COATING AND FOULING LIQUIDS

This is the tank alarm for difficult liquids that foul, coat, or crystallize on other types of sensors. Capacitive sensing easily tolerates thick deposits of dirt, sludge, viscoes coatings, or crystallization.

BEST FOR LIQUIDS THAT COAT

There are other ways to sense fouling liquids such as conductivity probes, ultrasonics, optical sensors, photo-electrics, and thermal dispersion. Below is why we think capacitance is the best choice for fouling liquids.

Conductivity probes are susceptible to buildup onto the probes which will render them non-conductive, and therefore useless, in no-time. They have their uses, and we use them too, but not for dirty liquids.

Ultrasonic sensors are good for gauging level continuously, but they have to be programmed to the tank depth, dead-zone, and alarm points. Every time you want to adjust the anything, you need to connect a computer!  In addition, they are very prone to errors from turbulence and foam, floating debris.  Further, they have a dead-zone requirement of a couple of feet where nothing can be sensed. If they “miss” the liquid rising past the programmed level point and into the dead-zone, then there is no alarm! Especially bad for high level alarms. Finally, they must be placed far away from the tank wall, or else in a dedicated cylinder – really unsuitable for plating tanks where space is limited.

Optical sensors and photo-electric sensors both suffer from the same problem: if the lens gets coated, they don’t work. This unreliability makes it a no-go for us.

Thermal dispersion sensors can also be fouled: they tolerate more coating than optical sensors, but once the surface is coated thick enough, then you no longer have a functioning sensor.

Capacitance sensors, by design, sense through coatings up to 15 mm thick!  have no moving parts to break, and nothing to wear out. 

HOW CAPACITANCE SENSORS WORK

Capacitance sensors work by generating a capacitance field some distance away from the sensor with co-centric oscillating plates. If anything with a dielectric constant different that air enters this electrostatic field, it changes the capacitance of the internal oscillator and the medium – evey solids – is sensed. The more the dielectric constant differs from air, the greater the sensing range. Basically any conductive water-based liquids will be easily detected, whereas dry materials like plastics, will not.  This table of dielectric constants for various materials gives a long list of materials. The maximum extent of this sensing field in our sensors is 17 mm distance. So it is actually possible to use this as a non-contact sensor. However in practice we find its better to shorten the field so the alarm stops sensing even when there is a thin residue of liquid clinging to the sensor tip. 

VISCOUS LIQUIDS

These are also a great choice for viscous liquids like epoxy resins. A separate page on the capacitance sensor used with viscous liquids is here.