An Introduction to: Capacitance

This feature is the first in a series of occasional articles, outlining the particular characteristics of popular level measurement technologies.

The variety of level measurement devices on the market these days for solids and liquids can appear overwhelming. It is generally accepted, however, that particular technologies suit particular applications while being unsuitable for others. For example, while the technology exists to provide continuous high-accuracy level readings, this can be superfluous to requirements for many simple applications. In these cases, all that may be required is a reliable ‘on/off’ switch to alert operators when product in a storage vessel hits a certain level.

While this is a simple requirement, these switch points must be reliable. If they fail, this can lead to overspills or vessels running empty, which in turn can have severe consequences for production lines. Therefore, the reliability of a level switch is a notable concern for operators. 

Rotary paddle switches have long been a popular, low-cost option – the technology behind this device has barely changed in decades, and for specific uses, they are undeniably useful. They are a simple mechanical device that is typically mounted at the top or on the side of a vessel. Once installed, the inserted rotating paddle or blade at the end of a probe rotates continuously until contact with the product material stops the blade. 

However, the limitations of rotary paddles are notable. Both the internal motor and paddle blade are prone to wearing out and breaking from the resistance of the product; if the sealing on the probe decays, the product may get into the motor and electronics, causing them to fail. Level switches may regularly go untested, and rotary paddle switches are rarely if ever fitted with an actual functional testing mechanism. Therefore, the first time an operator may be aware of the malfunction may be when the material starts spilling out of their vessel.

Probe failures with this technology are too commonly accepted when they should signal that operators should investigate the scope for improvements. Instead, operators usually replace on a like-for-like basis, which, while it may offer the reassurance of knowing what one is getting, will not improve performance or cost-effectiveness.

The fact is that for many critical tasks, rotary paddle switches are not durable enough or reliable enough.

Hycontrol, a company with over 35 years of experience in the manufacture and supply of level measurement equipment, frequently recommend capacitance technology as a replacement for rotary paddle switches in most industrial applications. A single capacitance switch will have the functional lifetime of several rotary switches, given the propensity of the latter to break and require replacement. Indeed, while the initial single unit cost is undeniably higher, the improved reliability and reduced maintenance costs of the capacitance switch make it a competitive alternative in the medium term, and in many cases a more cost-effective long-term option.

Capacitance probes are a compelling choice for both solids and liquid switching applications. They function by creating a low-voltage, high-frequency signal along the probe, creating a small current flow between the probe tip and the ground plane. As the level of the measured material comes into contact with the probe, this alters the capacitance measurement between probe and ground plane, which the switch then interprets to trigger an alert for the user.

All materials have a dielectric constant. This is its ratio of electromagnetic permittivity versus the permittivity of free space. Effectively it means the ability of the material to store electrical energy. During operation, the capacitance device observes when the dielectric constant at the probe point shifts from that of empty air to the set dielectric alarm point. When this happens, the alarm is triggered. While in some cases, factors such as temperature can alter dielectric values, measuring an environment where such considerations do not fluctuate will mean that capacitance is a very reliable form of level control.

Traditional capacitance switches had a critical technology limitation that required the use of specific transmitters or sensors dependent upon the dielectric constant and conductance of the measured product or material. Nowadays, as a result of technological advances, this is no longer the case, and modern capacitance units such as Hycontrol’s ME Series use the same transmitter and sensor for all applications with adjustable sensitivity, greatly simplifying the selection process. In most cases, once the probe has been installed, wired and calibrated (a relatively simple adjustment), users can effectively forget about it.

Capacitance probes do have their limitations besides the issue mentioned above with environmental changes altering dielectric constants. For example, stickier products may coat the probe over time, resulting in a reduction of sensitivity – as a simple and cost-effective replacement for mechanical level switches, they are a proven technology. As with all level monitoring technology, a regular service and maintenance regime will extend the probe’s functional life and ensure the optimum level of operation. Capacitance probes are durable and flexible devices that can be mounted from the top, side, or bottom of a vessel, and allow for reliable monitoring of a wide range of products across a variety of industries.

ME Series Capacitance Probes - Datasheet
An Introduction to: Capacitance