Ultrasonic Sensor For Liquid Level Measurement (1).docx

Ultrasonic Sensor For Liquid Level Measurement Abstract :-

The need for accurate and reliable sensors is growing in so many markets around the world, such as industrial and manufacturing, automotive, medical, energy, and smart grid applications. The designers and end-users of these sensors are counting on newer technologies that use less power and deliver higher accuracy -- technologies that will eliminate moving parts and allow sensing in remote locations and in lessthan-ideal conditions. One of the most effective areas of sensor technology is ultrasonic, the science that measures the time interval between an ultrasonic signal that is sent and received, or what is commonly referred to as "timeof-flight" (TOF). TI is leveraging its ultrasonic expertise to deliver new signal conditioning solutions to fluid level sensing, fluid identification, flow metering, and distance sensing customers with its latest products (TDC1000 and TDC7200) based on time-to-digital converters (TDCs). This application note provides an introduction to how Texas Instruments Ultrasonic Sensing solutions (TDC1000 and TDC7200) can be applied to popular applications such as liquid level sensing, flow sensing, and fluid identification.

Introduction In the industrial production process, it is important to measure and control the height of liquid media stored in containers for production safety. In order to meet different measurement conditions, a variety of liquid level sensors have been developed. At present, there are radar sensors, ultrasonic sensors, radioactive isotope sensors, electronic sensors, thermal liquid level meters, optical liquid level meters, hydraulic pressure gauges, and so on. In some special fields such as petroleum, chemical, and aerospace, the measurement entails more requirements for methods and instruments, especially when a container is to be stored at high temperature or high pressure, and contains inflammable, explosive, highly corrosive, or very volatile liquid inside. In these cases, the detecting sensors cannot be installed in the container directly. An alternative is to use ultrasonic detection technology which can achieve a noncontact and non-immersion measurement without damaging the physical structure and integrity of a container. Therefore, ultrasonic inspection can be used to determine the liquid level and can provide a guarantee of the safety of the detecting process. Generally, these methods can be classified into four types by the realization principle . Each method has its own application conditions and limitations. The first type is based on sound speed, in which the liquid level is obtained by measuring the time difference between the emission and reception of waves; the accuracy of these inspections is easily affected by the pressure and temperature in the container. The second type is penetrative methods, in which the liquid level is determined through comparing the attenuation of ultrasonic waves before and after they transmit the gas–liquid medium, but the sound waves can easily be affected by internal impurities and bubbles in the liquid, which may lead to a weak reception. The third type is based on Lamb waves, in which the liquid level is detected through comparing the propagation characteristics; however, these methods require complex initializations and strict conditions, as described in reference. The fourth type is ultrasonic impedance methods, in which the liquid level is determined by comparing the attenuation time of echoes or by comparing the transmission coefficients of sound waves, but the sensitivity is relatively low. In the

actual measurement, an appropriate method should be chosen according to the specific requirements of the detection environment.

2.Need Of The System 2.1 Tank Level Measurements Senix ultrasonic sensors are commonly used for tank level measurement and control. They offer cost-effective, continuous level measurement and a wide range of features to handle almost any tank level management task. Unlike mechanical level indicators and many electronic level measurement technologies, ultrasonic sensors don’t have to touch the liquid to measure it so the risk of sensor fouling or liquid contamination over time is eliminated. Here are some of the most common applications for ultrasonic tank level sensors

2.2 Tank Level Display In the most basic application, a tank level sensor may simply transmit continuous liquid level data to a process meter or to your PC running SenixVIEW software. A simple display will provide tank level in feet or meters. With more advanced displays, you can convert simple level measurements to engineering units like gallons, liters, pounds or kilograms. ToughSonic sensors, with simultaneous outputs, allow you to select digital or analog output for display while other output(s) provide control input. Display versatility is just one of the many ways that non-contact tank level sensors out perform traditional mechanical level indicators

2.3 Tank Level Alarms Many tank level applications require alarms to be sounded when levels get above or below a certain point. All ToughSonic and ToughSonic CHEM sensors include 2 solid state switches that can be configured to activate alarms at pre-determined levels. Sensor switch outputs are independent of other outputs so, for example, you can maintain a continuous tank level display and alarm settings using the same level sensor.

3.Block Diagrams

1.Working For single transducer in fluid level, fluid identification, and distance applications, the TDC1000 AFE excites the sensor and detects the echo once it returns, as shown in Figure 1. The AFE excites the transducer by hitting it with a series of pulses and with frequencies in the range of kHz to MHz. The TDC1000 allows a maximum number of 31 pulses to excite the sensor, and its frequency can have a range from 31.25 kHz to 4 MHz. The excitation of the sensor is marked with a START pulse, while the echo is denoted with a STOP pulse. The difference in time between START and STOP time-of-flight (TOF) indicates the fluid level, fluid ID/concentration, and distance.

Figure 1. How Ultrasonic Works for Single Transducer Measurement For dual transducers applications such as flow meter (Figure 2), the TDC1000 AFE utilizes the transducers in a pitch-and-catch fashion. In this method, transducer A is excited by the TDC1000 while transducer B acts as a receiver to generate STOP pulses. The time-of-flight (TOFAB) between excitation and STOP pulses indicates the distance between the transducers. A differential ToF, TOFAB – TOFBA, needs to be done to find the flow of medium (water or gas) between the two transducers.

