Info

Fig. L Graph showing how with temperature,

can vary thermistor, and sometimes it is enclosed in an envelope of some kind, glass for example.

General advantages and disadvantages

For all uses in general, thermistors have certain advantages and disadvantages over other devices. Advantages include small size 0.006 to 0,1 inch in diameter, and their low specific heat allows them to draw virtually no heat from the object being measured. Temperature differences as small as 0.0001 degrees F have been measured.2 Their high resistance permits adequate impedance matching with associated equipment, and reduces effects of lead wire resistance changes on temperature readings.

This is not all for free, however, and we must be aware of a few disadvantages of using thermistors. One problem is the nonlinear resistance—temperature relation which requires main calibration points, raising costs. When a thermistor is compared to a well made platinum resistance—temperature sensor, ie thermistor will show poorer calibration stability. Another disadvantage

Fig, 2, Simple application oj the thermistor t Current meter type of temperature measurement, is that most thermistors of the oxide type show aging effects in that resistance increases with time. This problem can be solved by preaging which is accomplished by exposing the thermistor to a temperature that is higher than the temperature at which it is to be used. This will enable you to do very precise work, Also, enclosing the unit in glass will reduce chemical effects on aging.

A few uses and applications

R. F. Turner3 lists and describes many applications for the thermistor, some of which seem rather unusual. First we shall discuss basic methods of measuring temperature, and later take a look at a flow meter and vacuum gauge that can be made with thermistors.

The biggest advantage of the thermistor in temperature measuring is remote elect ri-cal readout If you would like to tell the operator on the other end of a QSO what the local temperature is without going outside, a thermistor thermometer is a great convenience. Similarly, if you would like to monitor the temperature in the final of your transmitter without opening it up and feeling around, then read on.

Fig. 2 shows the simplest application of the thermistor. T is the thermistor, E is a source of voltage, Rx is the calibration control, and M is a milliammeter or rnicroam-meter, R7 is set and the meter scale is calibrated in degrees. A well regulated constant voltage source is required, and a zener diode from Poly Paks, or some other low cost source will meet this requirement. Make the meter as sensitive as possible to limit the current flow in the thermistor because the thermistor will dissipate power just as any resistor will, and the heat resulting from this will effect the resistance. I noticed no problems of this type while using a fifty microamp meter, and this may be all that is necessary for your application.

Fig. 3 shows an ohmmeter measuring the resistance of the thermistor. This is the way that most experiments start. A chart can be made to relate meter reading to temperature, but this is not very convenient. Fig. 4 shows the most commonly used circuit. Ru R2j Ry, and T make up a Wheat-stone bridge. Ro, which is calibrated in degrees, is adjusted so that no current flows in the meter, and a very accurate reading can be made. This is not so convenient as just looking at a meter, but it is the best method for very precise measure-

t_OW RANGE DC

ohmmeter

Fig. 3. The thermistor as a low current ohmmeter, merits. I prefer the ordinary current meter method, first described, for my own uses, because it is very simple, and it works. None of these methods is beyond the ability of the average amateur and should provide interesting experiments, if you are looking for a gadget that is rather interesting, simple, and useful.

Fig. 4. Bridge type temperature measuring device.

Another device that can have applications for the amateur is a flow meter. Fig, 5 from Turner shows a typical circuit of a flow meter that can be used to measure either liquid or gas flow. Heat will dissipate more rapidly from an object when it is in a flow than when it is in static surroundings. A bridge circuit is used to measure the difference in resistance between 11 which is out of the flow, and T2 which is out of the flow, but in the same medium. Iii this ease the bridge is balanced initially with R^, with the flow still may have a scale calibrated in rate of flow. This method is much simpler than a mechanical method, is free from wear and vibration, and results

in almost no pressure loss. Perhaps the more adventuresome experimenter could devise a

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Fig. 5. Typical circuit of a flotv meter to measure either liquid or gas flow,

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