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A General Overview of Capacitance as a Method of Point Level Detection |
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| Posted By : Rich Tavis | |||
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The level control industry, like the rest of the world, is
becoming more technologically sophisticated.
As people begin to understand more about capacitance probes, we
are beginning to see a shift away from electromechanical devices, such
as rotary paddle switches and pressure sensitive switches.
And why shouldn't we? Capacitance
probes are more affordable than ever, and the advantages of solid state
equipment over electromechanical speak for themselves. Basic
Principle of Operation Proximity Switches You may be familiar with a cousin of the capacitance probe: the proximity switch. Proximity switches also function on the capacitance principle, but cannot be as widely applied as true capacitance probes. In the case of a proximity switch, the two conductors are both incorporated into the switch itself. This means that a metal vessel wall or grounding plate is not necessary to function as the second conductor, but it also places the two conductors very close to one another. These sensors only look at a very small area when determining presence or absence of material. There are a number of factors, such as: build-up, dust, conductivity, moisture, durability, and dielectric constant, which will cause problems for proximity switches. These devices do not have the features, mechanically or electronically, of a true probe, and are inherently less reliable. In the right application, they are a low cost alternative to an industrial level sensor; but in most applications, proximity switches are no substitute for capacitance probes. Conductivity Conductive materials which leave residue after they have fallen away (most liquids, for instance) require a coated or sleeved probe. This insulating coating keeps a conductive material from grounding the probe to the vessel wall. Any conductive residue that builds up from the vessel wall to the active portion of an uncoated (bare metal) probe will short out the two conductors. It would be like pressing our two conductors right up next to each other, making it impossible to sense anything between them. This is not a problem in the case of a full vessel, because the indication that you get from a shorted probe is “presence of material”. However, if the product leaves a conductive residue that coats all the way back to the bin wall after the level has dropped below the probe, the unit will still indicate “presence of material”. A few materials can leave a heavy enough residue that is so conductive; it is like putting a metal pipe around the probe that it cannot see through. Carbon black and molasses are two examples. Even sleeved or coated probes have difficulty with these materials. If a small section of the probe can remain uncoated, however, and keep it from grounding all the way back to the bin wall, then the unit will work. Sensitivity Generally, a probe should only need to be calibrated once at the time of installation. When you calibrate the probe, you are telling it what the dielectric constant of the ambient environment (air) is, and what the dielectric constant of your material is (estimate: low, medium, or high). When the unit senses the shift in capacitance from air to material or vice versa, it sends a relay signal. The dielectric constant of air is 1. Different brands of probes have different sensitivity ranges. Some probes can sense material with a capacitance shift of 1/2 picofarad relative to air. This represents a very small change in capacitance. The density of a material can play a role in the effectiveness of capacitance sensors. Extremely light and fluffy materials (packing peanuts for instance) may have so much air in the product that the probe does not see enough of a capacitance shift from air to detect the material. The dielectric constant of the material may be within the probe’s range, but there may not be a high enough concentration of material to air for a given volume. If we were to compress the material, increasing the density, the probe may have no trouble sensing it. Shielding Many capacitance sensors have shield built into the probe assembly. A shield (or guard, as it is sometimes called) is designed to overcome problems resulting from sidewall build-up, build-up along the probe assembly, or bridging between the sidewall and the probe. This is generally a concern only with side mounted units. A shield is a portion of the probe, which is non-sensing. Usually, the first five or six inches of the shaft as it extends out from the enclosure make up the shield. The shield is designed to provide a “dead zone” up next to the bin wall, which will ignore any build-up that occurs on the vessel.
