Level measuring with a capacitance level transmitter
Today we are going to discuss the use of a capacitance level transmitter. To hit it off though, we will first take a look at a more general problem of the automation industry in order make the subject more relatable.
For most variables in process automation, you have plenty of principles you can use and vendors that want to help you. Each vendor will highlight a product using a certain principle and possibly tell you to avoid others for this or that reason.
Setting the hype aside, you need to keep in mind that the best device for your application transcends accuracy or price. Of course, your process needs an accurate, stable, and long-lived device. And you don’t want to pay too much for it. But your company needs local support, quick service, or remote assistance from the vendor too. These facts will help you choose the right solution for you.
One day we may find a solution that you can set and forget. However, today is not that day.
A good example
After you’ve spent a while in this business, you’ll have a good idea which devices will work best in your process. For instance, if you need the level in an ice cream tank, then you’ll want a non-contact device. That way, you’ll reduce the risk of contamination and avoid certain types of process connection. Makes sense, right?
Nonetheless, most ice cream tanks today use pressure transmitters with flush connections and hygienic approvals. Because even though a non-contact solution like a free-space radar looks better for the process, the industry typically uses transmitters designed for food application.
The pressure device will operate just fine, but if you have unstable product density, it can affect the readings. A change in the density will increase the error margin and reduce accuracy. If you choose a free-space radar, density won’t affect the measurement.
But wait, you might say. Those tanks usually have agitators, and those disrupt radar devices, right? Not exactly. Yes, most such tanks will have agitators. However, ice cream has a high dielectric constant that negates that effect. Cool, isn’t it?
Capacitance level transmitter
As I mentioned in a previous newsletter, I haven’t seen many capacitance level transmitters in the field. Although the principle isn’t new, you won’t find these devices everywhere. If you have a non-conductive product, then you’ll need to calibrate the zero and span with the tank empty and full. If you have a higher conductive medium, then the factory calibration should do.
On the upside, these flexible devices work in non-conductive and conductive media. However, you have a range between 1 and 100 micro-siemens per centimeter where these devices don’t work as well. And the capacitance level transmitter should make contact with the product, which puts them behind other technologies. Anyway, let’s see how the device works.
As it says in the name, “capacitance level transmitter” it operates as a capacitor. The probe acts as one of the capacitor plates and the metallic vessel wall as the second one. We call the medium that separates these plates the insulating material (dielectric constant, K). The capacitance, or storage capability, of the capacitor (C) depends on the plate areas (A), their distance apart (d), and the dielectric constant (E).
C= E(K A/d)
- C = capacitance in picofarads (pF)
- E= constant
- K = relative dielectric constant
- A= effective area of the plates
- d= distance between the plates
When you have a tank with the level at zero percent, then you have only air as your insulating material. As the product begins to fill the vessel, that material will change to a liquid or solid, thus changing the capacitance. So the capacitance measured by the sensor is directly proportional to the level of the tank.
This principle can work in many process conditions, such as slurries, foams, high pressures, and more. And the dielectric value will determine if you should use a partly or fully insulated probe. For instance, on a capacitance level transmitter measuring a value up to 5, you might use a partly insulated probe. But if the value goes higher than 5, then you should deploy a fully insulated probe.
Remember, you need two plates. Usually, you have the sensor probe as one and the metallic vessel wall as the other. However, you can still use this principle in a plastic vessel. You just need a concentric tube in the tank or a sensor with double rod electrodes.
Continuous measurement and level switches
This principle applies to continuous measurement and level switches. When you have a continuous measurement, the probe should cover the whole measurement range. With a level switch, the installed rod only detects level in a particular area.
Many vendors offer these solutions on the market – Endress+Hauser, VEGA, OMEGA, Siemens, and others. Depending on the vendor, you may have to get a model for liquids or solids, with fully or partly insulated material, in PE, PTFE, PFA, and more.
Capacitance level transmitters are simple yet flexible. If you need to calibrate the device in the field for non-conductive media, then you might have a bit of difficulty. But in conductive media, it installs fairly easily. Scale out a device and compare its pros and cons to others to decide the best for your process.