PLA manufactures four types of SmartDiver probes with five sensing technologies offering our customers a sensing technology for every level measurement application.
Although, the vast majority of applications are served well with our standard ultrasonic probe (figure 1), there are a few applications that require a different technology.
Figure 1. SmartDiver standard probe (combines ultrasonic attenuation and acoustic impedance methods of measurement).
In this article, we will explore the different types of technologies that are used in the SmartDiver probes and discuss in detail the specific applications for which each technology is best suited.
Ultrasonic attenuation (suspended solids)
This is one of the technologies in our standard probe sold with most of our units to measures suspended solids. In this method of measurement, a piezo resistive element embedded inside a PEEK sensor ball sends an ultrasonic pulse.
This energy is sent through the slurry to a reflector and some of the energy is received back (figure 2). If there are no solids, most of the energy sent is received back at the transducer. As solids are introduced, that energy is attenuated up to about 200 gpl where no signal is received back at all (saturation).
Figure 2. Sound attenuation and suspended solids measurement
Ultrasonic sensors can take up to 130 deg C.
These sensors cannot accept any air what so ever. If your thickener is fed by, for example, a cyclone or you have a cavitating pump or a feed pump with failing mechanical seals drawing air, then these sensors will not work. Hence, for applications where there is entrained air a more suitable technology will be light attenuation.
Acoustic impedance (density)
This method of measurement is shown in figure 3. A pulse (red arrow) is sent out by a transducer (piezo element). In air, no energy transfers off the face of the sensor and all the energy is reflected back (green arrow) to the transducer as the energy has nowhere to go.
As the sensor is placed into something denser, for example water, some of the energy is allowed to come off the face of the sensor and go into the water and away.
As the medium gets denser, for example in a slurry, more energy is allowed to pass off the face of the sensor and into the slurry and less energy is reflected back to the transducer. This method measures how much energy comes back from the interface between the transducer and the slurry. The more energy back, the less dense the medium.
Figure 3. Acoustic impedance and density measurement
Acoustic impedance will not give you any clarity reading or even light interface. But it will give you a great profile into the mud so you can see not only your mud depth but also the mud profile.
PLA trends both ultrasonic attenuation and acoustic impedance while diving with the ultrasonic diver probe. In figure 4, the density profile (green) kicks in when the attenuation profile (blue) is saturated. The combinations of both methods of measurement is shown in figure 5.
Figure 4. SmartDiver trend – attenuation and density
Figure 5. Standard SmartDiver probe technologies – combined sound attenuation and acoustic impedance
Light attenuation – (4 beam)
This technology uses two LED’s and two light receivers (figure 5). Looking just at a single light source and a single receiver first, we get attenuation of that light as solids are introduced between the forks. Higher solids, less light. Now looking at three forks, the third is a receiver also. The set up picks up the same light but over a shorter distance. In this way, we can compensate for scale build up on the forks as the same light is received over two distances. Now introducing the 4th fork, the second LED. This does a cross check of what the other LED produced but now the distances are reversed.
Advantages of light attenuation are that they are relatively inexpensive, easy to understand and operate. The measurement is immune to scale. This means that the clarity reading will not drift with scale and that we can “pick” an interface (and mud) target without needing to take a reference reading in the clear liquor as we have to do with all other sensor technologies. Hence, if we do a dive and we are reading high solids in the overflow, then its high solids in the overflow. There is no possibility that an operator can reset a sensor dirty fault and zero the reading in dirty overflow as can and does happen with fibre, conductivity and ultrasonic sensing mud divers.
The down side of this technology is that light attenuation can measure only up to 20 gpl so you are heavily reliant on the heavy mud tilt switch for your true mud reading. As they also have electronics with the sensor, they can only be used in applications up to 80 deg C.
Figure 5. Light attenuation sensor
This sensing technology for level measurement is probably the cheapest of the bunch, but can only be used in applications where the mother liquor (carrier liquor) is conductive i.e. caustic at a good concentration (last washer for instance would not be a suitable application for a probe with this technology).
Suspended solids interfere with the conductivity reading of the liquor and the conductivity drops as the solids increase. This sensor is less susceptible to entrained air than ultrasonic but it is still susceptible. The downside of this method of measurement is that you lose any clarity reading as the clarity is the mother liquor concentration and this varies.
Fibre optic backscatter
This sensing technique is only used on applications where we have entrained air and require a solution that is high scale and temperature resistant. For example, certain applications in the alumina industry, namely those tanks that have an atmospheric flash vessel on the tank feed that draw air into the tank when the tank is colder than the slurry being fed.
A probe with this technology produces a nice clarity reading and it is capable of running up to 200 gpl suspended solids. Another good feature is that because it runs fibre to and from the window where the back scatter occurs, there is nothing to fail so the sensor has great longevity. However, the fibre optic cable may be more susceptible to damage with the continuous winding on and off the drum.
Even though most of the SmartDiver applications can be covered by our standard ultrasonic method of measurement, in some very specific cases a different technology is required. Factors such as scale, entrained air, and temperature will determined if other technologies such as conductivity, light attenuation or fibre optic backscatter are a better fit. Discuss with your PLA representative about the best technology for your specific application.
For more information contact our technical team at email@example.com or call us on +61 3 9786 1711