Lasers come to level measurement

Laser-level technology is expanding options in sensor applications, and CONTROL’s favorite sensor expert, David Spitzer, writes about the good and bad points of this new measurement technique.

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Laser-level MeasurementBy David W. Spitzer, Spitzer & Boyes LLC

 

NON-CONTACT, level-measurement sensors, such as laser, non-contact radar and ultrasonic technologies, were developed for applications where contacting sensors were seen as problems, either through false readings or high maintenance. Non-contact sensors are generally located above the material and aren’t directly exposed to the material. These sensors are exposed to the environment of the material in normal operation, and can come in contact with the material when the level of the material is excessively high. Level sensors mounted externally to the vessel, such as nuclear, ultrasonic (bottom-up), and laser (sight glass) technologies, do not come in contact with the material.

All non-contact, level-measurement systems consist of a level sensor located above the material surface that emits a signal and processes the returns (reflections) of that signal. These technologies measure level continuously, but they do so at essentially one point in the vessel. This generally doesn’t pose a problem for liquid applications where the gas/liquid interface is horizontal. Liquid applications with multiple interfaces affect the performance of ultrasonic level devices, while low-dielectric-constant interfaces affect the performance of radar level devices.

In addition, in solids applications, material entering and leaving the vessel affects the solid/gas interface. For example, a rat-hole may form as solid material leaves the vessel. If the level measurement reflects a point in the rat-hole, the measured level will decrease (as expected). However, if the material remaining on the sides of the vessel falls and fills in the rat-hole, the level will abruptly (and unexpectedly) increase. If possible, sensors should be located such that the measured level represents the actual level, while avoiding rat-hole affects. Multiple sensors or scanning sensors may be needed if an appropriate location can’t be found.

Similarly, laser-level measurement sensors emit a laser beam towards the material, and measure the remnants of the beam reflected from the material. These systems determine the level of the material by measuring the time that the laser beam takes to travel to and return from the material. The distance between the sensor and the material can be calculated as one-half of the measured time-of-flight multiplied by the speed of the laser beam. Mechanical dimensions can then be used to determine the percent level in the vessel.

Laser-level technology isn’t limited by the dielectric constant of the material (as is radar-level technology) nor its propagation velocity in the vapor space (as is ultrasonic-level technology). In addition, the laser beam emitted by the sensor doesn’t diverge much, so it can target smaller areas than radar- and ultrasonic-level technologies. Further, in some applications, laser-level transmitters can be aimed to sense the level in locations that can be difficult to measure using other technologies, such as inside the chute of a bin.

However, degradation of the laser beam’s strength between the sensors and the material can cause laser-level measurement systems to fail. Degradation can occur at the sensor, in transit to/from the material, and at the surface of the material. Dirt, dust or other coatings on a laser transmitter/receiver can cause the received laser signal to be weak. Because accumulations over time are normal for the process, routine maintenance may be required to keep the transmitter/receiver operating properly. In many applications, the sensor may be shielded in a tube and/or continuously purged with gas to keep it working. Similar concerns exist when the laser beam travels through a sight glass that can become dirty and cause attenuation of the beam.

Also, accuracy can be degraded based upon the surface on which the laser beam is reflected. For example, the laser beam will likely measure the top of a layer of foam by reflecting off the top of the foam. If the foam is transparent to the laser energy, the beam may reflect from the foam/liquid interface and measure the liquid level. Translucent foam might cause the level measurement to represent a location within the foam. Further, foam conditions may vary over time and cause erratic level measurements.

 

     FIGURE 1: LASER LEVEL APPLICATIONS
  Laser Level Applications
 

Lasers can be used in many non-contact applications where radar and ultrasonics don’t work.

 

Many laser-level measurement sensors use Class 1 lasers that don’t generally pose a hazard under normal operating conditions (See Figure 1). Other sensors use Class 3 lasers that can pose a risk of eye injury when viewed with the naked eye for more than a moment.

Jim Van Rems, vice president of Riegl Laser Measurement Systems says, “Laser level measurement systems are among the most expensive level-measurement technologies available, so they tend to serve niche markets where no other level measurement technology is viable.” Measuring the level of molten glass and metals at temperatures as high as 2000 C, is one such application, according to Van Rems.

Other typical applications include measuring the level of wet or dry material in tall tanks that pose problems for radar and ultrasonic level technologies. Van Rems warns against installations where the vapor space is opaque to light and/or contains fine particles, reflective surfaces and/or electric charges. For example, the level of flour is difficult to measure using laser level technology because it contains fine particles that reflect light and contain an electric charge that causes light to be obstructed, reflected, and collect on the sensor. Steam is another example of a vapor that reflects light.

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