Lab2 Thermistor

Laboratorio de Instrumentacin

Mecatrnica

ITESM Chihuahua

Thermistor Calibration

Laboratorio de Instrumentacin Mecatrnica

2

Calibration equipment

3

The thermistor and a reference thermometer are placed in

an insulated mug. Different mixtures of hot and cold water

are used to set the water temperature inside the mug.

DMM Leads

(Ohmeter mode)

(or Thermocouple lead

of the DMM,

temperature mode )

Calibration equipment

Thermistor probe ( 10k

thermistor) Thermocouple probe

4

Measuring resistance

5

You will need to use the bench multimeter to measure the

resistance of the thermistor. Make sure that the probes are

plugged into the high and low jacks labeled for voltage and

resistance, not current. By convention, black probes are

used for the ground and red probes for measured voltage.

Although this polarity does not matter for measuring the

thermistor resistance, it is good to get in the habit of

connecting the probes up correctly.


6

You will need to select the resistance measurement (the

button labeled ) and automatic continuous measurement.

If the MAN for manual or TRIG for trigger lights are on

on the display, try fiddling with the buttons (shift-trigger

may help). Better would be to read the DMM Users Guide.

Depending on what sort of probe is connected to the

multimeter, you may be able to connect them directly to

the thermistor wires, or you may need to use alligator clip

leads to connect them.


7

For making precise measurements, it helps to subtract off

the measurement of the wires connecting up to the device

being measured, by shorting them together and recording

that measurement.

There is another technique that does an even better job of

compensating for wiring resistance: using 4-wire

measurement and Kelvin clips. For an explanation, see

http://www.allaboutcircuits.com/vol_1/chpt_8/9.html

Measuring Resistance

8

Although the DDM is capable of 4-wire measurement, we

do not have the Kelvin clip probes for simultaneously

measuring current and voltage, so this technique is not

available to us. In any case, the resistances were looking at

on the thermistors are large enough compared to the wire

resistance that we dont really need the extra accuracy

obtainable with 4-wire measurement.

Measuring Resistance

9

You will want to measure the temperature and resistance of

the thermistor at the same time (since the uninsulated

water baths will equilibrate to room temperature fairly

quickly). Having one person hold and read the temperature,

while the other records the temperature and resistance

makes the recording easier.

You need to both record the measurements in a lab

notebook and type them directly into a computer file

(Excel spreadsheet).

Calibration procedure

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Once the equipment is assembled and ready to use, perform the following steps to obtain the calibration data.

1. Fill the coffee mug with water at a desired temperature. Mix hot and cold water as necessary.

2. Insert the reference thermometer and the thermistor probe into the water in the mug.

3. Wait for the thermistor probe signal and the reading of the reference temperature to stabilize.

4. Record the temperature and the thermistor resistance. (Notebook and Excel)

5. Return to step 1until you record the thermistor resistance for the range of 0 to 100 celsius, going up and down.

If your group has two mugs, you can be refilling one mug while you wait for the reference thermometer and thermistor probe to come into equilibrium with the water in the other mug.


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Temperature (oC) Resistance

(temperature going up)

Resistance

(temperature going

down)

0

10

20

30

40

100

Thermistor T-R Curve

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NTC (Negative Temperature Coefficient) thermistors

have the advantage of a very high sensitivity to

temperature changes, but the disadvantage of an

aggressively nonlinear characteristic.

The following curve shows the resistance of a typical NT

C thermistor device over a temperature range from 0 to 5

0C.

NTC thermistor T-R curve

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Steinhart-Hart Equation

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Thermistors T R curve can be described in equations. The most commonly used form is the Steinhart-Hart

Equation shown below:

1

= + + ()3

Where Rt the thermistor resistance at temperature T (Kelvin);

A, B, C the thermistors constants. Manufacturers may provide typical

values of the A, B, and C coefficients, or you can calibrate these values

for better accuracy.

NTC thermistor T-R curve

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From the previous curve, resistance value changes from a

ratio of 3.0 to a ratio of 0.5 within 0~50C.

The change is most rapid at low temperatures, giving

great resolution for determining the corresponding tem

perature values there.

At higher temperature, resistance changes relatively less

with temperature and the measurement resolution will

be relatively poor.


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It is not hard to calibrate your own response curves,

if you have an accurate temperature measurement

standard. For your calibration , three points need to

be selected, two close to the ends of the operating

range and one near the center.

Insert the three pair of resistance values and

temperatures to the equation to form three equations.

Solve the equations for the unknowns (A,B,C).

http://www.thinksrs.com/downloads/programs/Therm%20Calc/NTCCalibrator/NTCcalculator.htm


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Compare your A,B, and C coefficients with the

manufacturers coefficients.

Compare the experimental T-R curve of the thermistor

agianst the T-R curve of the manufacturer.

Compare the experimental T-R curve of the thermistor

against the T-R curve with your coefficients.

Repeat everything for the next circuit

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Additional Notes

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http://www.digikey.com.mx/Web%20Export/Supplier%20Content/api-technologies-1171/pdf/api-ntc-engineering-notes.pdf?redirected=1

http://www.omega.com/Temperature/pdf/44000_THERMIS_ELEMENTS.pdf

Documents

Lab2 Thermistor