Technical note

A thermostatic

system

for

biomedical

applications*

Keywords--lnstrumentation Temperature control

Fig. 1 shows the general diagram of the regulator composed of a water tank of 25 litres, with Perspex walls, isolated externally by means of polystyrene foam (styropor) for good thermal isolation. The refrigeration unit is composed of a sealed Freon compressor connected to a copper-tube spiral forming the refrigerating element, the length of which is approximately 150ram. The spiral is within a cylindrical brass container, 350 mm in height by 80 m m in diameter, open at the top end and containing a mixture of ethylene glycol and water.

TEMPERATURE control, to within very close limits, is a frequent problem in biomedical experiments. In many cases, the problem consists in controlling the temperature of a water bath to a certain value and within desired limits. Then the water is circulated to the system in which we want to maintain a constant temperature and which, as a rule, has a small thermal capacity. The present note describes a system, constructed with c o m m o n materials and components, consisting of a bath whose temperature can be controlled between 15 ~ and 60~ with a variation < + 0' 01 deg. C.

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110V"u .

T =l ,oy I

thermostat

'~

infrared

\

compressor

[

brass tube--

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-F

- thermometer -L_

spiral- / mixture-- /

=:3

water bath

Fig, 1 Diagram of regulator

1N5060

,12V 35s ,12V ,

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contact thermometer

Fig, 2 Control unit *First received 5th September and in final form 24th October 1973

Medical and Biological Engineering

January 1975

121

Within the brass container, and in the mixture, there is the capillary of a gas thermostat which controls coarsely the temperature of the mixture between 0 ~ and 15 ~C. The quantity of heat stolen from the bath may be adjusted by the degree of immersion of the container into the bath water. A small 1750 rev/min mixer makes the temperature uniform. To control the temperature to the desired value, a 250W infrared lamp, with internal reflector, is used. The lamp irradiates heat directly into the water and is controlled by a mercury thermometer, logic units and a relay. The advantage of using an irradiator, instead of an immersed resistor, is the elimination of overshoots and the improvement of the precision of temperature control.

8"-

- ,12V

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Fig, 3 NOR unit

122

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NORunit

2+

2e--~

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Fig. 4 Output unit

The control unit (Fig. 2) consists of a 12V d.c. power supply, two logic units and a relay. The advantage of using logic units is that the mercury thermometer switches ON and OFF smaller currents (4-5 mA) than those of the relay, and hence contact sparking is avoided and better reliability is obtained. Figs. 3 and 4 show the diagram of the logic units used in the system. One is a NOR unit and the other is an output unit able to command relays up to 300 mA, 12V. FRANCISCO V. LACAZ C. D. STAMOPOULOS Institute of Biomedical Sciences University of Sao Paulo Brazil

Medical and Biological Engineering

January 1975