This application involves the monitoring of wastewater discharge within a geothermal power generation facility. The primary requirement is to measure the liquid level within a weir box to calculate water flow rates. The application presents specific challenges including very low hydrostatic pressure ranges, requiring high sensitivity, and the presence of geothermal byproducts which necessitate a robust sensor design to prevent clogging and corrosion. The selected solution utilizes a screw-in hydrostatic level transmitter featuring a capacitive ceramic sensor and a flush diaphragm to ensure long-term stability and accuracy in polluted media.
The client requires a measurement solution for a geothermal water discharge system located at a power generation facility. The specific application involves measuring the hydraulic head (water level) inside a weir box. In open channel flow applications, a weir, typically a V-notch or rectangular cut-out—is used to obstruct flow, creating a pool of liquid upstream. By measuring the height of this liquid above the weir crest, the volumetric flow rate can be accurately computed using standard hydraulic formulas.
The technical requirements for this installation are particularly demanding due to the extremely low level range involved. The maximum height of the liquid to be measured is only 100 millimetres (approximately 10 mbar of pressure). Accurately resolving level changes within such a small span requires a sensor with exceptional sensitivity and signal stability, as standard piezoresistive sensors often struggle with signal-to-noise ratios at such low pressures. Furthermore, the medium is identified as wastewater from a geothermal source. This implies the water likely contains suspended solids, minerals capable of causing scaling (such as silica), and potentially corrosive elements. The sensor must be externally mounted via a threaded connection to integrate into the existing weir box infrastructure, and it must output a standard 4-20mA signal for integration with the plant’s flow computer or PLC.
To address the challenges of measuring low-level geothermal wastewater, the LMK 351 low range wastewater level transmitter was selected as the optimal solution. This device is specifically designed for measuring small system pressures and performing level measurements in containers. The core advantage of this transmitter lies in its own-developed capacitive ceramic sensor element. Unlike standard sensors, capacitive ceramic technology offers superior performance and higher overpressure capability in very low pressure ranges, making it ideal for the client’s minute 0-100mm (scaled to 0-400 mmH2O) measurement requirement.
A critical feature for this geothermal application is the mechanical construction of the process connection. The LMK 351 is supplied with a G 1 1/2″ pressure port that facilitates a flush mounted sensor. Usage in viscous, pasty, and polluted media is made possible because of this flush mounting, which eliminates cavities where geothermal sludge or mineral deposits could accumulate and block the sensor face. The diaphragm itself is constructed from 99.9% Aluminum Oxide (Al2O3) ceramic, which provides excellent chemical resistance and mechanical robustness against the abrasive nature of wastewater.
For the electrical interface, the solution includes a specialized vented cable. Since the measurement range is so low, variations in atmospheric pressure could significantly skew the reading. The vented cable ensures the back of the sensor element is referenced to ambient atmospheric pressure, cancelling out barometric changes. The unit is sealed with FKM seals to ensure compatibility with the aggressive nature of the media, and the housing is rated for environmental protection, suitable for the humid conditions typical of weir box installations in environmental engineering. The 2-wire 4…20 mA output signal ensures seamless integration with the client’s control system for continuous flow monitoring.
LMK351 Screw-In Low Range Wastewater Level Transmitter
- SKU ID: s1-lmk351-0009
- Part Number: 471-0400-1-3-TR0-M00-1-1-C-000
- Pressure Range: 0 – 400 mmH2O
- Output Signal: 4-20mA, 2 wire
- Accuracy: 0.35% FS
- Electrical Connection: PUR cable outlet, with vent tube (IP 68)
- Cable Length: 2m of cable
- Process Connection: G 1 1/2″ male DIN3852 with flush diaphragm
- Process Connector Material: Stainless Steel
- Diaphragm Material: Ceramic Alumina Al2O3 99.9%
- Media Exposed Seals: FKM