thermistor calibration

living with the lab

© 2013 David Hall

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• setup includes a laptop, fishtank system, thermistor circuit, thermometer, and cup• the cup is filled with cool water, tap water, and warm water to produce three

calibration points

experimental setup

digital thermometer

thermometer and thermistorare submerged in cup

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circuit

10kΩ

5V

analog pin 4(measures voltage across 20Ω resistor)

response of Cantherm MF52 series thermistordigikey part #: 317-1258-ND

(resistance = 10kΩ at 25˚C)

0 5 10 15 20 25 30 35 400

5

10

15

20

25

30

temperature (˚C)

resis

tanc

e of

ther

mist

or (k

Ω)

variation of 0 to 1023 analog value with temperature• as temperature increases, the electrical resistance of the thermistor decreases• this causes a decrease in voltage across the thermistor• this causes an increase in voltage across the 10kΩ resistor• this causes the 0 to 1023 output of the analog pin to increase with increasing temperature

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calibration sketchvoid setup() Serial.begin(9600); // use a baud rate of 9600 bps pinMode(1,OUTPUT); // set pin1 as an output (pin1=TX) Serial.write(12); // clear screen & move to top left position Serial.write(129); // move cursor to row 0, position 1 Serial.write("Temperature Sensor"); // print a text string starting at (0,1) Serial.write(152); // move cursor to row 1, position 4 Serial.write("Calibration"); // print a text string starting at (1,4) Serial.write(189); // move cursor to row 3, position 1 Serial.write("analog input="); // print text string at (3,1) Serial.write(22); // turn cursor off to keep screen clean

void loop()

int analogT; // declare analogT as an integer analogT=analogRead(4); // read the voltage at analog pin 4

Serial.write(202); // move cursor to row 3, position 14 Serial.print(analogT); // print the analog input reading (0 to 1023) Serial.write(" "); // overwrite previously printed numbers

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CAUTION: Be careful with hot water!

fill the cup about 2/3 full of water at three different temperatures

calibration method

cool water(water fountain is a good source)

tap water warm water(mix hot water with tap water, around 50°C is good)

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• insert thermistor and thermometer into water while running the calibration program• wait until “analog input” values from thermistor circuit reach a steady value• record this temperature and the “analog input” printed to your LCD

obtain the cool water calibration reading

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repeat for room temperature and warm water readings

hot water potpress unlock and then

press the dispense button

when finished, return your cup and thermometer

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use Excel to determine your calibration equation • example values are shown in the table below• use the temperatures and “analog input” values that you collected earlier

temperature analog inputoC (0 to 1023)

12.2 38422.1 48457.4 782

283.79calibration equation

10 15 20 25 30 35 40 45 50 55 600

100

200

300

400

500

600

700

800

900

f(x) = 8.70911742650142 x + 283.791310663273

temperature (˚C)

outp

ut o

f ana

logR

ead

use five digits for these fitting constants

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• allow system to reach steady state for room temperature sample• collect 20 “analog input” readings taking a reading every 5 seconds• enter these readings into Excel and compute the standard deviation, σ.• when you write your temperature control sketch, you will set your UCL as the

setpoint plus 3σ and your LCL as the setpoint minus 3σ• for the example below, 3 is equal to 5; if your 3σ is less than one, then set 3σ=1

time

anal

og in

put 520

510

500

490

heat=off heat=off heat=off

heat=on heat=on heat=on

setpoint = 505LCL=setpoint-3 = 500

UCL=setpoint+3 = 510

determine the random error in analog input

10

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using the calibration equation

calibration equation

if the setpoint is 27oC and 3=5, then the temperature control values are . . .

283.79

The arrows indicate the order that the control parameters are computed.

using algebra, manually invert the calibration equation so you can compute temperatures based on the analog input values returned by the Arduino

quantity T (°C) analogT

UCL 27.6 524

setpoint 27.0 519

LCL 26.4 514

control strategy• if analogT > UCL (or 524) then heater must be off• If analogT < LCL (or 514), then heater must be on

𝑇=0.11482 ∙𝑎𝑛𝑎𝑙𝑜𝑔𝑇−32.585inverted equation

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CAUTION!Your heater can easily get hot enough to burn you and cause a

fire. NEVER leave your system plugged into a wall outlet when you are not actively testing its performance. You should always be present when the system is plugged into a wall outlet.

Only test your fishtank in an area with a fire extinguisher and an active smoke alarm nearby.