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SensingDesign complete (stages 1–6, rev A), not built, not validated

HeatFlag

One heat colour the whole crew can see, and a heat log that writes itself

A solar pole station measures WBGT and heat index on site, shows the whole crew one colour on a pole light (green / yellow / red / blue = stop heavy work), and writes the heat log with readings, alerts, acknowledgements and QR-logged breaks. Advisory only: early warning and evidence, not a safety or compliance certification.

Who buys it
Safety manager or owner at a mid-size construction, roofing, landscaping or solar subcontractor in TX, CA and the Southwest (50–500 field staff).
Why it pays
For 3 sites and 60 workers: about $4k/yr in hard dollars (avoided claims, serious-event and citation expected value) and about $17k/yr including productivity and supervisor time (est.), against $1,047 per season.
Target price
$349 per station per heat season (May–Oct) rental, all-in; or $899 + $15/month in season (target)
First-minute wow
One light the whole crew can see from anywhere on site, and a break log that fills itself in.
See the CAD designConcept page
HeatFlag concept: the idea and where each part goes
The concept. The CAD design is below, under Printable CAD.

What the research found

And how this design answers it

  • US workplace rules trigger on heat index or air temperature, not WBGT (school athletics rules do use WBGT; see FieldFlag) → logs both every minute and flags on the stricter of the state rule-set and the WBGT work/rest level
  • Cheap sensors make WBGT inaccurate → standard 150 mm copper globe with PT1000, SHT45 in an 8-plate shield, ultrasonic wind; accepted only at MAE ≤ 1.0 °C vs a Kestrel 5400
  • LiFePO4 can't charge above 45 °C in Texas sun → shaded, chimney-vented battery bay and a charger NTC cut-off
  • Theft on open sites → cable lock through every clamp, security screws, tilt and lid tamper alerts, GNSS tracking and a 150 m geofence
  • Liability if a worker collapses → "advisory only" everywhere, raw readings and rule-set version logged with each flag, hash-chained daily logs
  • Concept extras (tinyML, LoRaWAN, e-ink, siren) → cut; blue = stop plus the beacon buzzer

Use case

Heat rising from yellow to red, with breaks enforced

CrewSupervisorSafety managerHeatFlag cloud

  1. 01Trigger: at 10:40 WBGT reaches 25.3 °C for two 1-minute samples and the beacon flashes yellow, with an SMS to the supervisor
  2. 02Detection: at 12:55 WBGT reaches 28.4 °C, the flag goes red and the app starts a 15-minute-per-hour break clock
  3. 03Alert: the beacon chirps at each break; crew scan the QR placard, and a break more than 10 minutes overdue texts the supervisor (20 minutes: the safety manager too)
  4. 04Action: at 15:05 WBGT reaches 31.6 °C and the flag goes blue: stop heavy work

Outcome: Every flag change, acknowledgement and break is logged with the readings that caused it; about $3.5k/yr in avoided claims and serious-event expected value per 60 workers (est.)

How it works

Architecture

  1. 01

    Sense: SHT45 in a radiation shield, PT1000 in a 150 mm black globe, Renke ultrasonic wind at 1.7 m

  2. 02

    Decide: device computes heat index and an approximate WBGT; the cloud computes Liljegren WBGT and the authoritative flag

  3. 03

    Send: nRF9151 LTE-M, MQTT over TLS with CBOR payloads, 7-day on-device buffer and back-fill

  4. 04

    Tell: Patlite 4-tier beacon + buzzer, SMS/push to supervisors, QR break pages, PDF/CSV log export

Hardware

Real parts, buyable today

Bill of materials: about $821 per pilot station including the $198 Patlite and $80 globe; volume target ≤ $300 (est.).

BlockPartWhy this part
Compute + radioNordic nRF9151 SiP (LTE-M/NB-IoT + GNSS)Pre-certified cellular plus GNSS for theft tracking
Air temperature / RHSensirion SHT45 in a printed 8-plate shieldBenchmarked against a Davis 7714 shield
Globe temperaturePT1000 + MAX31865 in a 150 mm copper globeStandard globe size, so no small-globe correction
WindRenke ultrasonic anemometer (RS485 Modbus)No bearings; wind dominates natural wet bulb
BeaconPatlite LR6 4-tier + buzzer, TPS55340 24 V boostOne colour visible from anywhere on site
Power20 W panel, BQ24650 MPPT, 12.8 V 12 Ah LiFePO4About 22 Wh/day budget and about 5.6 days autonomy (est.)

Mechanical

Printable CAD

17 parts, generated from code and checked as closed solids. Pick a part to inspect it, or download the STL files to print.

See the CAD parts render
HeatFlag printable parts, rendered from the CAD
Render of the printable parts from the actual CAD. Not yet printed or photographed.

Loading the CAD…

Real CAD · drag to rotate · scroll to zoomReal CAD · swipe sideways

Open risks we are working on

  • WBGT accuracy is unproven until the 3-day Kestrel side-by-side test passes (MAE ≤ 1.0 °C)
  • On 43 °C days the battery bay may pass 45 °C at midday and charging pauses
  • Patlite current and polarity are unverified, and they size the power budget and driver
  • Kestrel or Perry could copy it; the edge is speed, the beacon, the break log and price
  • Pilot BOM of about $821 needs a volume path to ≤ $300 for margin
  • Thresholds not yet checked against the current ACGIH booklet; carrier certification cost and product liability insurance open

Design stage only: nothing on this page is for sale. Specifications and prices are targets.