programmable or specific controller?

[justwhy2003 0]

hi im investigating a project where the level of liquid in a tank was measured using conductive level points as digital inputs to a control system starting and stopping pumps accordingly. i am looking to replace this input with a pressure transmitter to indicate the level in the tank but it would be desirable to use the same digital signals at the same levels to switch the pumps at the same points. previously if there were three different capacities of tank for different applications, the probes could be moved up or down physicially to compensate. with a pressure transducer being stationary at the bottom of the tank, this compensation may have to be integrated in the control logic. a small controller is required that conatins the parameters required for each tank size and can dedicate the appropriate parameters to the controller for each application easily eg terminal jumpers or dip switches. my problem is whether to use a programmable controller such as a PLC/PIC or does anyone know of a dedicated controller that would suit such an application, has anyone by mad chance encontered a problem such as this. anyway, any input appreciated thanks justwhy2003

[Myles 0]

On a tank system, running pumps the norm would be to at least fit low level probes for run dry protection on the pumps. As for high levels good to prevent overfilling, spillages etc. The pressure transmitter on the vessel is a good idea but not so easy to use as high and low levels, it can drift over time, products with different SG would give different switch points and it can also fail. As for a controller there are many many choices, depending on how you intend to use the system would depend on your choice. any operator interface etc?

[P Daniil 0]

What kind of fluids are stored in the tanks? There are many other types of sensor you can use. For such an application I would first look for a capacitive type sensor.

[justwhy2003 0]

the liquid is waste water, a problem with the conductive probe was that when the level fell in the tank, the foam on top of the water was coating the probe and giving inaccurate readings, this problem may be replicated on a capacitive probe. i have encountered this problem once before where such a probe was used to measure level of oil.

[P Daniil 0]

It depends on the physical shape and layout of the capacitor system. For such a system I would suggest an insulated rod type hanging immersed in the monitored fluid. We have succesfully used such sensors in potable water as well as ship grey water tanks. Capacitive sensors are less sensitive/suited for oil detection due to the lower dielectric constant of oil.

[milldrone 0]

I don't know what your waste water is like but I'll tell you about mine and what works for me. I work at a wood working facility that prefinishes some of it's product. The waste water is paint cleanup. And it has to be evaporated. The paint coats and builds up on anything that you use to sense the liquid level. what I have found to work best is to setup a "bubbler" which is nothing more than a hose (that can be replaced when necessary) that has a slow steam of air supplied to it that bubbles out at the bottom (lowest point you need to measure). A pressure transducer is monitoring the air pressure on the hose. this gives me an analog signal for the PLC. If you were to use such a system then you would need to set up "Programmable limit switch logic" to give you adjustable setpoints. Can your foam ever overflow and cause any problems? Vaughn

[OZEE 5]

Believe it or not, I do exactly the same thing except for fluidizing flour. As soon as the whole vessel of flour is fluidized, you can easily detect a pressure change on the fluidizing air supply.

[Rodney 21]

I have just completed a project for a customer which was using pumps to pumpout a well. They used ultrasonic for there switching level and that was waste water. Rodney

[nic00 0]

Were just about to switch to ultrasonic but in regards to flow

[paulengr 44]

First, I have heard of people using wire guided radar sensors to get a level reading while ignoring foam. I've also heard of doing ultrasonics "upside down". Second, I've done simple pressure transducers. The trick is to get a big fat chemical seal and mount it flush. You can get them from Omega and Rosemont. If the diaphram on the chemical seal is big enough, the usual "plugging" issues go away. Standard pressure sensors have just a little dinky hole that gets plugged very easily. In addition if it matters, the seal is a replaceable unit and you can get dP cells too so if you need liquid density, simply use a dP cell the same way (pressure divided by height is proportional to density).