Selecting the Correct Level Sensor Technology

Table of Contents

Introduction
Level Measurement Selector
Tank Installation Guide for Massa Ultrasonic Liquid Level Sensors
     Purpose
     Overview
     Installation Problems

Introduction

The sight glass is a manual approach to monitoring condition levels in inventory or process tanks, and therefore it continues to be a preferred method for a wide range of industries. Real time monitoring to reduce inventory costs and improve process efficiency has been facilitated, thanks to the adoption of lower cost tank gauging indicators and level sensors. Integrated electronics, which enabled advanced sensor control, provides significantly lower cost benefits and is now readily available with traditionally hard wired or wireless communications.

Although a host of measurement technologies are available for today’s system control and maintenance engineers, one fact of business remains unchanged – “purchase the technically acceptable, least cost technology to get the job done correctly and reliably.” Level measurement methods include, but are not limited to:

Floats – These devices operate on simple principal of buoyancy and are useful for course measurements, where coming into contact with the material of interest is not a major problem. Floats have lately been migrating to magnetic technology, which improves performance but at the same time increases costs. However, floats involve moving parts, which can pose a risk with certain conditions or materials for reliable readings.

Magnetic gauges – These devices are similar to floats in that they come into contact with the material being measured, but they tend to depend on strong permanent magnets riding an auxiliary column. This is so that when the float shifts, the change in height location is conveyed by the magnetized shuttle. Magnetic gauges can be optimized around fluid specific gravity and are capable of handling high pressure and temperature.

Hydrostatic devices – These devices usually work on the principal of differential pressure and are very popular. They are considered to be accurate, but have to be submerged in the container being monitored to work. A variety of these devices are available including displacers, bubbler-type, and most commonly, pressure transducers. For accuracy, proper calibration and set-up conditions are important. Hydrostatic devices offer a wide range of measurements and excellent accuracy levels at a moderate price.

Radar level transmitters – These devices are typically through air or guided wave. They are similar to other sonic technology, microwaves are beamed down to the target and reflected back to the sensor antenna. Round trip time is measured to acquire distance. Air radar technologies are known for their long range, relatively long dead band, high device cost, and ability to work with headspace vapor. Air radar, in certain cases, may exhibit beam divergence related structural interference and set-up can become tedious. These issues aside from cost can be eased with guided wave technology in which the signal travels along a stiff probe that is contacting the target material.

Capacitance sensors – The function of these sensors is based on the assumption that the fluid determined has a dielectric constant and the capacitance change differs with fluid level. The measurement is produced by an uninsulated rod or insulated rod, referenced either to the target material or a reference probe or target material. When there is a change in capacitance, the corresponding level change can be established.

Ultrasonic level sensors – These are non-contact devices long known to be the workhorse in various industries because of their ease of use, blend of technical capabilities, and lower total cost of ownership. Ultrasonic sensors, similar to air radar in concept, propagate an ultrasound sound pulse from a piezoelectric transducer that bombards the measurement target and is reflected back. The round trip time speed of sound, which is typically in the range of 40 kHz to 300 kHz, is measured to determine the distance to the object. The sensor is temperature compensated and is typically used in air-based atmospheres. However, it may work effectively in the presence of specific headspace vapors based on chemical concentration.

Continuous monitoring or ultrasonic level measurement control is common with water, diesel fuel, pastes and solid material powders, chemical liquids, pellets, or bulk products. Typically known as air transducers, ultrasonic sensors, or level transmitters, this technology is reliable and low cost. Sonic technology continues to be a preferred level measurement method for chemicals, agricultural, mining, and water and wastewater applications.

Level Measurement Selector

Ultrasonic sensors can be grouped according to the operating frequency that works best for any given application. Since wavelength is an important factor for determining optimal ranging capability, short range applications like smaller tanks, bins, and totes only need to range upwards of 6 to 7 feet (2 m) but also need a short deadband of only a few inches. For such short range applications, a moderate operating frequency like 150 kHz is optimal. For medium range applications, for example 20 feet (6 m) operation closer to 90 kHz, is preferred. A longer distance up to 30 feet or even 50 feet can be achieved by reducing the frequency to 40 to 50 kHz, typically. However, frequency is not the sole factor. Other considerations like beam focusing, sound pressure, and sensor control algorithms play a role in the design. From an application viewpoint, target materials are the major factor. The following table provides an overview on what technology is best utilized where.

