sludge indication

i have a flow stream that is mostly water, but has a little oily film to it. whenever the main operator is on vacation we suck the tank dry and a sludge enters the flow stream. i am looking for a density type meter that will indicate when i am starting to suck up the sludge. a 4/20 output or contact closure [with setpoint] will do. trying very, very hard to stay away from
the nuclear instrumentation.

any help would be greatly appreciated.



Richard Belknap


Why not just put some kind of low level alarm indicator on the tank? This would prevent the tank from getting low enough to suck sludge. In our building we pipe city water to the tanks and use a float system(like your bathroom toilet) to automatically add water. We put a solenoid valve on the water line then plug that into the float. The float is then plugged into 120VAC wall outlet. When the float "trips" it opens the valve and lets water in. Height is adjustable.

Low tech, but it works great.

Good Luck,

Koen Verschueren

You can buy mass flow meters from Krohne, Yokogawa, Endress & Hauser, ... These meters have a 4-20 mA output and digital outputs but are expensive.
I don't know what kind of pump you use. If you use a system with fixed pipes then you will probably have an almost constant motor current (with water), and this will change if the sludge passes the pump (increase). Maybe you could do some tests with a simple current measurement (switch).
Try installing a conductive low level probe in the tank. The water will conduct, the oil on top doesn't. Keep pumping until low level (empty tank or oily film).

This is often used for emptying oil tank bunds of rainwater.
Try for vibrational density meters and ultrasonic. The ultrasonic systems are widely used in water treatment. The 7828 tuning fork density meter will install in any pipe size from 1" up and has a 4-20mA output of density or %solids. used in a variety of industries including mineral processing and washing plants and water treatment it is used in underflow pumping applications on some quite diverse applications. Be carefull that in some applications, especially where contaminant levels are low, the temperature compensation algorithms may be especially usefull.
Many coriolis mass flow meters (as another respondant has suggested) also measure density by the resonant frequency method...the term "coriolis density measurement" as used in one ISA handbook on instrumentation, implies nothing more than a mass flow meter which exploits its operation at resonant frequency to measure density as well as mass flow. Some are very accurate at reference conditions but be sure that if you choose this type of device to investigate the quality of density measurement and signal processing provided. Designed primarily as mass flow measurement devices they may not all provide the performance of a dedicated density meter and as flow through devices may not be suitable for your application if your pipe diameter is at all significant.
Finally, the success of any density system may depend on the contamination level and the difference in density between the water and the sludge.
If the contamination is genuinely "oily" you might also try website for an oil in water float type device or