MTS Sensors offers analysis of pros & cons associated with various sensors

There are many physical and environmental variables that affect the selection of an optimal level-monitoring solution for chemical processes. The criteria include:

• Physical state (liquid, solid or slurry).
• Temperature.
• Pressure.
• Chemistry.
• Dielectric constant of medium.
• Density or specific gravity of medium.
• Agitation, acoustical or electrical noise.
• Vibration, mechanical shock, and the tank size and shape.

There are also application-specific constraints of price, accuracy, response rate, calibration ease, physical size and mounting, agency approval and control. All of these properties should factor into choosing the best possible liquid-level sensor for chemical applications.

Magnetostrictive Sensors

Magnetostrictive level sensors use a time-based magnetostrictive position-sensing principle. The momentary interaction of two magnetic fields induces a sonic strain pulse in a specially designed waveguide within the sensing element.

While one magnetic field is generated by a permanent magnet sealed inside of a float, the other is generated from an “interrogation” current pulse applied along the waveguide. Traveling at ultrasonic speed, the resulting strain pulse follows this waveguide, and is detected at the head of the sensing element.

The position of the magnet is then determined by accurately measuring the elapsed time between the interrogation pulse application and the arrival of the strain pulse. The absolute position reading never needs recalibration or re-homing after a power loss.

Ideal for accurate continuous-level liquid measurement in storage and shipping containers, magnetostrictive level sensors can measure product and interface tank levels, yet require the use of two proper floats based on the specific gravity of the liquid.

When choosing float and/or stem materials, it is imperative to match material requirements with that of the liquid being measured. Some magnetostrictive sensors even incorporate temperature measurement, thereby granting one sensor the ability to simultaneously measure three process variables.

The magnetostrictive technique is popular for custody transfer applications because of its accuracy, but it can additionally conduct commercial transactions. It is also frequently applied on magnetic sight gauges.

In this variation, the magnet is installed in a float that travels inside a gauge glass or tube. The magnet operates on the sensor, which is mounted externally on the gauge: Boilers and other high temperature or pressure applications take advantage of this.

Pertinent to the chemical industry, magnetostrictive technology can withstand harsh environments, including extreme temperatures, foaming liquids or caustic fluids, while maintaining performance. These sensors detect liquid level—not foam level—and maintain linearity regardless of temperature changes in the vessel.

Hydrostatic Pressure Sensors

There are two main hydrostatic pressure sensor types: head, which are externally mounted to the bottom or side of the tank, and submersible.

Both measure liquid levels by determining the pressure caused by its weight, which is based on the distance from the bottom of the tank, the pressure exerted by the liquid in the tank and the reference pressure from the air above the liquid in the tank (which may require a second sensor). Built-in circuitry correlates the pressure into a 4- to 20-mA output signal proportional to the liquid level. 

Submersible pressure sensors are intended for open-air applications, so they are designed with electronics sealed inside. For these sensors, using chemically compatible materials is critical to ensure proper performance.

Although they can reach extreme depths for measurements, they are not the most accurate, especially if not fixed in place. 

Additionally, more than one pressure sensor can be used to form differential pressure sensors—these sensors calculate measurements based on each sensor’s input in order to gain accuracy while measuring mass, density and volume. Since these sensors measure increasing pressure with depth and specific liquid gravities vary, they must be calibrated for each application.

Large temperature variations, moreover, cause changes in specific gravity, which should be accounted for when the pressure is converted to level. Designed to keep the diaphragm free of contamination, these sensors ensure proper operation and accurate hydrostatic pressure level measurements.

Capacitive/RF Admittance Sensors

Capacitive level sensors operate on the principle that a capacitive circuit can form using the liquid that needs to be measured. The capacitance of the circuit formed then changes as the liquid replaces air as the medium of the capacitor.

Continuous-capacitance level transmitter operation depends on liquid conductivity. For conductive liquids, the sensor measures capacitance between its probe and the tank wall. If the tank isn’t conductive, a ground can complete the capacitor. The sensor measures change in the liquid rises and falls, causing the capacitance to increase since the liquid is more conductive than air. 

The setup changes when the liquid is an insulator. For isolative liquids, the sensor has a metal shield on the capacitive probe exterior. Rather than the tank wall, the sensor uses the metal shield to complete the capacitor. The liquid level measurement is based on decreasing capacitance as the liquid rises. 

Continuous-capacitive sensors are not highly affected by temperature or pressure. And while most sensors require calibration and a continuous-dielectric constant, auto-calibration sensors can automatically adjust to changing dielectric constants. Despite all technology advancements, though, capacitive sensors are not very accurate.

Capacitive sensors don’t contain moving parts, which facilitates use and cleaning. They can further be designed for high temperature and pressure applications. On the other hand, danger exists from the buildup and release of a high-voltage static discharge that results from the movement of low-dielectric materials. Yet this danger can be eliminated with proper design and grounding.

Ultrasonic/Sonic Sensors

Ultrasonic level sensors emit sound waves and operate on the principle that liquid surfaces reflect these waves back to the source, and that the transit time is proportional to the distance between the liquid surface and the transmitter.

Ultrasonic sensors can additionally provide point level control, continuous monitoring or both. Another advantage is that it is non-contact, thus it doesn’t require immersion of any element into the sensed liquid.

However, ultrasonic sensors need specific circumstances to work their best. They need a clear line of sight for the sound waves, for example, or else they bounce off the object and give inaccurate readings. Sounds waves are also sensitive to temperature, pressure, moisture and air composition. All these variables affect the speed and ability of sound to propagate through air.

Turbulence, foam, steam, chemical mists (vapors) and changes in process material concentrations also affect ultrasonic sensor response.

Turbulence and foam prevent the sound wave from being properly reflected to the sensor; steam, and chemical mists and vapors distort and/or absorb the sound wave; and variations in concentration change the amount of energy in the sound wave that is reflected. Ultrasonic sensors are immune to several properties of the liquid being measured—its viscosity, electrical properties and the temperature of the solid, for instance.

When choosing a liquid-level measurement technique specifically for a chemical application, there are more than a dozen technology options. Yet none can be evaluated simply on one factor, such as cost or accuracy.

Not every method is equally smart for every application, so it is important to carefully weigh the most significant needs across the board and select the technique that is able to stand up to the chemicals and processes involved.

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