Use of pressure transmitters to monitor fluid level is actually a very popular technique. Rivers, reservoirs, wells, tanks, or other fluid containers, can be monitored by measuring the hydrostatic pressure.
When a hydrostatic pressure transmitter is positioned in a vessel, it measures the pressure caused by the weight of the liquid column vertically above it. For an accurate reading, the transmitter needs to be located at the lowest point of the vessel, typically mounted or laying on the bottom, known as zero level. The measuring cell of the pressure sensor detects small changes in hydrostatic pressure, which increases or decreases according to the filling level. As their installation point becomes the reference for zero level, vessels with unknown depths can be analysed and monitored, making hydrostatic pressure transmitters ideal for measuring resources such as underground water reservoirs, rivers and lakes, as depths can vary heavily, especially in cases of drought, heavy rain or floods. Hydrostatic level measurement is unaffected by the size and shape of the vessel due the hydrostatic paradox (that is, the pressure at a certain level in a liquid is proportional to the vertical distance to the surface), but the specific gravity (or the liquid density) should always be factored in to calculate the level of liquid.
In most applications the liquid is subjected to atmospheric pressure. Readings are commonly compensated by using a vented cable which allows the sensor to adjust to any changes in atmospheric (or barometric) pressure. This is achieved via a small nylon tube that runs through the cable and is particularly important for the measurement of low levels of liquid, where changes in atmospheric pressure can significantly affect the true reading of the level. For example, at high altitude where low atmospheric pressure is present.
The ESI range of hydrostatic pressure transmitters has become an increasingly popular solution for level measurement as a result of its simple efficiency and high durability in unsteady process conditions. The products are characterised by their media resistance. Typically manufactured from 316L stainless steel with polyurethane jacketed vented cable, they are suitable for immersion in most media. Other materials such as plastic, titanium and Hastalloy C or nylon and PTFE cable may be used to ensure compatibility with whatever medium is being measured. The moulded electrical connection ensures that the medium does not enter the unit itself, even at submersion depths of several hundreds of metres.
The advantages of using ESI Hydrostatic pressure transmitters are that the vessel does not need to be disturbed or adapted. The sensor is simply lowered in to the media and converts the acting pressure into an electrical signal. Most users will typically require a 4-20mA output, enabling signals to be transmitted over long distances. Cable can be supplied in continuous lengths of up to 500m.
In most applications the sensor can be connected to a system via a simple IP67 rated terminal box, with glands to prevent water ingress. In humid applications, where moisture may potentially enter the vented tube, it is advisable to connect via a desiccant chamber. The desiccant absorbs moist air, preventing it from travelling to the sensing element and causing a short circuit.
Hydrostatic level transmitters can be permanently immersed in boreholes for measuring the depth of the water table for example. The water table is the underground depth at which the ground is totally saturated. Deep boreholes with an average diameter of 25mm are drilled and left to fill to the level of the water table. A small diameter submersible pressure sensor is lowered down the borehole to accurately measure the level. The PR3441 and PR3442 models are perfect for this application, with slim diameters from 16mm.
For hazardous applications, such as bore holes, wells, shafts or refineries, ATEX and IECEx certification is available, confirming that the products are safe for use in similar circumstances where there is a risk of explosion. For example, monitoring the water levels in coal seam gas wells.
Most recently, the PR3441 model has gained Marine approval from DNV GL for use on commercial shipping vessels.