To call the shielded portion of the probe assembly “inactive” is somewhat misleading. The shield does generate a field with the same frequency as that generated by the active portion of the probe. This forces the active field out and around the shield field, causing the probe to examine a large area around itself rather than just the area immediately surrounding it. This allows the probe to ignore build-up along the probe assembly. Depending on the material, it is often possible to completely coat the probe with an inch or two of material all the way back to the bin wall and still recognize it as build-up, not “presence of material”. Probes that do not have shields can often be desensitized to ignore some build-up, but not nearly as much as a shielded probe. Variations
on a Theme Flush Mounting:
In applications where intrusiveness and/or durability are a concern, most probes have a flush mounting option. On a flush mounted probe, the rod probe assembly is replaced by a flat disk which mounts flush (or nearly so) with the bin wall. Some flush mounted probes even have shields. The shield on a flush mount comprises an outer ring around the active probe. Just as with a standard probe assembly, the shield on the flush mount forces the active signal out and around the shielded portion, back to the bin wall. The goal here is the same as before: to force the active signal to look at a larger area, allowing it to see through build-up. Keep in mind, a flush mounted probe will never ignore build-up as well as a rod type shielded probe. In some applications, an adapter may be required to bring the face of the unit truly flush with the inner face of the bin wall. If the bin wall is very thick, and/or the probe is mounted on a hopper bottom, material can build up in the crevice that may exist.
Coated Probes and Temperature Considerations: As we mentioned before, most conductive materials require a sleeved or coated probe assembly. The sleeve is most often a Delrin or Teflon plastic tube that fits over the probe. Probes can also be coated with Teflon or Kynar, but coatings scratch and wear off much easier than sleeves. One scratch in a coated probe and it functions as if there were no coating at all. Usually, probes are made of 316 stainless, and sleeve and coating materials are typically FDA approved, so food grade applications are no problem for most standard capacitance probes. The only exception would be a probe that has Fortron or Vyton components. Most manufacturers have moved away from these materials, but there are a few who are still using them. Teflon sleeved or coated probes are usually used for high temperature applications, but we need to be aware of the temperature that we expose the unit’s electronics to as well. The electronics on a standard, integral probe are located just outside the vessel, so we add the temperature of that environment to the heat which transfers through the vessel wall and through the probe assembly, to arrive at our ambient electronics temperature. The maximum that most probe electronics can withstand is 165-185° F. If the temperature that the enclosure will be exposed to is greater than this, then you may want to consider using a probe with remote electronics. Remote probes may also be necessary in high vibration environments. If the ambient electronics temperature is only slightly above the recommended range, then it may be possible to lag the enclosure out from the vessel a few inches to reduce heat transfer. Remember that we are talking about ambient electronics temperatures. Actual process temperatures can often be well above 300° F before electronics temperature becomes an issue. Lags are also used to extend the probe into the vessel in certain applications. A six-inch concrete vessel, for instance, might require a lag in order for the probe not to sense the concrete wall that it is inserted through as “presence of material”. In this case, the lag functions like a shield that is truly “inactive”. Other Probe Options: When top mounting a capacitance probe, it is often necessary to extend the probe down to the desired sensing level. Any probe requirement that exceeds 4 or 5 ft. in length may be a good application for a cable probe. Flexible cables generally offer the same advantages as regular solid probes, without acting as a lever arm for material shifts within the vessel. Before you ask the question, the answer is yes; the cable probe is grounded back to the vessel wall if material pushes it over against the sidewall. This does not present a problem though. Remember that the indication given by a shorted probe is that of a covered condition. If material is present to push the cable over to the bin wall, then there is, in fact, a presence of material. When the material falls away, the cable will fall back to its hanging position. The probe is no longer shorted, and the unit indicates no presence of material. If you have a very turbulent application, the cable probe can be tethered to the vessel bottom to keep it from being thrown around. Just remember that a nonconductive spacer must be used between the cable probe and the tether, otherwise we are grounding out the probe again. Shorter probes may also be necessary in certain applications where flush mounting is not an option. Unshielded probes can generally be cut down to 2˝ - 3 inches, but shielded probes are much more difficult to shorten. If you will recall, a shield makes up the first 6 or so inches of a probe, and we need to leave 2˝ - 3 inches of active probe. This leaves us with a minimum probe length of 8˝ inches. The shield itself can be shortened, but it may require