3.3 Calibrating Instruments

• Calibration is a comparison between a known measurement and the measurement you achieve using the instrument
• This checks the accuracy of the instrument, especially for higher readings
• An example is checking whether a meter (e.g., voltmeter, micrometer, ammeter) reads zero before measurements are made
  • This helps avoid zero error
• To calibrate a thermometer means to put the correct mark of readings at the correct place so that other temperatures can be deduced from these marks
  • An uncalibrated thermometer may not read 0 °C for the freezing point of water, or 100 °C for its boiling point, but we know these values to be accurate

Calibration Curves

• Calibration curves are used to convert measurements made on one measurement scale to another measurement scale
• These are useful in experiments when the instruments used have outputs which are not proportional to the value they are measuring
  • e.g. e.m.f and temperature (thermocouple) or resistance against temperature (thermistor)

• For example, the calibration curve for a thermocouple, in which the e.m.f varies with temperature, is shown below:

A curve of voltage against temperature can be used as a temperature sensor

• The calibration curve for a thermistor looks like:

 Thermistor calibration curve

• The accuracy of all measuring devices degrades over time. This is typically caused by normal wear and tear
  • Calibration improves the accuracy of the measuring device

Worked Example

ANSWER: A

• The voltmeter has a systematic error as the reading it gives is always greater than the true value
• If the true value is zero, the voltmeter would give a value greater than zero
• Therefore, the curve doesn’t pass through the origin (0,0) as this would indicate that the reading is the same as the true value, and not greater - this rule out graph C
• So, when the true value is zero, the meter would give a reading greater than zero. This is either graph A or B
• The systematic error varies with voltage
  • So, the amount by which the meter reading is greater than the true value changes

• Therefore, graph A is correct, because the difference between the meter reading and the true value increases with voltage

Exam Tip