thermistor calibration living with the lab © 2013 David Hall

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living with the lab 3 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 thermistor are submerged in cup

living with the lab 4 circuit 10kΩ 5V analog pin 4 (measures voltage across 20Ω resistor) response of Cantherm MF52 series thermistor digikey part #: ND (resistance = 10kΩ at 25˚C) 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

living with the lab 5 calibration sketch void 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 }

living with the lab 6 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 waterwarm water (mix hot water with tap water, around 50°C is good)

living with the lab 7 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

living with the lab 8 repeat for room temperature and warm water readings hot water pot press unlock and then press the dispense button when finished, return your cup and thermometer

living with the lab 9 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 temperatureanalog input oCoC(0 to 1023) calibration equation use five digits for these fitting constants

living with the lab 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 analog input heat=off heat=on setpoint = 505 LCL=setpoint-3  = 500 UCL=setpoint+3  = 510 determine the random error in analog input 10

living with the lab 11 using the calibration equation calibration equation if the setpoint is 27 o C and 3  =5, then the temperature control values are... 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 quantityT (°C)analogT UCL setpoint LCL control strategy if analogT > UCL (or 524) then heater must be off If analogT < LCL (or 514), then heater must be on inverted equation

living with the lab 12 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.