Calibrating Instruments (Edexcel IAL Physics)

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 A voltmeter gives readings that are larger than the true values and has a systematic error that varies with voltage. Which graph shows the calibration curve for the voltmeter?

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 Hint You will be expected to use a calibration curve for the Core Practical 12: Calibrate a thermistor in a potential divider circuit as a thermostat