L & J Engineering

Servo Level Gauging What Make's it Work

The MCG 1500SFI Servo Gauge is a state of the art gauge that uses a combination of mechanical and electronic components to detect the level of product, interface between product and water, density and temperature in a customer’s storage facility. It utilizes a displacer for measurement of product level, water level, and density. The displacer differs from a float in that it sinks in a liquid instead of floating. The general principle of operation comes from the observation that a material’s weight changes when it is submerged in a liquid. The weight of the displacer in one product differs from its weight in another product. Therefore, it is easy to determine when a displacer is in the air, immersed in a product or immersed in water. We can also tell by its weight when the displacer moves from one medium to another (i.e. the displacer moves from air into a product) and determine very precisely when the displacer is at the midpoint between products.

The MCG 1500 SFI Servo Gauge Basics: The displacer is connected to the servo gauge via a cable wound around a drum. The drum is a precision machined spool with a continuous groove to guide the displacer cable as the displacer is raised or lowered. The drum is located inside the drum housing at the rear of the servo gauge. Since the displacer is hanging from the drum, the weight causes a rotational force to be transferred to anything connected to the drum. The force is transferred through a magnetic coupling to a shaft located in the electronics chamber of the servo gauge.

Attached to the shaft is a strain gauge. The strain gauge is the part of the MCG 1500SFI used to measure a force exerted by the displacer. The strain gauge is constructed from a thin metal tab with a pressure sensitive resistive element attached to each side. As the force on the shaft changes, the strain gauge flexes causing the resistive elements to expand or contract and therefore causing the measured resistance to change. Since the weight of the displacer differs for air, product and water, the measured resistance also differs.

The next part of the process involves the Intelligent Scale Board (ISB). The resistance from the strain gauge is calculated and sent on to the CPU board through a bi-directional infrared data link. The Intelligent Scale Board allows for fine tuning in light or heavy products. The gauge can be calibrated for each application “on the fly” by using the MCG 2150U Remote Calibrator without violating the explosion proof rating of the gauge.

To supply power to ISB the gauge makes use of two coil, the Bobbin Coil and the Gear Coil, acting as a transformer to transfer AC power to the ISB. The information the CPU board receives from the Intelligent Scale Board is used to interpret the relative weight (see next page) of the displacer and the position of the displacer with respect to the product (i.e. submerged, above or at the interface). The CPU board also controls the movement of the stepper motor. The Stepper Motor turns the gear coil, which turns the strain gauge, the shaft, the magnetic coupling and the drum to wind the cable and raise or lower the displacer. Attached to the stepper motor is the Optic Encoder. The encoder sends information on the position of the motor, which also corresponds to the position of the displacer.

The values for relative weight of the displacer are displayed as the “analog” value on the gauge’s LCD display. The values displayed are inversely proportional to the weight of the displacer. If the displacer is submerged in liquid the weight is less but the number on the LCD display is greater. If the displacer is hanging in air the weight is greater and the value on the LCD is smaller. The number is referred to as the analog count. We aim for 110 in air (Analog zero) and 1100 in water (Analog span). Between the two numbers is the surface or interface level at 500 analog counts.

To find the surface of a liquid, we use a stepper motor to adjust the position of the displacer so that the display always shows an analog number of 500. In a similar way, the level of the product/water interface is found by adjusting the position of the displacer so that the displayed analog value is 990.