Laser Level Measurement principle

The least-common form of echo-based level measurement is laser, which uses pulses of laser light reflected off the surface of a liquid to detect the liquid level. Perhaps the most limiting factor with laser measurement is the necessity of having a sufficiently reflective surface for the laser light to “echo” off of. Many liquids are not reflective enough for this to be a practical measurement technique, and the presence of dust or thick vapors in the space between the laser and the liquid will disperse the light, weakening the light signal and making the level more difficult to detect.

Working principle of Laser level measurement

Laser transmitters used for level measurement basically work upon the same operating principle as ultrasonic level sensors. The only difference between two methods is that the former employs speed of light whereas the latter makes use of sound waves speed. Laser level measurement devices can work with variety of materials such as bulk solids, slurries, and opaque fluids like dirty sumps, milk, and liquid styrene etc. Just like an ultrasonic sensor, a laser transmitter is also mounted at the top of the tank which then directs a short pulse of light in downward direction towards the surface of the process liquid. This light is then reflected back to the sensor. The measurement system includes a timing circuit which is used to determine the time taken by the light signal to return back and hence finds out the distance between the sensor and the process material. A figure depicting the working of a typical laser level transmitter. Three types of laser technology are commonly used in the process control market today: pulsed, continuous-wave (frequency-modulated), and triangulation. It is important to beaware of the fundamental operating differences between the technologies and the suitability of each for specific types of applications.

Time of flight

This technique uses the time light takes to travel to a target and back, but the time for a single round trip is not measured directly. Instead, the strength of the laser is rapidly varied to produce a signal that changes over time. The time delay is indirectly measured by comparing the signal from the laser with the delayed signal returning from the target. Calculated using the time and the speed of light outlined by the formula, D = (C x T )/ 2 where D = distance to the target (m) C = speed of light (3 × 10 8 m/sec) T = time of flight of laser pulse (sec).

Confocal chromatic:

This uses use a white light source to accurately measure the distance to surfaces. The essence of our confocal chromatic imaging principle is the accurate detection of colours from light that is reflected back from target surfaces. The white light is focused onto the target surface by a multi-lens optical system. These lenses disperse the light into monochromatic stages (colours) along the measurement axis. A specific distance to the target is assigned to each colour’s wavelength in a factory calibration. Only the wavelength which is exactly focused on the target is used for the measurement. This light reflected from the target surface is transmitted from the probe, through a confocal aperture and onto a spectrometer which detects and processes the spectral changes and calculates distances. These distance measurements are transmitted at high speed via Ethernet communications protocol

Triangulation

One method for accurately measuring the distance to targets is through the use of laser triangulation sensors. They are so named because the sensor enclosure, the emitted laser and the reflected laser light form a triangle.

The laser beam is projected from the instrument and is reflected from a target surface to a collection lens. This lens is typically located adjacent to the laser emitter. The camera views the measurement range from an angle that varies from 45 to 65 degrees at the centre of the measurement range, depending on the particular model.

Applications :- Granular solids, nontransparent liquids, slurries, and molten metals. Inaccuracy:-  ± 5 mm ( ± 0.2 in). Range:-  0.2 to 250 m (0.5 to 800 ft) Price :- $800 to $3000 USD

Laser transmitter advantages and disadvantages

Advantage of laser level transmitter 

Following are the major advantage  of laser level measurement techniques:

