# 6.2 Calibrating Instruments

## 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? • 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

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 ### Get unlimited access

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