Choose the Correct Level Sensor

Answer a number of key questions to identify the most appropriate choice

By Brian Sullivan, Valin Corp.

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At process plants, a significant percentage of measurement devices aren’t correctly matched to their application, leading to decreased quality and consistency of the operation. Often, the source of this problem is the assumption that one type of level measurement sensor suits multiple applications. For example, a float sensor may serve its purpose adequately in a completely liquid environment but, when the liquid contains particles or suspended solids, the circumstances change, compromising the accuracy of the sensor. Maintaining a high level of output quality in any plant requires selecting the proper level measurement device for each application. There’s no “one size fits all” solution.

Unfortunately, companies and individuals often are tempted to skip a lengthy and involved sensor selection process for a cheaper or faster outcome. This is ill advised, though. To ensure picking the right sensor for long-term durability and safety, engineers must consider all possible factors in a design scenario. After all, in a dangerous application, an incorrectly specified level sensor easily could become the source of a major incident.

A common question is, “Are all these detailed questions really worth it for deciding on a simple level sensor?” The answer is “yes” — because there’s no simple sensor.

THE SELECTION PROCESS
You must consider several factors to ensure the accuracy and effectiveness of a level measurement device in a particular application. These factors include the design conditions, the specific media the sensor will contact, how the information gathered from the device must be transmitted, and what additional accessories are needed to complete the operation. Properly considering each of these criteria will lead to choosing a sensor that can provide more accurate inventory, increased product quality and maximized output due to a lack of disruptions.

The first order of business in the selection process is to narrow down the application for which the sensor will be used. In other words, what will its job be? This could be as simple as giving a visual readout or as complex as serving in a multistage automated response system. Answering this question requires knowledge of how the gathered information will be used. Must the sensor continuously monitor the level of a volatile substance? If so, a continuous level sensor is necessary. On the other hand, if the aim is to stop a tank from overflowing or spilling, a point-level sensor might suffice. You easily can determine the complexity and versatility required of a sensor by first assessing the application.

The next step is to determine design conditions; these are crucial in choosing the correct option. Questions to ask at this stage include:

What material(s) will the sensor contact? A sensor must be compatible with the fluids or materials to which it will be exposed. If the sensor will contact any caustic, corrosive or aggressive materials, it must be able to withstand them while maintaining proper functionality. For example, a metal sensor used to measure critical process fluids could release metal ions or particles and contaminate the fluid. In this situation, selecting a sensor made out of a fully compatible material is best.

Are solids or liquids being measured? This is an extremely important question that must be considered upfront. Float-type sensors normally are a good solution for measuring liquids. However, when measuring solids, float-type sensors are impractical.

The following is a list of popular sensor types and the materials with which they are compatible:
• mechanical float sensors —fluids only;    
• electromechanical (tuning fork or staff-based) —solid substances only;
• ultrasonic — both solids and fluids; and
• radar — both solids and fluids.

Remember, other design conditions besides the type of substance being measured could cause one of these sensors to perform better than another. The material being measured is just one factor to consider in device selection.

Where and how will the level sensor be placed — internally or externally? This question is important from a logistical standpoint. External sensors could require additional plumbing and installation costs while internal sensors may throw off production amounts or limit tank capacity. Some systems incorporate an internal sensor and an external display to convey information. In this case, always consider the requirements of both the internal and external components.

Is the material at rest or in motion? In a silo or storage vessel, a material may be stagnant at all times except during filling or extraction. In contrast, in a mixing tank, for instance, a substance may experience constant motion or agitation. In the latter case, ensure the sensor can withstand the movement of the material.

What temperatures and pressures will the sensor face? When considering temperature, sensors generally fall into two categories: those built to withstand extreme heat or cold, and those limited to common room temperatures. For example, a metal sensor will withstand an extremely hot environment much better than a polytetrafluoroethylene one. As far as pressure, most sensors can handle 0 to 100 psi — but an extremely high pressure environment or vacuum could cause rupture or malfunction.

What is the density of the material? This question applies almost exclusively to fluids. Those that are fairly dense or contain floating solid particles often require more-complicated level measurement sensors. So, for example, a simple float sensor will suffice in a water cooler tank but may experience difficulties measuring rough crude oil because of the viscous nature of that fluid. A material also may include a mix of fluids such as oil and water. In such a case, a capacitance sensor can accurately measure the mixture. A magnetic float sensor with the buoyancy factor adjusted to the material would work, too.

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