  • “Lasers have virtually no beam spread (0.2° beam divergence) and no false echoes, and can be directed through spaces as small as 2 in.
  • Laser transmitters are capable of providing accurate results, even in processes involving heavy vapors and foam.
  • They can be conveniently used with vessels having multiple blockades. Moreover, these devices can work with distances as high as 1500 ft.
  • “For high-temperature or high-pressure applications such as in reactor vessels, lasers must be used in conjunction with specialized sight windows to isolate the transmitter from the process. These glass windows must pass the laser beam with minimal diffusion and attenuation and must contain the process conditions.”
  • As compared to ultrasonic and microwave waves, the wavelength of light waves is very short, due to which they experience no virtual beam divergence, hence a laser beam can be targeted on a point without facing any intervention from the nearby entities.
  • “The laser radiation propagates through the medium at the speed of light. This means that very high update rates are achievable, with none of the ‘lock-in’ problems that can be experienced when using ultrasonic instruments.”
  • Laser level measurement is absolutely independent of the pressure or temperature variations of the medium such as air. Consequently, the speed of light passing via any gaseous medium does not vary at all. Even in level measurement applications involving unidentified and frequently varying gaseous densities such as petrol or gas tanks, laser level transmitters offer accurate results with no calibration issues.
  • In comparison to other level measurement techniques like ultrasonic, radar, nuclear or mechanical methods, laser technique is considered to be highly developed in technology.
  • Laser level transmitters provide very quick and accurate response than other techniques. Besides, they are simpler to apply.
  • Unlike radar level and ultrasonic level technology, the performance of laser level transmitters is remains unaffected by the dielectric constant of the process material or propagation velocity in the vapor space respectively.
  • The laser beam transmitted by the sensor doesn’t undergo much divergence hence it can aim and sense smaller areas as compared to than radar and ultrasonic level measurement devices.
  • Furthermore, laser level transmitters can be applied in areas where other level measurement technologies fail to work, for example, inside the chute of a bin.




Disadvantages of laser level transmitter 

Few disadvantages associated with the use of laser level transmitters are mentioned below:

  • Use of laser level measurement techniques prove to be very expensive.
  • The strength of the laser beam transferred between the laser level transmitter and process material may sometimes get degraded due to which the functioning of the level detection system can suffer. This degradation can happen at any of the three stages: at the sensor/detector, in transit between the sensor and the process material, or at the surface of the process material.
  • Presence of dirt, dust or any other type of coating on a laser level transmitter can seriously affect the performance of these devices since it weakens the strength of the laser signal. Hence, routine maintenance must be carried out carefully for preventing accumulations and coatings over time and proper functioning of laser level measurement devices. For some applications, these sensors can be enclosed inside a tube or cleaned with gas to keep them in a working condition.
  • Besides, their accuracy can also get affected depending upon the surface of the process material which reflects back the laser beam to the sensor. “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.”
  • Foam conditions usually changes with time and results in inconsistent level measurement readings.

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Technical Specification of Laser level transmitter 

Sl No.  Instrument Details Description
1 Tag No. LV-1421
2 Service Chemical
3 Tank No. RJ-1232
4 Range 0.5 to 100 m (2 to 330 ft)
5 Resolution 5 mm (0.2 in)
6 Typical accuracy ±11 mm (0.4 in)
7 Measuring beam Laser wave length: 905 nm, eye safe
8 Laser beam divergence < 0.35°
9 Environmental conditions XYZ
10 Operating temperature –40° to +60°C (–40 to +140°F), up to 280°C (535°F)
11 Process pressure –1 to +2 bar (29 psi)
12 Output HART/ 4 to 20 mA, NAMUR compliant
13 Power supply Powered from the loop  4 to 20 mA, 16 to 42 V DC
14 Enclosure material Powder coated aluminum (standard), 316L stainless steel
15 Protection class IP66 / IP67/ Nema 4 (SS version 4X)
16 Process fitting Flange (ASME 2”, DN50)
17 Wetted parts  316L SST
18 Display  Integrated 128×64 pixels LCD display with TTG (Through-The-Glass) interface
19 Approvals CE, ATEX, IECEx, FM, 3A
20 Manufacturer ABB
 




Laser level transmitter mounting guide line

 

Electrical connection laser level transmitter 

HART terminal – 2 wires

HART 2 WIRE SYSTEM
HART terminal with heater option – 2 + 2 wires

Laser level transmitter Manufacturer

  • K-Tek Corp. (www.ktekcorp.com)
  • Optech Inc. (www.optech.on.ca)
  • Thermo MeasureTech (www.thermomt.com)
  • ABB

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