Conductive level probe query

[justwhy2003]

Hi Has anyone heard of or use a conductive based level probe that has pairs of contacts at incremental levels on a single probe to give different switching levels to a control system, ive attached a drawing to help explain what i mean. The vast majority of conductive level probes i can find have two or more eloctrodes like a fork that will have different height prongs for different switching levels. I know that these probes are available. Also has anyone had any experience with RF based level probes, im not sure of the theory but im guessing the probe vibrates at a frequency that is damped by the rising liquid producing an output. Any help appreciated Justwhy2003 Conductive_probe.bmp

[ECSI]

Multitrode makes a level probe with 10 sensors and the probe can be 1 metre to 9 metres long. http://www.multitrode.com.au/products/probe/overview.cfm

[robh]

We have a couple of Joslyn Clark conductive level sensors in our plant. I will have to look at them to get a part number. They look like an ice cube relay and mount in an 8 pin socket. You attach wires to the desired probe length ( I think that we made our probes out of stainless steel). We use them to turn an asco valve on for fresh water make up in a tank. They work fairly well as long as the probes stay clean. Edited August 21, 2007 by robh

[paulengr]

I've used ultrasonic extensively along with pressure sensors (even the bubbler type). I have a couple RF types. This is generally better in various crappy environments. There are two types. One is outright radar and doesn't have a probe. The other type is "wire guided radar". It generates pulses but directs them down the wire. The pulse is microwave frequency. When it hits a large change in dielectric constant (like the surface of a powder/granular material/liquid), this causes a reflection. The unit just measures the time delay between sending the pulse and the reflection. Very simple and very immune to "caking" materials compared to the others.

[BobLfoot]

Paul - how does your radar type sensor work with powders which tend to generate "dust". We have problems with caking on the probe and with dust falsing our ultrasonics.

[justwhy2003]

Paul Thanks. Do you have a part number for the RF probe? Ive been looking at one made by a company in the USA but the tanks we use arent metallic so the probe requires one stainless steel cable as the plate and one teflon coated cable as the opposing plate. As the application of the probe is for waster water, a worry is that solids may stick to the probe and alter the true reading. Any thoughts? Justwhy2003

[paulengr]

Sorry...I was a little wiped out when I wrote that. I've had a project go badly and been camping out in the plant 12 hours a day...err, make that night. Just got back on "normal" hours. Drexelbrook, now part of Ametek, makes ultrasonic, radar, and wire-guided radar probes. Ametek is a big name in the power utility business. They tend to make very solid stuff, but it does tend to be a little pricey at times. The wire guided radar probes aren't cheap (they will set you back close to $5K) but they definitely work. In fact for your application, I think they used to make one that could also distinguish between a "nonfirm" surface like a sludge blanket and the underlying actual fluid level if you're dealing with activated sludge tanks. Having a nonmetallic tank really isn't a problem either. Only conductivity probes that use the tank as a ground have that issue (point sensors). The big reason that I've used them in the past is when you are dealing with caking or "dusty" powders, especially ones that don't have a nice, smooth surface. Things that this works well with include Portland cement, sand (wet or dry), clay pigments, and trona or soda ash. All of those are granular materials that fly in the face of flowability, and totally confound any kind of noncontact sensors whenever you least expect it. The far cheaper alternative if it's just a tank with a decent surface to reflect sound waves off is an ultrasonic probe. I used to recommend Milltronics but they were bought out and then the company went seriously downhill. At this point I'd suggest taking a look at Hyde Park, now owned by Schneider. They have a decent probe with an analog output with a distance capability of 8 meters for under $1K. This is always my first choice for level measurement if there aren't compelling reasons to use an alternative. If you need longer ranges than that, Drexelbrook's stuff is every bit as nice as the old Milltronics stuff. There are three more variants for just liquids. There's the traditional conductivity probe. It consists of 2 cables separated by a spacer. As the liquid rises and falls, the resistance between the cables changes. If the liquid is nonconductive, then the dielectric constant changes, leading to a capacitance probe. The in-between version uses a sinusoidal probe signal and measures the impedance at various frequencies. This is different from all the above "pulse" based technologies. I've never used any of these mostly because I work in "dirty" industries (mining, foundry). These tend to be somewhat troublesome if you don't have a relatively clean and predictable (in terms of electrical characteristics) fluid. Considering the costs, even with clean fluids, again, I tend to lean towards ultrasonics. Finally, there's laser and it's low frequency cousin, radar. Laser has a massive range advantage but can't tolerate dusts very well. Radar is almost as accurate and more tolerant of dusts but tends to be expensive. Almost forgot...bubbler tubes. Simple, almost totally immune to crap buildup, and cheap. Simply attach a compressed air line to a pipe with a pressure regulator. Adjust the regulator so that you see just a few bubbles coming up when the tank is full. Then put a pressure sensor in the line. The air isolates the pressure sensor from the fluid and makes the line self-cleaning. The level is equal to pressure / (density * G, the acceleration due to gravity). These are extremely popular in the waste water business...to the point that they forget that there's other technologies out there. There's even a variable density type. Use 2 bubbler tubes with different lengths. The pressure difference is directly proportional to density...write out the equations for 2 bubbler tubes with a known height difference and solve for density.