Calibration is the operation of comparing the readings of a measuring instrument with the values given by a reference standard, without modifying the instrument.
Discover FizziQ
How to measure it in class
With the FizziQ app, it is possible to calibrate the smartphone’s light sensor using a reference light source.
Steps:
- Open FizziQ and select the light sensor (lux). Place the smartphone in total darkness and verify that the displayed value is close to 0 lux.
- Place the smartphone at a fixed distance from a lamp of known power and note the brightness value displayed by FizziQ.
- Compare the measured value with the theoretical value calculated from the lamp power and distance (inverse square law).
- Repeat for several distances and plot the calibration curve. Determine if the sensor response is linear and identify any deviations.
Scientific activities on this topic
Several experiments easily achievable with a smartphone, tablet, or computer allow understanding the principles of measuring instrument calibration.
- 1: Evaluating sensor quality - Analysis of sensor precision: https://www.fizziq.org/en/activities/evaluate-the-quality-of-a-sensor/
- 2: Uncertainty - Analyzing measurement uncertainties: https://www.fizziq.org/en/activities/uncertainty/
- 3: Altimeter - Create an altimeter with an atmospheric pressure sensor: https://www.fizziq.org/en/activities/altimeter/
- 4: Tuning forks - Study of tuning fork frequencies throughout history: https://www.fizziq.org/en/activities/tuning-forks/
Learn more
The origins of calibration
The word “standard” comes from the Frankish “stalo,” which designated a reference post. From the Middle Ages, each city had its own length and weight standards, engraved in the stone of markets. The French Revolution unified measurements with the meter, originally defined as one ten-millionth of the quarter meridian of the Earth.
The international calibration system
Today, the calibration chain starts from the BIPM (International Bureau of Weights and Measures) in Sevres, near Paris. National standards are compared with international standards, then serve as references for accredited laboratories. This unbroken chain is called metrological traceability.
Calibration of MEMS sensors
MEMS (Micro-Electro-Mechanical Systems) sensors in smartphones are factory-calibrated through automated testing. An accelerometer is subjected to known accelerations, a barometer to reference pressures. Correction coefficients are stored in a non-volatile memory of the sensor.
Orders of magnitude of calibration errors
A calibrated laboratory thermometer has an error of less than 0.1 degrees C. A smartphone accelerometer has a typical error of 1 to 3% of the measured value. A smartphone barometric pressure sensor has an accuracy of approximately plus or minus 1 hPa after factory calibration.
Formula
Calibration error is calculated by the difference between the measured value and the reference value:
E = V_measured - V_reference
Meaning: E: calibration error (in the unit of the measured quantity) V_measured: value displayed by the instrument V_reference: value given by the reference standard
Application examples
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Verification of a kitchen scale with a 500 g calibration weight
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Checking a stopwatch by comparing it to an atomic clock via internet
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Verification of a thermometer in an ice-water mixture at 0 degrees C
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Testing an automobile speedometer on a road section of known length
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Comparison of a household barometer with local weather station data
FAQ
Q: What is the difference between calibration and adjustment? A: Calibration compares the instrument to a standard without modifying it. Adjustment goes further by adjusting the instrument to correct observed deviations.
Q: What standard to use in class? A: Simple references can be used: ice water at 0 degrees C for temperature, marked weights for mass, verified graduated ruler for lengths.
Q: Is a calibrated instrument necessarily accurate? A: Calibration reveals errors but does not correct them. The identified corrections must then be applied, or the instrument must be adjusted.
Q: Should a smartphone be calibrated? A: Smartphone sensors are factory-calibrated, but calibration in class allows verifying their reliability and learning the scientific approach.
Q: Does calibration eliminate all errors? A: No. Calibration identifies systematic errors, but random errors (noise) persist and must be addressed by statistical methods.
Related concepts
Adjustment - Uncertainty - Precision - Systematic error - Metrology - Standard - Accuracy