Detecting liquid levels with conductive Level Rod-Probes

Liquid levels in process and stor- age tanks need to be measured and monitored, since unwanted variations in these levels (due to evaporation or removal of the pro- cess liquids) must be corrected. In this respect, there are two different general tasks:

• control of the level in order topermit automatic execution of process operations (such as dosing of liquids)

Possible deposits of encrustation between the tips of the probe can be avoided by ensuring that the difference between the probe-rod lengths is at least 60 mm.

The levels of non-conductive or poorly conducting liquids, in which level rod-probes cannot be used, can be controlled and monitored by our float switches.

The level rod-probes are available in many different versions:

• with two to five rods for detec-tion of one to four different levels

• with or without an integratedtemperature sensor.

A suitable electronic controller applies a low alternating voltage to the probe rods. A current then flows from the electrically conductive tips of the rods and through the conduc- tive liquid to the reference electrode, called the ground rod. The electrical circuit is closed. If the liquid level drops below the tip of a probe rod, the related electrical circuit is opened. The electronic controller detects the two states “current” and “no current”.

The ground rod must be at least as long as the longest rod. An addi- tional ground rod must be provided for level rod probes coated with PTFE-Compound, if the distance between the tips of the minimum and maximum rods is more than 1000 mm.

• monitoring of the level in order to prevent possible damage (dry-running, heating without suf- ficient liquid) to the devices (pumps, heaters) installed in the tanks or to prevent an overflow of the process liquid from the tanks.

You can assure the safe control and monitoring of the liquid level in your tank by using level rod-probes. Since these are purely passive devices, suitable electronic controllers are needed.

Level rod-probes operate on the conductive principle, which means that they can be used only in electri- cally conductive liquids (conductivity >4 μS). Encrustation and contamina- tion in the tank normally have no effect on the function of the probes.