The slurry is a combination of two types of matter- solid and liquid. To measure the level of any slurry in metallurgy or ore mining, its composition plays an important role. There are multiple techniques that provide level measurements. They are broadly classified as contact-based and non-contact based. Since contact-based level measurement technology is highly receptive to corrosion along with a risk of reaction due to different types of elements mixed together in the slurry, the non-contact based technology is the most sought-after in the industry. There are five different types of non-contact based technologies available for level measurement nuclear, laser, weight, radar, and ultrasonic. Among all the technologies available, ultrasonic is the most widely used.
Ultrasonic level transmitter is the go-to level measurement technology due to its simple operation, cost-effectiveness, and easy installation. At the same time, ultrasonic is completely non-contact based so its application suits any type of liquid irrespective of its composition. When it comes to the slurry, different metal compositions and alter the accuracy of measurements if other measurement technologies are chosen. For example, nuclear technology is applicable to relatively small vessels. Also, safety and licensing are an issue. The reactive nature of the metals makes it an unviable option in the case of reactive elements in the slurry. Laser and weight technology have been used mostly in the selective niches of powders and solids in bulks, and small vessels respectively. Radar technology is another alternative to ultrasonic however, the high cost involved in more precise options make it a less favourable choice.
As the name suggests, Ultrasonic level transmitters use ultrasonic waves to hit the surface of the liquid. It is based on the technology used by bats for flying and catching their prey. Ultrasonic waves are sound waves in the range of 20 to 200 kHz. The operation is based on calculating the time-of-flight principle which in turn translates the result into an electronic signal to signal the continuous level measurement.
The ultrasonic level transmitter consists of the level sensor, piezoelectric crystals, and control circuit. The level sensor receives the electric current and converts it into ultrasound waves. The piezoelectric crystals help in converting the electric current into ultrasound waves by oscillating at a high speed and these oscillations generate sound waves of very high frequency. These waves are then sent to the liquid surface and an echo is received back when the waves by the level sensor. There are 2 level sensors usually – one for sending and one for receiving.
The base of measurement of distance is the base of the tank or the vessel. The time taken by the waves to hit the surface and then return back is used to calculate the distance. To obtain the distance simply divide the time taken by the echo to be received by the receiver level sensor by two. However, there are a few considerations that might affect the time of flight for the waves to be received back in case of continuous level measurement.
Although ultrasonic level transmitters are feasible to use with almost all types of liquids and slurries, yet there are a few key points that should be adhered to while installation and set up.
- The type of vessel can affect the readings in the case of ultrasonic level transmitters. The shape of the vessel needs to be considered while setting up the level sensors. The ultrasonic level transmitter works accurately in small vessels, but for large vessels wide opening at the top is required. This is because the ultrasonic level transmitters have a dead zone that is very close to the sensor and it is not able to track ultrasounds in the zone. Especially, when continuous level measurements have to be taken, the accuracy can be significantly dropped. At times, the acoustic waves are received if the vessel material returns unnecessary echoes which can affect the readings.
- The pressure in the vessel can affect the speed of the waves. Hence, if the conditions in the vessel are changing it can affect the precision of the ultrasonic level sensor. There are a wide variety of sensor material available and the one that fits best the conditions in the vessel should be chosen.
- The temperature within the vessel should not exceed a very high or very low temperature. Temperature variations ranging in either extreme side of the spectrum can alter the piezoelectric crystals which in turn can cause deformations and continuous level measurements may reflect incorrect data.
- The position of the level sensor is extremely important in slurry level measurement and it should be directly above the surface at 90°. In case it is not followed, the echo might be missed altogether, or worse, the receiver level sensor might receive a bounced off echo deflected from the surface of the vessel.
- Avoid rotating devices and agitators inside the vessel as they generate additional echoes. These echoes can interfere with the echo of the sensor and give false readings. Additionally, there are echo compensating circuits available in case rotating blade devices are to be placed in the vessel.
- The condensation and humidity can affect the level sensor performance when recording continuous level measurements. To prevent this coating of moisture and condensation, place the sensor high enough to not be impacted.
- As the ultrasonic waves use a medium to travel, the medium itself can affect the results. In most cases, the medium is the air itself but if there are gases present in the vessel they can affect the readings as the speed of sound varies with different gases.
- Similar to gases, the presence of foam, dust, build-up on the sensor can alter the readings of the level sensor hence, these should be kept in mind while choosing the material of the sensor.
Ultrasonic level transmitters are widely used in slurry level measurements especially in open channel flow. Their versatility comes from the fact that it is cost-effective and non-contact based. The installation is often simple and easy to operate. Yet, mounting it in a capacity beyond the specified limit given by the manufacturer will not yield the desired results. Hence, adhering to the manufacturer’s guidelines are the best way to get optimum results.