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Whitepaper

How to select a Continuous Level Sensor

Geschrieben von Christensen, J. & Peterson, T. bearbeitet von mhd am 22. Mär. 2019
Using a continuous level sensor to monitor the material level in your storage vessels can help your operation maintain efficiency – preventing costly process interruptions and keeping you up-to-date on your inventory level. This whitepaper is intended to help operations choose a continuous level sensor without having to wade through volumes of technical data. It will briefly describe the sensor types available, address the pros and cons of each type, and explain what information you need to gather before working with a sensor supplier.

A continuous level sensor measures the amount of material in a storage vessel on a continual basis, rather than just indicating whether the material is above or below a certain point, as point level sensors do. This makes a continuous level sensor ideal for monitoring material inventory in your vessels to prevent downtime. Depending on the sensor type and supplier, the continuous level sensor can output data to a console or panel, send information to a PLC, HMI, or PC, or send information via SMS text or to the Internet for anywhere, anytime access. An advanced system using multiple sensors can report data from multiple vessels at your site or from vessels at multiple sites, making it easy to monitor your entire operation’s inventory status.

However, choosing the right continuous level sensor for your application can seem like ordering off a menu when you don’t speak the language. An overwhelming amount of information is available about the different types of sensors and the technologies they use. And a level sensor is just one of the many pieces of equipment you need to worry about. Knowing some basics about how different sensor types work and their pros and cons can help you determine which sensor you need.

Before getting into these basics it’s important to understand that a continuous level sensor, which is typically mounted on the top of a vessel, has a default dead zone (or blanking distance) that it cannot measure. The dead zone is the area between the highest point the sensor can measure and the vessel’s top. When the material reaches the bottom of the dead zone, the sensor will indicate that the vessel is full.

Dead-zone height varies by sensor type but can range from about 4 inches to about 36 inches. Most suppliers preset the dead-zone height in the sensor controller based on the sensor type, but a dead zone’s height can be increased if the application requires a lower full point.

Sensor Types

Types of continuous level sensors include weight-and-cable, 3D scanners, guided-wave radar, open-air radar, laser, and ultrasonic. Each type operates differently and has its pros and cons.

Weight-and-Cable

A weight-and-cable (or plumb bob) sensor works like an automatic measuring tape: The sensor lowers a cable with a weight (also called a bob or probe) attached to its end into the vessel. The sensor determines the material’s level by measuring how much cable has been let out when the weight reaches the material’s surface; then the sensor retracts the cable and returns the weight to the vessel’s top. Not strictly a continuous sensor, the weight-and- cable sensor is programmed to take measurements at predetermined intervals, such as every 30 minutes, once an hour, every 6 or 8 hours, or once a day.

The weight-and-cable sensor measures a single vessel distance or point directly below its mounting location and is highly accurate and reliable. The weight-and-cable sensor’s measuring range can be up to about 150 feet (approx. 46 meter) , and the dead zone is minimal, just 4 to 8 inches (approx.  102 to 204 mm) measured from the sensor’s mounting location to the weight’s tip when the cable is fully retracted.

Pros

The weight-and-cable sensor:

  • Isn’t affected by dust or other adverse process conditions
  • Has minimal contact with the stored material
  • Can be used in vessels up to about 150 feet (approx. 46 meter) tall
  • Is available in models that can handle temperatures up to 1,000°F (approx. 540°C)
  • Isn’t affected by material buildup
  • Isn’t affected by material characteristics, such as angle of repose or low dielectric constant
  • Can measure extremely light, signal-absorbing materials
  • Is approved for use in hazardous, high-dust locations
  • Can be equipped with an air purge to keep the mechanical components clean in very dusty conditions
  • Is simple to install and set up
  • Requires no calibration
  • Provides consistent, repeatable, and accurate measurements
  • Has a low purchase cost compared to the other sensors
Cons

The weight-and-cable sensor:

  • Doesn’t instantaneously respond to material-level changes
  • Measures a single vessel location distance or point
  • Isn’t recommended for use in high-pressure vessels
  • May require periodic maintenance
Guided-wave Radar

A guided-wave radar sensor uses time-domain reflectometry to measure the distance from the sensor to the material. For this sensing method, a low-power microwave signal is sent along a sensing probe (a cable with a counterbalance weight at its end) that acts as a wave guide, concentrating the radar signal within a small diameter around the probe. The sensor calculates the material level based on the signal’s flight time. The sensor’s cable diameter and length vary depending on the material’s characteristics and the vessel size.

The guided-wave radar sensor typically is for vessels up to 100 feet (approx.  30.5 meter) tall and can be used in powders, granules, pellets and other bulk solids. Dependent on the model, it can be used in materials with a dielectric as low as 1.3. The sensor measures the material level at a single point in the vessel (along the cable) between the bottom of the upper dead zone and the top of the lower dead zone. Guided wave radar works in high dust or humidity and is immune to condensation.

Pros

The guided-wave radar sensor:

  • Provides continuous level measurement
  • Provides highly accurate measurements
  • Is suitable for almost any vessel shape or diameter including narrow silos
  • Is available in high-temperature models up to about 800°F (approx. 430°C)
  • Performs well in vessels prone to changes in dust level, humidity, condensation, temperature, pressure, and material bulk density
  • Can be used in high-pressure vessels
  • Is relatively easy to install and set up
  • Virtually maintenance free
Cons

The guided-wave radar sensor:

  • Has a sensing probe that is in constant contact with the material
  • Measures a single point along the cable
  • Typically has a maximum cable length of less than 100 feet (approx.  30.5 meter) , limiting the measuring range
  • Not all models perform well in materials with a very low dielectric constant
  • May not be suitable for use with heavy or abrasive materials, such as large rocks, which are difficult to measure and can impose a high tensile load on the cable and damage it
  • Upper and lower dead zones vary by manufacturer and model
  • Has a relatively high purchase cost (but typically lower than open-air radar or 3D scanners)
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