Three different sensing types conductivity, ultra sonic/ ultra sound and capacitive sensing for water.

Theory of operation:

The capacitive sensor generally consists of two probes that act as the plates of a capacitor, and as the medium to be measured fills the space between the two capacitive plates; the capacitance changes due to the change in the dielectric of the medium between the plates. This “capacitor” is a part of an oscillator circuit that governs the frequency of oscillation in the oscillator circuit in the probe and this frequency is dependent on the value of the capacitor. The medium to be measured acts as the dialectic between the plates in the capacitor thereby changing the value of the capacitance and therefore the frequency of oscillation. It is mandatory that the dialectic constant of the medium be different than the dialectic constant of the material between the plates which is almost never a concern; as the dielectric constant of all liquids is different than the dielectric constant of air which is the medium replaced when the liquid rises between the plates. Otherwise there would not be any change in the value of the capacitance. This frequency change is then measured and is then correlated to the level of the material.

Accuracy:

There are several areas that can introduce errors into the accuracy of the measurement. 1-The probe must be calibrated for the characteristics of the medium being measured This is usually not an issue since good capacitive probe sensors have incorporated into their design a method of calibration for the particular medium being measured. 2-The dialectic constant of the material must not change over time or temperature. Unless the medium is a very exotic material, the change in the dielectric constant over time and/or temperature is very slight. Otherwise changes in the measured frequency will occur and give an error in the level measured. 3-If high accuracy is needed (smaller than a few percent), then high precision in the frequency measurement is required. A well-designed sensor assembly can easily achieve accuracies to less than 5%. 4-The change in frequency is not a linear function so the level will increasingly become more sensitive to errors at one end of the measurement range. Again with proper design these areas of potential error can be minimized so that a accurate assembly can be produced.

Applications:

This sensing method is used primarily to measure and demonstrate continuous graduations in liquid heights. For example, a range that indicates a change in level with 5% accuracy can easily be achieved. By using the industry standard of a 4 to 20mA output signal, interference from the surrounding industrial environment can be eliminated. This also provides an interface output that can be customized to the individual’s specific needs. This sensor type has particular use in the measurement of liquids that are corrosive; either acidic or caustic; or even flammable in nature. The probes (capacitive plates) can be manufactured from materials, or have protective coating applied that will resist any environment in which they are placed. With the proper selection of probe materials, environments that are not only corrosive in nature but also at extreme temperatures can be manufactured. Capacitive sensors are useful in confined areas as well.

Some areas that may find applications for capacitive probes are:

1-Monitoring of the levels in tanks that contain acidic or caustic liquids.
2-Maintaining required levels by interfacing the output of the probe to controlling pumps and/or valves.
3-Providing measurements of levels for inventory requirements.
4-Making measurements in environments that would preclude the use of other measurement methods.

Advantages:

The capacitive sensor is very useful for harsh liquid environments. It can also be mounted within a small volume and thereby be used in areas that are restrictive in space availability. Generally the probes will be sealed against the environment to which they will be exposed, thus making them useful in mediums that would otherwise corrode or render other sensor types useless. The design of the capacitive probe sensor can be such that the output can be of any form that meets the requirements of the end user. They can be designed in a modular form so that when maintenance is required, it is easily accomplished at relatively low cost.

Disadvantages:

The capacitive sensor relies on the change of frequency to determine the liquid level. Since the dialectic constant is different for most liquids, a means of calibrating the sensor must be supplied and the sensor must be calibrated for each different liquid being measured. This generally is a one-time occurrence which is accomplished during the installation process. If high accuracy is required, then the stability of the design and the details of the calibration process must be carefully followed.

PHYSICAL PROBE SENSORS