Continuous Level Technology Ultrasonic Radar Guided Wave Radar Hydrostatic Capacitance
Ease of Use Yes Yes No Yes No
Non-Contact Yes Yes No No No
Cost of Ownership Low High Hgh Low Low-Med.
Measurement
Level Y Y Y Y Y
Interface (liq./liq.) NR NR Y Y P
Interface (liq./sol.) P NR NR NR P
Volume Y Y P Y P
Flow (open channel) Y P NR NR NR
Level Applications
Changing density Y Y Y NR Y
Changing dielectric Y Y Y Y C
Turbulence Y Y C Y C
Aggressive Chemical Y Y Y Y Y
High Pressure C Y Y Y Y
Stream C C Y Y C
Solvents C Y Y Y Y
Foam C C C Y C
Buildup P P C C C
High Viscosity Y Y C C P
Dust P Y Y NR P
Solid Powders P Y C NR C
Granules/Pellets Y Y C NR C
Solid > 1" (25 mm) Y Y NR NR NR
High repose angle P Y Y NR NR

Y - preferred; C - condition dependant; P - possibe; NR - not suggested

Tank Installation Guide for Massa Ultrasonic Liquid Level Sensors

Purpose

This section helps in choosing proper mounting locations on tanks for Massa Ultrasonic Liquid Level Sensors, because inaccurate measurements may occur due to incorrect installation.

Overview

With Massa Sensors, a narrow beam of ultrasonic sound is transmitted that reflects from the surface of the liquid in a tank and returns back to the sensor (Figure 1). The distance to the liquid is determined by measuring the amount of time it takes for the echo from the transmitted sound pulse to travel to the surface of the liquid, and then back to the sensor.

Figure 1. Illustration showing a sensor mounted on a tank transmitting a conical ultrasonic beam that reflects from the liquid surface.

To obtain accurate liquid level measurements, the sensor should be properly mounted to the tank such that the echoes from the liquid surface return back to the sensor, and that objects in the path of the sound beam do not create “false echoes.”

The following paragraphs discuss some standard installation problems that should be avoided.

Installation Problems

Incorrect “Empty Tank” Reports, Due to “Off Angle” Echoes that Do Not Return to the Sensor

The Massa Sensor should be mounted level, meaning that the conical sound beam is perpendicular to the surface of the liquid. In case the axis of the sound beam is not perpendicular to the surface of the liquid, the reflected echo will not return back to the sensor for detection. Several factors contribute to an unleveled sensor problem, for instance, if the sensor is mounted “off angle” (Figure 2), the echo will go undetected. A similar situation is encountered if the tank is tilted so that the top and bottom are not level. In order to resolve this issue, the sensor can be mounted with self-aligning bulk head fittings.

Figure 2. Illustration showing a sensor mounted on a tank “off angle”, so that the reflected echo does not return.

Incorrect “High Liquid Level” Reports, Due to Obstructions in the Path of the Sound Beam

The ultrasonic beam path should be free from any obstructions as these may reflect the sound and promote a “false echo” to return to the sensor before the echo from the liquid surface. Figure 3 illustrates some standard mounting issues that must be avoided.

Figure 3. Illustrations showing how obstructions within the sound beam can produce “false echoes.”

In case the sensor is mounted such that the sound beam hits the tank wall before the liquid surface, any reflecting surfaces on, or near, the side of the tank can create a “false echo.” Some common examples of reflecting surfaces that create “false echoes” are structures mounted in the tank, such as ladders, or protruding or recessing tank seams. To resolve this issue, the Massa Sensor can be mounted closer to the tank’s center, so that the sound beam does not intersect any reflecting surfaces or objects.

Incorrect “High Liquid Level” or “Empty Tank” Reports When the Tank Is Being Filled

Tanks can be filled either from the top or the side. In the case liquid entering the tank from the side intersects the sound beam, it can either disperse the sound so that the echo does not return to the sensor, or reflect the sound and produce a “false echo.” Fluid entering the tank from either the side or the top can considerably agitate the liquid surface. This can disperse the sound beam, and the weak echo created may not return to the sensor. These two scenarios are illustrated in Figure 4.

Figure 4. Illustrations showing how filling the tank can disrupt the sound beam and produce “false echoes” or echoes too weak to be detected.

To resolve this issue, either the Massa Sensor must be mounted in a location so that the sound beam is not affected by the incoming fluid, or the liquid level measurements taken should be disregarded when filling agitates the liquid surface. Users should wait until the liquid level is sufficiently high enough to create detectable echoes.

Incorrect “High Liquid Level” Reports Caused by the Sensor Being Mounted Onto a Standpipe

A “false echo” can be reflected if a Massa Sensor is mounted onto a long standpipe that has a sufficiently small diameter for the sound beam to bombard the sides at the opening into the tank. The sensor interprets the echo from the opening, which arrives before the echo from the liquids surface, as a “high liquid level.”

To resolve this issue, a shorter standpipe or larger diameter standpipe can be employed, or alternatively the sensor’s sensitivity can be reduced.

Resolving Common Installation Problems

The previous examples of installation issues can be overcome as described if using "standard" general purpose sensor models. Massa also offers "plus" models able to overcome many of the issues using advance signal processing and set-up menus. Consult with a Massa Application Specialist to determine the best sensor for your application.

This information has been sourced, reviewed and adapted from materials provided by Massa Products Corp.

For more information on this source, please visit Massa Products Corp.

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