Sensors & Measurement · Study deck
Sensing with Existing Infrastructure
Imagine a school wants a rough room count without fitting a new sensor above each door.
Physics Phoebe is your guide for this deck.
After studying this chapter
Learning objectives
You will be able to:
- Explain: The mathematical gist.: Free-space loss in MHz is $20\log_{10}d+20\log_{10}f_{MHz}-27.55$, so solving for distance gives $d=10^{(27.55-20\log_{10}f_{MHz}-RSSI)/20}$.
- Explain: By watching the power meter, a computer can figure out which devices are turned on -- like identifying people by their footsteps!".
- Explain: A strain gauge with 0.01% annual drift accumulates significant error over 10-20 years -- enough to mask meaningful structural changes.
- Explain: Installing dedicated occupancy sensors in every room (2,400 rooms) would cost over $360,000.
Major section
A Clear First Route
The team must decide whether signs from its Wi-Fi system are good enough for that job.
- This page starts with one job.
- Last, choose reuse the old system, add a new sensor, or use both.
- A clue can track the wrong cause.
- A busy network may reflect software work rather than a busy room.
Major section
A Clear First Route (continued)
This first route is a guide to the main choice.
- The Practitioner sections add signal features, site tests, return on cost, and worked estimates.
- Under the Hood adds radio path models, drift, false causes, and code used for deeper checks.
- They do not reverse its main claim.
Major section
Start With the Measurement Story
Sometimes the best sensor is infrastructure that already exists: Wi-Fi, cameras, traffic loops, phones, meters, or building systems.
- The story begins by asking what proxy signal can be trusted and where it can mislead.
Major section
In 60 Seconds
This approach can reduce deployment costs by 10-100x while providing area-wide coverage.
- The mathematical gist.: Free-space loss in MHz is $20\log_{10}d+20\log_{10}f_{MHz}-27.55$, so solving for distance gives $d=10^{(27.55-20\log_{10}f_{MHz}-RSSI)/20}$.
Major section
Use Existing Infrastructure
Traditional IoT thinking: "We need temperature data -> Deploy temperature sensors.".
- Infrastructure-leveraging thinking: "We need occupancy data -> Use existing Wi-Fi routers.".
- The Wi-Fi router becomes a "free" presence sensor.
Major section
Design Guidelines
"What do you mean?" asked Temperature Terry.
- "Think about your Wi-Fi router at home," Max explained. "It sends out radio waves all the time.
- A smart computer can analyze those ripples and figure out that someone walked by, WITHOUT any cameras or motion sensors!".
- The infrastructure does double duty!".
Major section
Design Guidelines (continued)
the LED was amazed: "So the Wi-Fi router IS a sensor?".
- And your electricity meter is a sensor too," Max continued. "Every appliance in your house uses electricity differently.
- A hair dryer uses 1500 watts, a fridge uses 100 watts, a phone charger uses 5-25 watts.
- By watching the power meter, a computer can figure out which devices are turned on -- like identifying people by their footsteps!".
Major section
Campus Occupancy Monitoring
Installing dedicated occupancy sensors in every room (2,400 rooms) would cost over $360,000.
- Each Cisco AP reports associated client count every 60 seconds via SNMP.
- Key correction factor: Students carry 1.2-1.8 devices on average (phone + laptop).
- The infrastructure approach delivers the same HVAC savings at 93% lower capital cost.
Major section
Common Pitfalls
A single strain gauge only measures strain at its exact location.
- Infrastructure monitoring requires sensor arrays with spacing matched to the expected damage scale.
- Infrastructure sensors are often installed for 10-20 year lifespans.
- A strain gauge with 0.01% annual drift accumulates significant error over 10-20 years -- enough to mask meaningful structural changes.
Deck summary
Key takeaways
The team must decide whether signs from its Wi-Fi system are good enough for that job.
- This first route is a guide to the main choice.
- Sometimes the best sensor is infrastructure that already exists: Wi-Fi, cameras, traffic loops, phones, meters, or building systems.
- This approach can reduce deployment costs by 10-100x while providing area-wide coverage.
- Traditional IoT thinking: "We need temperature data -> Deploy temperature sensors.".
Retrieval practice
Recall check 1 of 4
Physics Phoebe says: answer from memory, then check your reasoning.
Q1A smart building wants to detect occupancy in 200 conference rooms. Which approach has the lowest total cost?
Show answer
Answer: B Wi-Fi infrastructure is already deployed for internet connectivity.
Retrieval practice
Recall check 2 of 4
Physics Phoebe says: answer from memory, then check your reasoning.
Q2A university wants to monitor building occupancy across 50 buildings to optimize HVAC energy use. Which approach would be most cost-effective?
Show answer
Answer: C Wi-Fi access point analysis leverages infrastructure that already exists in every building.
Retrieval practice
Recall check 3 of 4
Physics Phoebe says: answer from memory, then check your reasoning.
Q3Non-Intrusive Load Monitoring (NILM) identifies appliances by analyzing power consumption. Which appliance signature would be easiest to detect?
Show answer
Answer: A A 1500W hair dryer creates a large, unmistakable power signature -- a sudden 1500W spike when turned on, and a 1500W drop when turned off.
Retrieval practice
Recall check 4 of 4
Physics Phoebe says: answer from memory, then check your reasoning.
Q4Place each infrastructure-leveraging sensor where its evidence lives so you can judge which existing system can act as a trustworthy proxy.
Show answer
Answer: A Separate wireless signals, energy infrastructure, and civic acoustic or transport evidence so you can reuse installed systems while keeping each proxy's limits visible.
Q5Complete the WiFi RSSI-based proximity sensing:
Show answer
Answer: A WiFi RSSI can estimate distance using the log-distance path loss model.
Print reference
Answers 1 of 2
Answer key.
- B · Wi-Fi infrastructure is already deployed for internet connectivity.
- C · Wi-Fi access point analysis leverages infrastructure that already exists in every building.
- A · A 1500W hair dryer creates a large, unmistakable power signature -- a sudden 1500W spike when turned on, and a 1500W drop when turned off.
- A · Separate wireless signals, energy infrastructure, and civic acoustic or transport evidence so you can reuse installed systems while keeping each proxy's limits visible.
Print reference
Answers 2 of 2
Answer key.
- A · WiFi RSSI can estimate distance using the log-distance path loss model.