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. However, lasers have been applied with great success in measuring distances between objects.
Laser Level Measurement Principle :
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 is shown in the figure below.
Image Credits : Chipkin Automation System
Advantages of laser level measurement :
- “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 transmitters :
- 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.