Fire Pump Room Tank Monitoring: 26 GHz Radar and LoRa Level Transmitter Case Study
2026/10/27
The site is a fire pump room serving a logistics warehouse. The fire water tank is a concrete tank of 4 m depth, located in a separate tank room next to the pump set, with a fire control room 80 m away. The tank room is unheated, condenses water on every surface in summer, and the tank top is a closed concrete slab with a 600 mm inspection opening. Insurance and local code require the tank level to be monitored continuously and a low-level signal to reach the fire alarm panel.
| Field parameter | Typical value | Engineering consequence |
|---|---|---|
| Tank depth | 3.5–4 m usable | Range is short; dead zone and mounting height dominate the design |
| Tank top | Closed concrete slab, 600 mm opening | Sensor must be installed and serviced through a small opening |
| Room environment | Unheated, high condensation | Lens fogging and electronics corrosion are the main failure modes |
| Signal path | Tank room to fire control room, 80 m, two concrete walls | Wired path needs conduit through a fire-rated wall; wireless must pass two walls |
| Regulatory need | Low-level alarm + continuous indication | Dry contact output to the fire panel is mandatory, not optional |
- A mechanical float valve told operators nothing about the actual water volume; the first sign of a problem was a pump suction alarm.
- The fire panel needed a volt-free contact, but the first transmitter supplied only 4–20 mA, so an extra module had to be bought later.
- Condensation on the antenna lens caused a "lost echo" fault twice in the first rainy season.
- Battery devices were considered but the site did not want a maintenance routine that depended on battery replacement discipline.
The design target was a continuous level reading with a hard-wired low-level contact into the fire panel, plus a wireless link to the building management system as a secondary path. The chosen solution is a 26 GHz radar level transmitter in the inspection opening, 24 V DC loop powered, with two programmable relays, plus a LoRa wireless node powered from the same 24 V supply.
| Selection item | Specified value | Reason |
|---|---|---|
| Principle | 26 GHz non-contact radar | Unaffected by the water surface film and by condensation on the tank walls |
| Range | 0–6 m, dead zone 0.25 m | Covers the tank with margin and allows mounting above the maximum fill level |
| Outputs | 4–20 mA + 2 relays (low, low-low) | Relays drive the fire panel directly; the analog signal goes to the BMS |
| Power | 24 V DC from the fire panel supply with 2 h battery back-up | Fire systems must survive a mains failure |
| Protection | IP67 housing, IP68 antenna seal | Condensing, occasionally hosed tank room |
| Wireless | LoRa 470 MHz node, external antenna | Passes two concrete walls at low power; no SIM card cost |
- The bracket is fixed to the underside of the concrete slab through the inspection opening, with the antenna 300 mm above the highest fill level and clear of the inlet pipe stream.
- Relay 1 is set to the low-level alarm, relay 2 to the pump cut-out level; both are fail-safe (de-energised on fault).
- The 4–20 mA loop is wired in a separate conduit from the pump power cables to avoid induced noise during pump start.
- The LoRa node is mounted on the outer wall of the tank room with the antenna above the door line, and its signal is verified at the BMS cabinet before commissioning is signed off.
- The fire panel now receives a hard-wired low-level alarm and the BMS receives continuous level, so refilling is planned instead of reactive.
- Antenna condensation faults were eliminated by the larger 26 GHz antenna and a lens that is wiped during the annual service.
- Because the relays are fail-safe, a cable break shows up as an alarm rather than as a healthy reading — the failure mode is visible.
- No SIM card or data subscription is needed for the wireless path, which keeps the running cost at zero.
Is a wireless sensor acceptable for a fire system? Use wireless for indication and trending; the alarm contact should be hard-wired. Regulators and insurers expect the alarm path to be independent.
Can the sensor be mounted through a small opening? Yes, but check the antenna diameter against the opening and leave 300 mm clearance above the highest water level.
- Confirm whether the fire panel needs a volt-free contact, a 4–20 mA input, or both, before choosing the transmitter version.
- Specify fail-safe relay logic in the purchase order.
- Ask for the tank drawing with the inspection opening position — mounting is the usual cause of a failed first installation.
- Require IP67 housing and a sealed antenna for condensing rooms.
