9.2 Core Practical 12: Calibrating a Thermistor

Aim of the Experiment

• To calibrate a thermistor so it can be used as a thermometer

Variables

• Independent variable = temperature (^oC)
• Dependent variable = resistance of thermistor (Ω)
• Control variables:
  • Temperature gradient controlled by stirring

Equipment

• Power source
• Thermistor
• Fixed resistor
• Ohm-meter
• Bunsen burner, tripod and gauze
• Beaker filled with crushed ice
• Stirring rod
• Liquid in glass or mercury thermometer with range -10 - 100^oC

• Resolution of measuring equipment:
  • Thermometer = 1^oC
  • Ohm-meter = 0.01 Ω

Method

• Set up the equipment with the thermistor immersed in the ice, the ohm-meter connected to record the resistance of the thermistor by placing it in parallel across it, and the fixed resistor in series with the thermistor
• Place the beaker of ice onto the tripod, without lighting the Bunsen burner
• Measure and record the temperature and reading on the ohm-meter
• Light the Bunsen burner and keep to a gentle flame
• Stir the ice / water gently at all times to keep the temperature as even as possible throughout the beaker
• At approximately 5^oC intervals record the new temperature and resistance reading
• Continue until the water is boiling

Analysis

• Plot a graph of resistance against temperature
• Use the temperature graph to find the resistance at a given temperature

Evaluating the Experiment

Systematic Errors:

• Read the thermometer at eye level
• Check the zero error on the ohm-meter by connecting the leads across the terminals

Random Errors:

• Allow time for the temperature to reach equilibrium
• Stir the water before readings
• Ensure thermometer bulb is completely submerged in the water and level with the thermistor
• Turn off current between readings to avoid heating in the wires

Safety Considerations

• Risk of instability, use a stand and clamp to support leads
• Hot equipment and boiling water must be handled with care and allowed to cool down where possible
• Keep plastic cables / leads away from hot metal
• Check the voltage limit of the thermistor and stay within guidelines