Sensor Selection Essentials for Remote Tank Monitoring

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The tried and true (if not a bit laborious and less than accurate) archaic tank monitoring process is being disrupted by innovative Internet of Things (IoT) technology. The task of assigning an employee to physically report to a tank, put a stick in the tank, read the measurement and mark it down on a clipboard is being replaced by automated remote tank monitoring solutions. Technology has developed to a point where there are now more efficient ways to handle this mundane task, allowing businesses to better utilize their resources to handle other important task.

Although remote tank monitoring is considered to be a relatively straightforward application in the often complex world of the Internet of things, there are a number of key differentiators and considerations that must be addressed in order to successfully implement a solution.

 

Where to Start?

 

Tanks come in all shapes and sizes. Tanks can be fixed or mobile. Tanks may contain liquids, ranging from water to explosive chemicals. Or, they may be storing solids, such as grain or chemical reagents. While the contents of the tank involved is often the most important element in articulating the requirements for remote tank monitoring, there are many other elements that will need to be considered to ensure that issues of safety and regulatory compliance are addressed, while also delivering the most efficient solution for your business needs.

The following is a list of the many variables to consider when planning a remote tank monitoring solutions:

 

  • What is the shape of the tank?
  • How tall is the tank and how much can it hold?
  • What does the tank contain and what is being measured and monitored?
  • Where is the tank located? Is it mobile and do you need to track its location?
  • What temperatures can be anticipated at the tank location and does the substance expand or change properties with temperature?
  • Does the product in the tank create fumes that change the air density?
  • If the tank has an access point what is the NPT fitting size of the access point and is there anything around the access point that might interfere with a sensor?
  • Is there power available at the site or will it require batteries and/or solar equipment?
  • Does the location already have Internet access, or are cellular or satellite better choices?
  • How accurate of measurement is required? How critical is the accuracy of the readings and what is the acceptable margin of error?

 

Sensor Options

 

The correct sensor to use might seem like a true puzzle solving endeavour. Keeping in mind the questions that were addressed previously, the answer to your sensor requirements will not seem so daunting. In this section we will review the different sensor technologies available and under what circumstances you might want to consider using each one

 

Float Switch 

 

These are used to sense the level of liquid within a tank, and these sensors cannot be used on solids. Float Switches operate on a direct, simple principle. In most models, a float encircling a stationary stem is equipped with powerful, permanent magnets. As the float rises or lowers with liquid level, the magnetic field generated from within the float actuates a hermetically sealed, magnetic reed switch mounted within the stem. The stem is made of non-magnetic metals or rugged, engineered plastics. When mounted vertically, this basic design provides a consistent accuracy of ±1/8 inch. Multi-station float switches use a separate reed switch for each level point being monitored.

Users typically choose float switches and magnetic reed switches because they are hyper-reliable, simple and are an economical way of determining liquid level. These sensors can be made of a variety of material options and are compatible with most liquid media. These sensors are intended to actuate when media is at a single set high or low point.

There must not be any obstruction under the opening of the tank so that it does not interfere with the rod being placed in the tank, and ensure that no agitators can hit the sensor.

Float switches are considered low range in pricing.

 

Ultrasonic Sensors

 

These are normally used for non-contact level sensing of highly viscous liquids, or bulk solids. Ultrasonic sensors would also be a good choice for water treatment applications such as pump control or open channel flow measurement. The sensors work by emitting high frequency (20 kHz to 200 kHz) acoustic waves that are reflected back to the emitting transducer, which detects the wave.

Some things to consider before committing to this sensor are:

 

  • These sensors are affected by the changing speed of sound due to moisture, temperature, pressure, air density and fumes. These sensors must be configured to factor in these environmental parameters to provide more accurate readings.
  • Turbulence, foam, steam, chemical mists (vapors), and changes in the concentration of the process material can all impact the accuracy of readings.
  • Harsh chemicals can negatively affect the transducer if not housed in the proper material. In oil and gas applications the H2S vapours can destroy the seals on the transducer, so make sure that the sensor is rated for this environment if you plan on using them for this type of application
  • Proper mounting of the transducer is critical to get the most accurate readings; if there is any obstruction under the transducer you can experience false returns.
  • Ultrasonic sensors can be used to provide point level control, continuous monitoring or both.

