Picture a greenhouse team deciding when to water one bed.
Physics Phoebe is your guide for this deck.
sensortypesselection
After studying this chapter
Learning objectives
Sensor selection starts with the physical measurement and decision requirements.
Define measurand, range, and decision threshold.
Reject candidates that fail installed accuracy, environmental, interface, or power requirements.
Compare surviving candidates by lifetime cost and documented evidence.
Follow the systematic three-step selection process from requirements to evidence.
Major section
Start With the Measurement Story
A precise catalog number is not installed proof.
The useful question is not which sensor has the longest feature list.
A gateway is the local device or service that joins sensor data to a wider system.
This opening does not name one best sensor or score.
The winning part is the one that still supports the decision.
Major section
Field Conditions Drive Selection
The service path sets limits for battery life and access.
The 80 m cable or radio path adds a measured link-margin test.
A probe can be correct on a bench and still be the wrong choice.
It fails if it corrodes, drains a battery that staff cannot reach, or cannot send readings from its final place.
Soil-moisture sensor field cross-section showing dry topsoil, wet saline soil, probe corrosion, battery service route, gateway distance, and requirement cards.
Major section
Putting Numbers to It
For DS18B20, the project weights are accuracy 9, waterproofing 10, sleep current 7, cost 5, and humidity 3.
Its 5µA typical idle current must not be called a proven battery-life failure or pass.
A weighted score does not override a failed accuracy requirement.
Weights encode your priorities, so changing a weight changes the ranking.
Major section
For Kids: Meet the Sensor Squad!
A sensor assembly inherits only the limits supported by its full installed evidence.
A sealed DS18B20 probe assembly may resist water; the bare chip is not waterproof.
The stated ±0.5 °C accuracy applies only over its specified temperature band.
Check full-system power and installed assembly before selection.
Major section
Nest Protect Sensor Selection
A product example does not validate another component or design.
Production example: Nest Protect combines several sensing functions.
Review the proposed carbon-monoxide sensor’s own datasheet and lifetime.
Keep safety evidence tied to the selected part and installation.
Production example: Nest Protect combines several sensing functions. This product example does not validate a proposed CO sensor; review that part’s own datasheet, lifetime, and safety evidence.
Deck summary
Key takeaways
A precise catalog number is not installed proof.
The service path sets limits for battery life and access.
For DS18B20, the project weights are accuracy 9, waterproofing 10, sleep current 7, cost 5, and humidity 3.
But there were SO many choices!
Sensor selection decisions that work at prototype scale can fail catastrophically at production volumes.
Retrieval practice
Recall check 1 of 4
Physics Phoebe says: answer from memory, then check your reasoning.
Q1A cheap sensor needs frequent field recalibration, while a dearer one stays within tolerance longer. What belongs in the purchase review?
ACompare unit prices alone, excluding recalibration labor and downtime.
BCompare references, labor, downtime, interval and lifetime error.
CCompare only the largest printed number on each sensor datasheet.
DSkip recalibration for both candidates because their factory settings meet the required tolerance.
Show answer
Answer: B Purchase price alone does not show the cost or reliability of keeping readings within tolerance.
Retrieval practice
Recall check 2 of 4
Physics Phoebe says: answer from memory, then check your reasoning.
Q2When comparing sensors, should you design based on "typical" or "maximum" accuracy specifications?
ATypical accuracy, because most samples may meet that figure.
BGuaranteed worst-case accuracy, so the design has margin.
CA midpoint between typical and worst-case accuracy figures.
DEither figure because the difference falls within the decision's useful precision.
Show answer
Answer: B Use a guaranteed worst-case accuracy limit under the intended operating conditions where the datasheet supplies one, then validate the installed system.
Retrieval practice
Recall check 3 of 4
Physics Phoebe says: answer from memory, then check your reasoning.
Q3For an outdoor station whose required temperatures stay inside the DS18B20's specified ±0.5 °C band, which listed assembled option should be checked for water resistance and installed accuracy?
ADHT22 -- it has +/-0.5C accuracy and measures humidity too
BBME280 -- it measures temperature, humidity, and pressure
CSHT31 -- it has the best accuracy at +/-0.3C
DA DS18B20-based sealed probe, if its selected assembly and temperature range meet the requirements.
Show answer
Answer: D This does not qualify it for the separate −20 °C requirement.
Retrieval practice
Recall check 4 of 4
Physics Phoebe says: answer from memory, then check your reasoning.
Q4What error budget determines whether candidate sensor readings are useful for the measurement need?
AEvaluate Accuracy Needs
BDefine Measurand & Range
CCheck Interface Compatibility
DVerify Constraints (Cost/Environment)
Show answer
Answer: A Its error budget decides whether readings are useful.
Q5Complete the sensor auto-detection and configuration:
Afound = i2c.scan()
Bfound = i2c.detect()
Cfound = i2c.list_devices()
Dfound = i2c.enumerate()
Show answer
Answer: A A lookup dictionary maps I2C addresses to sensor names.
Print reference
Answers
Answer key.
B · Purchase price alone does not show the cost or reliability of keeping readings within tolerance.
B · Use a guaranteed worst-case accuracy limit under the intended operating conditions where the datasheet supplies one, then validate the installed system.
D · This does not qualify it for the separate −20 °C requirement.
A · Its error budget decides whether readings are useful.
A · A lookup dictionary maps I2C addresses to sensor names.
With the corrected answer frame visible, compare the selection decisions. Lifetime service cost matters alongside purchase price. Use guaranteed limits at the required conditions. A sealed probe must be checked as an assembly. Start selection with the physical measurement, then accuracy, interface, and deployment constraints. A bus scan finds addresses; it does not approve a sensor.