expensive custom modification by the manufacturer. Capacitance technology is also used for continuous level measurement and inventory management. This branch of the capacitance family is quite different in it’s potential applications, and thus, should be the topic of it’s own article. Care must be taken when applying capacitance to bulk solid materials. Solids may present different material characteristics at different points in a vessel (density, particle size, temperature, etc.). These changes can have an effect on the capacitance of an environment at any given point. Calibration then becomes a floating reference as we move throughout the vessel. Readings from one point do not necessarily correlate to those from another. The versatility that probes display in the point level arena does not yet transfer to continuous level measurement. Radio Frequencies and Other Concerns There are those who will tell horror stories about probe failures and constant maintenance, but these stories originate from a time when capacitance technology was not nearly as advanced as it is today. Every method of level detection has its limitations, but today’s capacitance probe can handle the requirements of most applications. One of the significant improvements that have occurred with capacitance probe technology is the shift away from radio frequencies. The radio frequency range runs from 9 kilohertz to 300 gigahertz. Today, we are able to produce probes that operate well below that range. This accomplishes two things: greater temperature stability for sensitivity settings, and no interference from other radio frequency emitting devices. The most common complaint that you will hear about the probes of old, is that they have to be recalibrated all of the time. Because of the circuitry that was used on early probes, they had the potential to drift out of calibration with swings in temperature. Thus, if the unit was set to a high sensitivity setting, a temperature shift of less than ten degrees might change the calibration setting by more than one picofarad. The other problem associated with radio frequencies is interference. Perhaps you have seen signs that read “No Two Way Radios Beyond This Point”. Any equipment that emits RF signals, such as two-way radios, can trigger false alarms from probes that operate within the RF range. There are still units on the market which do operate in the RF range, so if either of these issues is a concern, be sure to do your homework before deciding on a probe manufacturer. There is a common misconception that all
level controls are the same... they’re not.
Different models may use different methods to arrive at
approximately the same goal. Certain
units may work better than others in a particular application.
As a general rule, liquid applications are easier than solids. This is because material characteristics vary at different
points in dry bulk storage while liquid characteristics tend to remain
constant throughout. Thus, a
probe that was designed for use in liquids may not work as well in solids.
Probes that were designed for solids, on the other hand, can handle
most liquid applications with ease. Depending
upon your application, it may be to your advantage to ask a few questions
of the manufacturer before purchasing a probe.
Problem
Applications: Things to Look For · Low Dielectric Constants - Very few products have such low dielectric constants that they pose a problem for capacitance probes, but they do exist. Light and fluffy materials are sometimes difficult to sense, because of the ratio of air to material for a given volume. If the material around the probe is mostly air, then it may not see enough of a capacitance change from an empty vessel to recognize the material. The sensitivity of the probe will also be a factor in these situations. Probe sensitivities range from 1/2 to 2 picofarad shifts from ambient. · Varying Materials - If several materials with widely varying dielectric constants are kept in the same vessel, it may be difficult calibrate the probe to sense them all accurately. Usually, if the probe is calibrated to sense the material with the lowest dielectric constant, then it should work fine for all of the materials. A similar problem can arise when trying to sense a product with a low dielectric constant, when steam is present in the vessel. The probe must be set to a point which is sensitive enough to detect the material, but not sensitive enough to see the steam. · Static Discharge - Different probes have different levels of static protection. If the unit is not adequately protected, a static electric discharge may damage the unit’s electronics. This is especially important in applications where product is being pneumatically conveyed. If static electricity is a concern, be sure that the unit has adequate protection. ·
Conductivity - Capacitance probes can handle most conductive
materials, though a sleeved or coated probe may be necessary.
Just remember that if the product leaves a conductive residue all
the way back to the vessel wall, the unit will not function correctly.
If this is a concern, discuss the application with the
manufacturer. There are ways of overcoming many problems, so never assume that capacitance probes are not a viable option without consulting the level control manufacturer. Chances are, they have seen similar applications and may have ways around the problem. Very often, capacitance probes are the best option for point level detection. Though they have their limitations, probes are still the industry’s most versatile point level control.
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