Ultrasonic sensors are considered mid-range in pricing depending on the substance being monitored. When properly specified, ultrasonic sensors are typically one of the easiest types of level sensors to install and setup.

 

Radar Sensors

 

These sensors are designed for applications requiring non-contact liquid level measurement, in which a higher level of accuracy is required. These sensors can be adjusted for variables such as materials, tank configuration, and system interface. They are ideal when vapour, dust, or a foaming surface prevents ultrasonic-wave measurements from occurring.

These sensors use microwave-pulses to track any target material from the tip of the antenna to the bottom of the tank. The pulse widths, and sensitivity depend on the distance of the target from the antenna and the dielectric constant of the reflecting material.

This technology provides reliable, continuous pulses and are unaffected by environmental factors such as temperature, vacuums, methane, steam, pressure, carbon dioxide, vapours, and condensation.

It is critical to note that the tank must have a metal bottom to capture the microwave signal.

These sensors are the most expensive choice of tank level sensors and are often two to three times the cost of ultrasonic sensors.

 

Pneumatic Level Sensors

 

Utilized in hazardous areas where there is no electricity and in applications involving heavy sludge. These work by having a compression of a column of air against a diaphragm which then actuates a switch.

These sensors are suitable for use with highly viscous liquids such as grease, or water-based and corrosive liquids. Readings are impacted by the liquids specific gravity. If this varies due to a mixture of chemicals then readings will be inaccurate. They are also affected by the temperature of the liquid.

They have the additional benefit of being a relatively low cost technique for point level monitoring.

 

Magnetostrictive Level Sensors

 

These are similar to float type sensors in that a permanent magnet sealed inside a float travels up and down a stem in which a magnetostrictive wire is sealed. Magnetostrictive level and position devices charge the magnetostrictive wire with electrical current, when the field intersects the floats’ magnetic field pulse is generated. The pulse travels back down the wire at the speed of sound. Just like ultrasound or radar the distance is measured by time of travel from pulse to return.

These sensors are ideal for high-accuracy, continuous level measurement of a wide variety of liquids in storage and shipping containers.

They require the proper choice of float based on the specific gravity of the liquid.

The accuracy of the magnetostrictive technique makes it ideal for “custody- transfer” applications.

 

Capacitive Level Sensor

 

Are quite good at sensing the presence of a wide variety of solids and organic liquids. The technique used is frequently referred to as RF for the radio frequency signals applied to the capacitance circuit, but these are not radar sensors.

Dual-probe capacitance level sensors can also be used to sense the interface between two immiscible liquids with substantially different constants.

These sensors contain no moving parts, they are rugged, simple to use, and easy to clean, and can work in high temperature or high pressure applications.

A danger does exist from build-up and discharge of a high-voltage static charge that result from the rubbing and movement of dielectric materials. This danger can be eliminated with proper design and grounding.

A significant limitation for capacitance probes is in tall bins used for storing bulk solids. These applications require a conductive probe that extends to the bottom of the measured range. Long conductive cable probes (20 to 50 meters long), are suspended into the bin or silo, and are subject to significant mechanical tension due to the weight of the bulk powder in the silo and the friction applied to the cable. Installations such as these will frequently result in a cable breakage.

 

Conclusion

 

When determining the right sensor for your remote tank monitoring needs, it is important that you have complete information on the tank(s) being utilized, their location and the contents of the tanks that you are monitoring.

Using remote tank monitoring solutions can free up your support resources, reduce service costs, production downtime, and unnecessary truck rolls, allowing you to plan better, and operate in less of a reactive state. As a result, the payback is very quick on your investment. Without remote tank monitoring, business-impacting events such as equipment shutdown, lost production, and costly spills and compliance violations can occur unnoticed. When 24/7 production is your goal, and safety and compliance are critical, remote monitoring is a must.

Asset Monitoring solutions are experts in tankmonitoring and can provide the right support that you need in making the correct decision for your remote tank monitoring needs. eTankdataTM is an end-to-end solution that meets your needs and your budget.

 

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