How does the dielectric constant of my material affect my capacitive sensor?
Capacitive sensor basics
Capacitive proximity sensors are a noncontact option capable of detecting both metal and nonmetal targets. These sensors react to changes in capacitance caused by the presence of these targets. Capacitive sensors are available in cylindrical as well as surface mount housings and provide a sensing range from 1 mm to 50 mm. Many of these models feature a sensitivity adjustment potentiometer that allows the user to fine-tune the sensor to a specific application.
Capacitive sensors are commonly used for sensing targets such as plastics, liquids, powders, and granular materials. If chosen correctly, a capacitive sensor is able to sense through nonmetallic objects to detect a target behind them. This ability is very useful in level measurement applications when sensing a liquid or granular material through a container wall.
The sensitivity of the capacitive sensor is affected by the dielectric constant of the target material being detected. The higher the dielectric constant of the material, the more sensitive a capacitive sensor is to that target. Displayed in the chart below is the dielectric constant of some common materials:
Putting it together
As seen in the chart, liquid targets generally have a higher dielectric constant than dry granule targets. This makes liquid level measurement a very common application for a capacitive sensor because of the sensor’s extreme sensitivity to liquid targets.
On the right side of this chart are some common materials used for the containers that would allow the sensor to “see-through” to the material located within. Glass and plastics would be the best option for this purpose as they have a very low dielectric constant value which translates to a low sensitivity for the capacitive sensor.
Materials with a high dielectric constant are detectable at a longer distance than a material with a lower value. This also plays an important role in applications where the target is being detected through a container wall. The material being sensed behind the wall needs to have a dielectric constant that is greater than the material of the container. For ensured operation, the dielectric constant of the target material should be about double the dielectric constant of the container wall.
Application: Sensing water level through a sight glass
In this example, the desired target was much higher than the “see-through” material so the thickness of the glass would not be so critical. If the two values are more similar, then one would need to consider the thickness of the outside wall.
The dielectric constant of water is 80.4 and the glass is 3.7. Water has a dielectric constant of more than 20 times that of the glass so there would be no problem detecting the water while ignoring the glass.
Application: Sensing sand through a PVC container using a capacitive sensor with a fixed 5 mm range
The dielectric constant of sand is 4.8, while PVC is 3.4. Since these two values are very close, the thickness of the PVC becomes extremely important. The best way to determine the maximum thickness of the container wall would be through testing within the application. For this example, it was determined that the thickness of the PVC could be 3 millimeters or less. These results can vary depending on the make-up of the sand being used as a target.
The dielectric constant does have a significant effect on the operation of capacitive sensors. It is very important to consider this value when designing an application using this sensor type. When properly selected, capacitive sensors are an ideal choice for the following production applications:
- Level control (liquid or bulk solids)
- Plastic parts detection
- Wood detection
- Inspecting packaging procedures