pressing pushbutton /1, which is normally closed, will remove the phase advance and allow the temperature bath to approach the new temperature within a few minutes. In practice, P~ should be held until the bath temperature is within 1~ of the new temperature. The remainder of the circuit provides the water bath reference temperature and determines the forward loop gain. The 20 k f~ potentionmeter sets'the reference temperature. Voltage follower A4 isolates Vte~,p from this reference voltage. Amplifier A4, transistors 7"1 and T2, resistor R4 and the two 1Mf~ resistors form a very high gain current operational amplifier. A4 is the controlling element while T~ and Tz amplify current. The controlled voltage is the Tz emitter voltage. Because this voltage must equal the product of emitter current and the 0-1 resistance, controlling 7"2 emitter voltage also controls Tz emitter current and consequently t.e.d, current. The internal feedback loop through Ra forces this controlled voltage to depend only on the reference voltage and

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amplified monitor voltage V~e=~and to be insensitive to variations in the characteristics of 7"1 and T2. Resistor R3 is selected so that T2 will become fully conducting when the water bath temperature exceeds the reference temperature by I~ Resistor R4 matches operational amplifier A, to the output transistors T~ and T2. R4 is 220kfL The 10 /~F and 150 tff electrolytic capacitors stabilise the high gain-current amplifier consisting of A4, 7"1 and 7"2. D. F. NOBLe

Department of Physiology & Biophysics University of Mississippi Medical Centre Jackson, Mississippi 39216, USA D. PEARCE

Department of Physiology College of Medicine East Tennessee State University Johnson City, Tennessee 37601, USA Reference BraD, GOFDON, Jun. and AKmA, TAMAMURA,P. 171 The cambion thermoelectric handbook. Cambridge Thermionic Corporation, Cambridge, Mass. USA.

Medical & Biological Engineering & Computing

May 1978

Water-bath temperature controller.

Med. & Biol. Eng. & Comput., 1978, 16, 334-336 Technical note Water-bath temperature controller Keywords--Thermoelectric devices. Cooling, Servomecha...
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