www.rechner.com / faqs / Basics /

Capacitive Sensors Basics

Jump To:

Capacitance and Fields
Hysteresis
Conductivity
Flush vs Non-Flush (Shielded vs Non-Shielded)

Capacitance and Fields

Capacitive sensors work by detecting the capacitance of a material (the amount of charge the material can hold). When a threshold amount of capacitance is reached, the sensor will switch states and give an output.

The capacitance of a material is based on three characteristics: (1) the dielectric constant of the material, (2) how far away the material is from the sensor, and (3) how much of the material is inside of the sensor’s field.

1. Capacitance increases the higher the dielectric constant of the material.
   
    
   
   
   
   
   
   
   
2. Capacitance increases the closer the material is to the sensor.
   
    






3. Capacitance increases the more material there is in the field (average capacitance of the material and the air inside of the field). This rule applies also to the density of the material (higher density = more material.)
   

Adjusting the bias of the field created by the sensor allows the user to control how much material is inside of the field before it switches which allows the sensor to be programmed in a way so that low dielectric material will not cause the sensor to switch.

Likewise, the sensor can be biased in a way that a specific amount of material can be inside of the sensors field without causing it to switch. This is useful for detecting objects though plastic containers, glass, or cardboard.

Adjusting the sensor changes how much material can be inside of the sensors field.

Hysteresis

Hysteresis is a “latching type effect” that causes a sensor that is on (detecting) to stay on even after the object has moved out of the sensor’s field.

This is useful in some cases and is often built into inductive sensors so that the sensor will not chatter (switch on and off repeatedly) when the object is right at the edge of its detection range.

The effect of hysteresis is amplified the more the sensing field is extended. Rechner Sensors capacitive sensors generally have a hysteresis of less than 10% at their ‘Normal Sensing Distance (Sn).’

Conductivity

Conductivity exaggerates the amount of capacitance that is front of the sensor by bridging the effect of the sensors field to a larger volume. This will cause capacitive sensors to become stuck in an ON state when only a tiny amount of material in front of the sensor is connected to a larger mass.

Ketchup is a good example of this issue. If a sensor mounted through the side of a tank has residue on the side wall that extends to the main volume of ketchup further down the tank, then the sensor will stay ON.

Rechner has a solution to the problem. The LevelMaster-Series (KS-700/KS-800) is not affected by the issue of conductivity.

Examples of conductive materials: brines, batters, ketchup, mayonnaise, and vinegar or salt-based viscous fluids.

 

Flush mount vs Non-Flush mount

Standard capacitive sensors come in 2 different styles which are differentiated by the shape of the sensing field that the sensors create.

Non-Flush mount sensors have larger fields and further sensing distances.

Flush mount sensors can by mounted flush with the side of a container and are shielded on their sides so they do not detect at their sides.