A manufacturer of specialised aerial firefighting equipment sought a more durable liquid level sensing solution for their helicopter-mounted water tanks. The existing vented hydrostatic sensors were experiencing reliability issues, with the primary failure point being the submersible cable. The dynamic nature of the tank’s deployment and retraction system was causing the fragile atmospheric vent tube within the cable to be pinched or crushed. A new sensor was required that could withstand this mechanically demanding environment while providing accurate level measurement in tanks containing fresh water, seawater, and chemical fire retardants.
- Application: Continuous level measurement in helicopter-mounted aerial firefighting tanks.
- Problem: The existing sensor’s vented cable was susceptible to crushing during tank operation, leading to sensor failure.
- Requirement: A submersible hydrostatic level sensor with a highly robust, crush-resistant cable, ideally with a diameter around 9 mm.
- Solution: The LMK387 submersible level transmitter, configured with a heavy-duty 9.0 mm diameter TPE-U sheathed cable and a robust ceramic diaphragm.
A specialist manufacturer of aerial firefighting systems for helicopters required a highly reliable method for measuring the liquid level within their deployable water tanks. The application presents a unique set of challenges, combining harsh media exposure with significant mechanical stress. The tanks are filled with various liquids, including fresh water from lakes and rivers, corrosive seawater, and chemical fire retardants. Furthermore, the operational environment is extremely dynamic; the entire tank system is subject to the rapid movements and vibrations of the helicopter, and the sensor itself must endure the pressure transients from fast filling and dumping cycles.
The central issue confronting the engineering team was the premature failure of their existing vented hydrostatic level sensors. While the sensors provided adequate accuracy, their standard 7.5 mm diameter polyurethane (PUR) cable proved to be a critical vulnerability. The integral capillary tube, which provides an atmospheric pressure reference for the gauge pressure sensor, was frequently being pinched or crushed during the tank’s operational cycles. This damage would block the vent path, leading to inaccurate readings and eventual sensor failure, compromising mission-critical data. The manufacturer identified the need for a replacement sensor with a fundamentally more robust cable design, similar to a successful 9 mm cable upgrade they had implemented on other instrumentation.
The proposed solution was the LMK387 series submersible level transmitter. While this sensor offers a range of features, the key deciding factor was its availability with an optional heavy-duty, 9.0 mm diameter cable protected by a tough Thermoplastic Elastomer (TPE-U) sheath. This significantly thicker and more durable cable construction provides superior mechanical protection for the internal wiring and the essential atmospheric vent tube, directly addressing the primary cause of failure in the application. The 5-metre cable length was specified to accommodate the installation and interface with their existing control system.
Beyond the cable, the sensor’s core technology was well-suited to the demanding conditions. The LMK387 features a capacitive measuring principle with a high-purity (99.9%) flush ceramic diaphragm (Al2O3). This diaphragm is exceptionally robust, capable of withstanding the dynamic pressure changes seen during operation. Its material composition, paired with a stainless steel 1.4404 (316L) housing and FKM seals, provides the necessary chemical resistance to handle intermittent exposure to seawater and retardants. The customer confirmed that a daily wash-down protocol made the 316L stainless steel a suitable material choice despite occasional saltwater use. A pressure range of 0 to 0.25 bar, equivalent to 2.5 metres of water head, was selected to cover the tank’s capacity. The sensor’s standard 4-20mA, 2-wire analogue output allowed for straightforward integration into the helicopter’s existing 28Vdc powered instrumentation system.
LMK387 High Overload & Thick Cable Submersible Level Transmitter
- SKU ID: s1-lmk387-2-92938
- Part No: 360-2500-1-1-C-1-1-42-3-005-000
- Pressure Range: 0 to 0.25 bar
- Reference Type: Gauge / Vented (atm to +P)
- Output Signal: 4-20mA, 2 wire
- Accuracy: 0.35 % FSO
- Installation Type: Submersible version with nose cone
- Housing Material: stainless steel 1.4404 (316 L)
- Diaphragm Material: Ceramic Al2O3 99.9%
- Media Exposed Seals: FKM
- Cable Type: TPE-U-cable (red, Ø 9.0 mm)
- Cable Length: 5m