An Introduction to: Vibrating Probes

Continuing our series of occasional articles on the characteristics and advantages of popular level control technologies.

Hycontrol’s customers and colleagues frequently ask why we recommend vibrating probes instead of capacitance probes or rotary paddles for powdered solid products, particularly in safety applications. Hycontrol supplies all three of these technologies, but over time our experience has led us to recommend vibrating probes as the first choice for safety-critical operations. There are several reasons for this, which we will explore shortly.

Before we discuss the advantages of this technology over other common choices, we will explain how vibrating probes work. A standard side-mounted vibrating level probe, such as the model pictured opposite, is built around a piezoelectric crystal in the back of the unit which vibrates the sensor blades.

There are several different designs on the market; Hycontrol manufactures a diamond-shaped model built around one vibrating probe inside another. This design is advantageous because it avoids the traditional problem of product building up and jamming between the two forks.

There are two key reasons Hycontrol recommends vibrating probes in safety systems. Firstly, they are very reliable, with no moving parts to break and wear out. Secondly, and crucially, they have ground-level testing as a core design principle; in other words, by pressing a button at the bottom of the silo, an operator can check that an installed probe is fully functional. GLT is an essential safety feature. The test button stops the inner probe from oscillating and vibrating. If a change is detected, the probe is working acceptably. Crucially, this technology is failsafe, meaning that a faulty probe will automatically trigger an alarm to alert users to a problem.

The operating principle is to detect a shift in the resonant frequency. The probe vibrates horizontally, giving a highly sensitive measurement. Immersion in the product causes the vibration to mute and the frequency to shift. The unit detects this change, triggering the switched output. There is an adjustable delay to prevent false triggers.

There are multiple configurations of vibrating probes which can be mounted to trigger at different points inside the vessel. For example, the cable version can be installed from the top of the silo and can extend to a length of 20 metres. The probe itself is sealed on the top and bottom and has a loading of approximately one tonne. If a height adjustment is required when mounted inside the silo, the cable can be coiled up and then cable clipped to make it the desired length.

This technology is ideal for powders, flakes, granules, and very light materials such as flour and cement. It can detect light, fluffy products weighing as little as 20 grams per litre. The probe’s constant vibration helps it to self-clean if any powdered product clings to it. Another advantage of the DP Series probe is that it is only sensitive at the tip so that, for example, when mounted through the side of a silo, an accumulation of product around the probe’s base will not cause it to switch.

We will now compare other popular devices used in powdered solids applications.

The most commonly-used option remains the traditional paddle switches, still abundant in the cement industry predominantly because they are the cheapest technology available. While the lower cost is undeniable, but there are some notable drawbacks. Firstly, the internal motor which rotates the paddle can wear out, or the paddle blade itself can break. There are seals in the shaft to stop the powdered product getting in which can wear out. If the paddle itself were to come loose or drop off, there is no mechanism to detect that. If monitoring a lightweight product, as the blade rotates, it can dig a hole, making itself insensitive to the actual level. These products range in price from $200 to $800, and some of them now have a basic form of test, but the problems associated with this technology is a considerable offset to the low price point.

The other comparable technology is capacitance, which operates on the dielectric. A lengthier discussion of this product’s benefits can be found in our previous blog.

In brief, immersing this probe into a product changes the field of capacitance, triggering a signal output. The only problem with this probe is that it does require some effort to calibrate it correctly for the monitored commodity. The operator must immerse it into the product, adjust the sensitivity, remove it, make sure it switches off, then put it back into the product to make sure it activates. Several repetitions of this process may be required to ensure that everything is working correctly. Such careful adjustment isn't always practical and is often time-consuming, which is why vibration (which requires significantly less calibration) is preferable in many instances.

Another possible problem with capacitance probes is that they measure dielectric, so a low dielectric product such as fly-ash will likely be very tricky for them.

So, if one is looking for a reliable level switch for a high-level alarm in standard safety applications such as overfill and over-spill prevention, or use on a silo protection application, Hycontrol will in a great many cases recommend vibrating probes. In summary: there is little to no maintenance required. There are no moving parts to break in the technology. It's unaffected by changes in the dielectric constant, as might be experienced with products such as cement. There's no calibration to carry out on the instrument – effectively you drop it into your vessel and plug it in. The vibration has a self-cleaning effect, which will counteract any dust and product build-up on the probe. And of course, for critical safety applications, it is failsafe.