Fig. 1 illustrates a typical application for a tank or pressure vessel. This could be a wastewater sump, water storage tank or any other application that requires the activation of a pump, solenoid, alarms or other electrically driven piece of equipment when water reaches a certain level. Here, a remote sensor is installed on the top of the tank and a single set of wires is run back to the control box. When the water level reaches the sensor to complete the electrical circuit, the pump or solenoid activates automatically.

Conductance actuated controls also are ideal for applications where action must be taken when fluid levels become too low. Then, the controls can trigger pumps or other devices to add water or another liquid to a tank, cooling tower or other vessel. A second probe sensor then can detect when the fluid reaches a desired level and shut down the system. This type of on/off or high/low application is called differential level control.

Consider using remote sensors installed on the side of the tank as shown in Figure 2. This illustration shows two single probe sensors used to report the level of the liquid to the control. In this pump up application, the lower probe would be used to signal pump on and the upper probe to signal pump off. Additional probes could be added to signal low liquid level and high liquid level.

There are a variety of conductance actuated liquid level controls that can be specified for use in many types of level control applications. Here are some features to look for in conductance actuated controls:

Control boxes constructed of plated and painted steel that conform to NEMA-1 General Purpose requirements. Suitable for ambient temperatures up to 120 F, they can be remote mounted on a wall or in a panelboard. The electric supply and circuits to the probes are separated by an isolation-type transformer that minimizes the shock hazard to personnel. Remote sensor housing made of brass with a painted, vapor-proof, die-cast electrical enclosure. Remote sensors are available with up to five probe connections and can be mounted on vessels with liquid temperatures up to 406 F. Look for a remote sensor which can be used in applications shown in Figure 2. This sensor has a brass body with integral electrode and Teflon sleeve insulator. The electrical enclosure is constructed of plated and painted steel and conforms to NEMA-1 General Purpose requirements.

For non-pressure applications such as vented tanks and open sumps, look for a plastic body remote sensor. This remote sensor will have connections for up to five probes and have an electrical enclosure that conforms to NEMA-1 General Purpose requirements. Probes are available in lengths up to six feet and are made of stainless steel with a Teflon coating that prevents buildup of scale. The coating also protects against false signals caused by shorting between the probes.

Controls with a rated sensitivity of 4,000 ohms are sufficient for most common liquids. Controls rated for 60,000 ohms make them suitable for distilled water and condensate applications. Look for controls that can control up to three separate functions, including single or dual level differential control at multiple on/off points, high alarm, low alarm, or high and low level alarm points. Some of the new microprocessor based controls come with a sensitivity rating of 60,000 ohms. The higher sensitivity rating simplifies selection. Since most conductive liquids are in this range, the chance of selecting the incorrect control is minimized.

The soon-to-be introduced McDonnell & Miller LPC-2000, for example, will offer as standard three separate control functions, including differential level controls with high and low level alarm points. Also included will be LED indicator lamps for each function. For stocking purposes, a single LPC-2000 control will take the place of four current Series PCH/PCL models and will simplify selection and provide greater versatility.

To select the proper control for your applications, first determine the number of functions: differential on/off control, low level alarm, high level alarm. Then select the number of probes and then the remote sensor. Remember to add one additional probe for grounding if the vessel is nonmetallic.

Hopefully these tips will help you select the best control for your needs. If you have any questions, contact your localMcDonnell & Miller representative.