← ACARA Digital Technologies Years 9–10

Sensor errors: faults, calibration and noise

A reading that is wrong due to a faulty or uncalibrated sensor or electrical noise, so check for stuck values, impossible spikes and offsets against a known value.

Part of: ACARA Digital Technologies Years 9–10 — Acquiring and Validating Data — Module 37

Understanding Sensor errors: faults, calibration and noise in ACARA Digital Technologies Years 9–10

What it does: Ice water is known to be 0 degrees. Subtracting the sensor's reading from 0 gives the offset to correct all later readings. This IF formula marks any temperature below -10 or above 50 for checking, which catches impossible spikes. A real temperature drifts a little over time. A value that never changes suggests the sensor is faulty or frozen, so the reading should be checked. 412 degrees is impossible for outdoor air. It is probably electrical noise or a fault, so flag it and check before using it. Electrical noise is random interference in the signal, often from other electronics, and it can cause sudden unreliable jumps in readings.

Common mistakes: A stuck sensor repeats one value, so the set of unique readings has exactly 1 item. The check should be len(unique) == 1, not 4. The limit is set far too high, so the impossible 480 reading would not be flagged. It should be a realistic maximum such as 50.

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This module's key terms are also in a printable crossword (Modules 255–329) and word search (Modules 226–329).

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Learning
Duo
A temperature sensor in a classroom shows exactly 21.4 for six hours straight, even though the room warmed up in the afternoon. What is the most likely cause?
A stuck reading from a faulty sensorThe room temperature really stayed identical
Matches
Sensor reads 3 degrees in ice water, so no action is needed because sensors are always accurate
↓
Assuming a sensor is always accurate. Ice water should read about 0, so the sensor has an offset and needs calibrating.
Applying
Meanings
Term
sensor error
Meaning
A reading that is wrong due to a faulty or uncalibrated sensor or electrical noise, so check for stuck values, impossible spikes and offsets against a known value.
Practice
=IF(MAX(B2:B8)>50,"Spike - check sensor","Readings OK")
Testing
Gaps
=(B2:B100)
Check Answer
Spots
2unique = set(readings)
3stuck = len(unique) == 4

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Quick Questions

1. A temperature sensor in a classroom shows exactly 21.4 for six hours straight, even though the room warmed up in the afternoon. What is the most likely cause?
A. A stuck reading from a faulty sensor ✓
B. The room temperature really stayed identical
A real temperature drifts a little over time. A value that never changes suggests the sensor is faulty or frozen, so the reading should be checked.
2. A school weather station logs 22, 23, 23, 412, 24. What should you do with the 412?
A. Flag it as an impossible spike and check it ✓
B. Keep it because every reading is automatically correct
412 degrees is impossible for outdoor air. It is probably electrical noise or a fault, so flag it and check before using it.
3. What does calibrating a sensor against a known value mean?
A. Taking more readings and averaging them without a reference
B. Comparing its reading to a trusted reference and adjusting any offset ✓
Calibration uses a known value, such as ice water at 0 degrees C, to find and fix a consistent offset in the sensor.
4. A thermometer is placed in ice water, which is about 0 degrees C, but it reads 3.2 degrees C. What does this show?
A. The sensor has an offset and needs calibrating ✓
B. The sensor is perfectly accurate
A known value of 0 degrees C compared with a reading of 3.2 shows a consistent offset of 3.2, which calibration can correct.
5. Which is a cause of electrical noise in sensor data?
A. The student typing the data into a spreadsheet correctly
B. Interference from nearby electrical equipment causing random jumps ✓
Electrical noise is random interference in the signal, often from other electronics, and it can cause sudden unreliable jumps in readings.

Match the Pairs

22.4, 22.4, 22.4, 22.4, 22.4, 22.4 → A stuck reading: the value never changes, which suggests a faulty sensor or a frozen logger
21.8, 22.0, 150.3, 22.1, 21.9 → An impossible spike: one value far outside what is possible, likely caused by electrical noise or a fault
Calibrate a sensor by comparing it with a known value and working out the offset → offset = 0.0 - reading_in_ice_water
Flag readings that fall outside the range the sensor can possibly give, as a check for spikes → =IF(OR(B2<-10,B2>50),"Check","OK")
Sensor reads 3 degrees in ice water, so no action is needed because sensors are always accurate → Assuming a sensor is always accurate. Ice water should read about 0, so the sensor has an offset and needs calibrating.
A sudden reading of 999 is kept because it is real data and must never be questioned → Keeping an impossible spike without checking it. Readings that are out of range may be noise or a fault and should be checked.

Key Term

sensor error
A reading that is wrong due to a faulty or uncalibrated sensor or electrical noise, so check for stuck values, impossible spikes and offsets against a known value.

Fill in the Blanks

1. =MAX(B2:B100)
Hint: Type the function that returns the highest reading, so an impossible spike stands out.
2. =IF(B3=B2,"Stuck","OK")
Hint: Type the comparison operator that checks whether two readings are identical, which can mean a stuck sensor.
3. =B2-E1
Hint: Type the operator that finds the offset between the sensor reading and the known value in E1.
4. =IF(B2>50,"Spike","OK")
Hint: Type the operator that tests whether a reading is greater than the impossible limit.
5. =AVERAGE(B2:B6)
Hint: Type the function that smooths out electrical noise by averaging nearby readings.

Spot the Error

Question 1
A readings = [21, 21, 21, 21]
B unique = set(readings)
C stuck = len(unique) == 4
D print(stuck)
Line C — A stuck sensor repeats one value, so the set of unique readings has exactly 1 item. The check should be len(unique) == 1, not 4.
Question 2
A MAX_TEMP = 5000
B reading = 480
C if reading > MAX_TEMP:
D print("Impossible spike")
Line A — The limit is set far too high, so the impossible 480 reading would not be flagged. It should be a realistic maximum such as 50.
Question 3
A known = 20.0
B reading = 22.5
C offset = reading - known
D corrected = reading + offset
E print(corrected)
Line D — The offset is how far the sensor reads above the known value. To correct the reading you subtract it, not add it: reading - offset.
Question 4
A window = [20.1, 19.9, 20.3
B total = sum(window)
C average = total / len(window)
D print(average)
Line A — The list is missing its closing square bracket, which causes a syntax error. It should end with ]. Averaging a window of readings smooths out noise.
Question 5
A previous, reading = 18, 18
B if reading = previous:
C print("Stuck sensor?")
Line B — A single = assigns a value, but an if test needs == to compare. A repeated reading is compared with == to spot a stuck value.

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