We have long standing expertise and knowledge in using capacitive measurement techniques in liquid dielectric and level sensing applications. Due to our extensive experience of designing customised measurement sensors for use in extreme operating environments, including high temperature and severe vibration, we can provide product developments for applications including motorsport, UAVs, off-road vehicles and industrial markets.
During the design process we use a range of modelling techniques such as Finite Element Analysis (FAE) which enable us to meet our customer requirements and relevant industrial standards.
Dynamic Range – circuit design to allow capacitance measurements over a wide dynamic range.
Temperature Compensation – use of reference circuitry to prevent measurement inaccuracies with temperature variation.
Environmental Robustness – allowing a wide temperature range at the sensor by locating the active processing circuitry remotely from the passive resonant circuitry.
Electrical Robustness – preventing damage to the measurement circuitry from electrostatic discharge (ESD) and ensuring it complies with relevant electromagnetic compatibility (EMC) standards.
Capacitance is the ability of a body to store charge, and is specified as the charge required to generate a potential difference. Capacitance is dependent on the body's geometry and material properties, and therefore any modification of these can be sensed by changes in the capacitance. A typical capacitor will have two conductive plates separated by a non-conductive gap, with larger capacitance being generated with larger plates and a smaller gap. In the case of fixed geometry the capacitance is related to the dielectric constant of the material in the gap between the plates, where a material with a high dielectric constant (e.g. water) will lead to a higher capacitance than a material with a low dielectric constant (e.g. air).
Capacitance is typically measured using resonant circuits; by combining the capacitor to be measured with an inductor or resistor the resonant frequency of the circuit will depend on the capacitance, therefore by measuring this frequency the capacitance can be deduced.
One of the main applications that we have used capacitance technology in is our range of level and dielectric sensors. For a level sensor the geometry of the system is kept constant (typically the conductive plates are in the form of concentric rods), but the dielectric of the material in the gap changes as amount of fluid in the gap changes with the liquid level. Large changes in dielectric constants between typical working fluids (typically around 100:1) and the range of lengths that we need to measure (typically around 2000:1) require a very wide range of capacitance to be measured. Our expertise in this area allows us to achieve this using a single measurement circuit, which in turn reduces cost and increases reliability. Dielectric sensors perform a similar measurement to level sensors, except the change in dielectric properties of the liquid are then being measured - rather than the amount of liquid. It is also useful to be able to measure the dielectric loss factor as the combination of these two measurements improves the specificity of the measurement, with typical applications in contamination and process control. For both liquid level and dielectric sensors the effect of conductivity of the liquid can be made negligible by the use of insulated electrodes.
It is very important that any capacitance measurement circuitry is robust against environmental and electrical interference. At extreme temperatures not only does the sensor need to function but the accuracy of the level sensing cannot be compromised, therefore the measurement circuit is designed to electrically auto-compensate for temperature effects from the components. To function at extreme temperatures the probe itself can be located separately from the sensing circuitry, typically this will lead to measurement errors due to noises and stray capacitance from any cabling. However our design uses high temperature analogue circuitry which is situated on the probe itself, allowing the signal processing for the sensor to be located remotely without introducing extra noise to the measurement. This high temperature analogue circuitry is based on a combination of mechanical and PCB design and component choice, with every part of the design being rigorously tested and successfully operating in the field with a working range of -40°C to +170°C.
Since many sensors are used in dangerous environments it is important that the circuitry is rated to relevant safety standards, we have therefore developed intrinsically safe (IS) rated versions of our capacitance measurement circuitry.
©2014 Gill Research & Development Limited. All rights reserved. Gill Research & Development Limited, Reg No. 3154453. Registered Office: The George Business Centre, Christchurch Road, New Milton, BH25 6QJ