Measuring tank levels in wastewater processing is a difficult task to carry out with repeated accuracy and reliability. Liquids containing contamination and suspended solids suggests that the best option for measuring liquid levels is to use a sensor that does not come into contact with the fluid itself, such as Ultrasonics, to try to minimise the potential of false reading caused by contamination build-up or corrosion.

However, the 7014 conductive sensor from Gill Sensors & Controls is challenging the assumption that non-contact is the best answer to the seemingly intractable problem of reliable level measurement.

First we take a quick look at 5 benefits of the conductive sensor and then some of the comparative drawbacks of ultrasonic devices.

Conductive

1. Eliminates false or inaccurate readings caused by residue build-up.
   By having a solid, cavity-free sensing probe with a homogenous non-stick FEP coating, the sensor does not suffer from residue build-up in sludge and foul water leading to inaccurate liquid level readings.
2. No periodic cleaning or maintenance required.
   With no moving parts and no holes or cavities to block, there are not any locations allowing build-up of residue and solids. This means that accurate and dependable level measurement can be delivered without the need for scheduled cleaning or maintenance.
3. Provides reliable measurement for difficult to measure conductive fluids.
   Unaffected by suspended solids and with ingress protection to IP69K, the 7014 Blackwater sensor will work with any conductive fluids in any installed environment. Mechanically robust enough to be unaffected by stirred or sloshing fluids and suitable for use with corrosive or acidic liquids or additives, the sensor will continue to perform with even the most difficult fluids.
4. Low installed and lifetime cost.
   As a fully integrated sensor with all the electronics built into the sensor the Blackwater sensor is easy to install. Not requiring any additional mechanical support within the measured vessel simplifies installation, keeping the installed costs to the minimum. With minimal maintenance requirements and no parts to wear or corrode the sensor delivers an extended service life with low cost of ownership.
5. Can be installed in all tank types, impervious to build-up on vessel walls.
   By using conductive technology, the sensor does not rely on the tank walls for its measurement function, meaning that its performance is not affected by any build-up or deposits that may accumulate. Supplied with tank profiling functionality, the sensor can be configured for all tank shapes providing a true volumetric output rather than a simple level reading.

Ultrasonic

1. Air temperature affects the accuracy of an ultrasonic sensor.
   Ultrasonic sensors use the speed of sound to measure the time taken for an ultrasonic pulse to travel from the sensor, to the fluid level and back to the sensor. However, as the air temperature changes, the speed of sound changes, by 0.17% per degree Kelvin. Unless the sensor can compensate for this change of temperature, then as the temperature varies, so will the accuracy of the sensor.
2. Strong wind or air currents can cause unstable measurements.
   If your ultrasonic sensor is located in an exposed, outdoor environment or where significant air turbulance occurs, this needs to be considered in sensor selection. When wind speeds rise above the 30 - 38mph range they can cause unstable measurements with a loss of signal. The ultrasound can be scattered or deflected such that no echo that can be measured is received.
3. Turbulence or foaming on the fluid surface can reduce signal strength.
   Ultrasonic sensors rely on the liquid surface to reflect the ultrasound pulse to generate a signal the sensor can process. If, during emptying or filling of the vessel to be measured causes turbulence or foaming on the surface of the fluid, this will scatter the ultrasound to such an extent the echo is not strong enough to be usable and provide a measurement. This fault can also be caused by improper aiming, high dust or sticky build-up on the transducer face.
4. Mounting location can affect the function of the sensor.
   In a parabolic storage tank, if you place an ultrasonic sensor in the middle of the tank, you will encounter problems with secondary echoes. Also you cannot position the sensor too close to the side wall or you could get interference from ladder rungs or support braces. Awareness of where the fill stream is located is also required or this too will obstruct the ultrasound pulse.
5. The sound path must be perpendicular to the measured surface.
   To achieve a reliable signal the transducer must be installed perpendicular to the monitored surface. If you wish to fit the sensor in a standpipe or nozzle there are dimensional and design features that must be taken into account so the ultrasound pulse is compromised in its effectiveness.