Calibration of a Thermistor Voltage Divider Portland State

Calibration of a Thermistor Voltage Divider Portland State University Department of Mechanical Engineering ME 121: Engineering Problem Solving

A Voltage Divider for Thermistors The thermistor is located in the upper leg of a voltage divider 5 V thermistor Analog input 10 kΩ Review: Why use the upper leg and not the lower leg? ME 121: Engineering Problem Solving page 1

Calibration Equipment The thermistor and a reference thermometer are placed in an insulated mug that is filled with a range of mixtures of hot and cold water. +5 V or digital output Insulated Coffee mug analog input Thermistor probe Voltage divider for Arduino input Reference Thermometer Note: The fixed resistor in the voltage divider is an integral part of thermistor calibration. Replacing the fixed resistor with another resistor, even one of the same nominal voltage, will change the calibration. ME 121: Engineering Problem Solving page 2

Calibration Equipment A digital kitchen thermometer provides the reference standard for calibration of thermistor probe. ME 121: Engineering Problem Solving page 3

Calibration Procedure Once the equipment is assembled and ready to use, perform the following steps to obtain the calibration data. A source of hot water is provided in the laboratory. 1. Fill the coffee mug with water at a desired temperature. Mix hot and cold water as necessary. 2. Insert the reference thermometer and thermistor probe through the hole in the lid of the mug. 3. Wait for thermistor probe signal and the reading of the reference temperature to stabilize. 4. Record the temperature and the output of the Arduino analog input reading. Store a large sample of readings so that statistics can be computed. 5. Return to step 1 until you have collected calibration measurements at 8 to 10 temperatures. Ideally you should randomize the order of the temperatures, i. e. , you should not take data only in order of increasing or decreasing temperature. ME 121: Engineering Problem Solving page 4

Calibration Procedure Use a table to record the nominal measurements along with any qualitative observations T (◦C) ME 121: Engineering Problem Solving V (Volt) Comments page 5

Calibration Procedure Suggestions for improving your results: • Let your system come into equilibrium before taking readings. Ø The thermometer and your thermistor have thermal mass: you need to wait for them to come into thermal equilibrium (i. e. , have the same temperature) with the water. Ø The insulated mug has more thermal mass than thermometer and thermistor. It too has to come into equilibrium. Ø To reduce wait time between measurements, use two thermos mugs. While one mug holds thermistor, thermometer and the water during the measurements, prep the second mug for calibration at another temperature to give the mug time to equilibrate with the water. When the measurements with the first mug are finished, switch thermistor and thermometer into the second mug. • Use a mug with a lid to reduce heat exchange with the ambient. • Randomize the order of temperature measurements ME 121: Engineering Problem Solving page 6

Calibration Equation The calibration process suggests a curve fit of the form V = f ( T ) : many voltage readings for each fixed temperature. We could use a polynomial for V ( T ). V = c 1 T n + c 2 T n − 1 + · · · + cn - 1 T + cn (1) However, to control the temperature of the fish tank, we will need T = f ( V ). T = a 1 V n + a 2 V n − 1 + · · · + a n - 1 V + a n (2) Curve fitting of the calibration data is discussed in another set of slides. ME 121: Engineering Problem Solving page 7