Figure 2. How Ultrasonic Works for Dual Transducer Measurement Further information on how ultrasonic is used in fluid level sensing, fluid identification, flow metering, and distance sensing applications will be discussed in the next sections. 2 Application No. 1: Fluid Level The TDC1000 is ideal for many sensing applications, and some of the most common involve managing fluids. Figure 5 shows a diagram for detecting fluid level in a container. Ultrasonic sensors are mounted on the bottom or top of the tank to determine the level of the fluid. The TDC1000, ultrasonic AFE, excites the non-intrusive transducer with 1 to 31 pulses. The sensor continuously transmits pulses of high frequencies (typically 1 MHz) into the fluid. Then the TDC1000 reports the time-of-flight (TOF) that the wave takes to transmit to the liquid surface, and reflect back to the sensor. Referencing the speed of sound in the fluid and using the equation TOF = (2* fluid level) / (fluid speed of sound), the exact distance of the liquid surface from the sensor can be calculated with high accuracy. For example, assume that the transducer is mounted at the bottom of a water tank, and the TDC1000 reports a TOF of 1 ms. Knowing that the speed of sound through water is approximately 1480 m/s at 25°C, the fluid level can be calculated as: TOF = (2*fluid level) / (fluid speed of sound) (1) Fluid level = (TOF * fluid speed of sound) / 2 (2) Fluid level = (1ms * 1480 m/s) / 2 (3) Fluid level = 0.74m (4)

Figure 5. Fluid Level Measurement Using Ultrasonic

For more information on how to select or mount ultrasonic transducers to the outside of a tank, see Application Note: How to Select and Mount Transducers in Ultrasonic Sensing for Level Sensing, SNAA266. In addition to this application note, there are videos, application notes, and other design tools available at http://www.ti.com/ultrasonic. Application No.2:Fluid Concentration:Now, let’s add to that level application, and see how fluid identification (ID) works. Fluid ID is the same asfluid level detection, except that the sensor is typically mounted on the side of the tank, as shown inFigure 6. Again, the TDC1000 excites the sensor, and then it reports the time the wave takes to transmit and reflect back. Since the exact distance is known, and time-of-flight (TOF) is measured, the speed of sound through the fluid can be calculated, and checked against a look-up-table to identify the fluid. For example, assume the distance of the tank is 0.1m and TDC1000 reports a TOF of 134μs, we cancalculate the fluid speed of sound as: TOF = (2*distance)/ (fluid speed of sound)

(5)

Fluid speed of sound = (2*distance) / (TOF)

(6)

Fluid speed of sound = (2*0.1 m) / (134 μs)

(7)

Fluid speed of sound = 1490 m/s

(8)

Using a look-up-table and knowing that the temperature is approximately 25°C, we can figure out that the unknown liquid inside the tank is oil.

Figure 6. Fluid Identification / Concentration Using Ultrasonic For more information on performing Fluid Identification or determining Liquid Contamination, see Application Note: Ultrasonic Sensing for Fluid Identification and Contamination, SNAA265. In addition to this application note, there are videos, application notes, and other design tools available at http://www.ti.com/ultrasonic.

Advantages:- Major advantages offered by ultrasonic level measurement technique are mentioned below:   

Ultrasonic level sensors are usually non contact type i.e. they do not make any contact with the process fluid under level detection. Besides, they consist of fixed components only hence require less maintenance They are usually mounted at the top of the vessel due to which they are less likely to offer leakage problems as compared to entirely wetted means.

Disadvantages:- Ultrasonic level measurement technique can not be suitably applied in all fields since use of ultrasonic level sensors includes few setbacks too. Many factors exist which have the tendency to influence the returned echo signal back to the sensor. Some of them include:     

Materials like powders etc. Heavy vapors Surface turmoil Foam Ambient noise and temperature

Besides, ultrasonic level measurement devices do not work satisfactorily in areas involving vacuum or high pressure conditions.

Limitations:1.Not Appropriate for Use with Every Liquid Medium:Ultrasonic level sensors work by emitting pulses of ultrasonic sound, which then bounce back to the sensor unit to establish where the liquid line is located within a tank or vessel. With some liquid media, however, these ultrasonic waves are absorbed rather than reflected. If you’re thinking about using an ultrasonic level sensor, be sure that your liquid medium reflects ultrasonic sound rather than absorbing it. 2.May Not Work with Agitated Liquids:Agitated liquids, turbulent liquids, foaming, sloshing, and other activity can hamper the performance of ultrasonic sensors. In applications where liquid agitation is common, ultrasonic sensors may therefore be a poor choice. This may not necessarily be the case, however: the integration of a so-called still pipe can allow for ultrasonic sensor use with some agitated liquids. 3.No Side or Bottom Mounting:Since ultrasonic sensors work by emitting sound waves directly at the liquid surface, top-mounting is necessary. This means that applications or facilities that require bottom or side-mounted sensors are not well-suited for ultrasonic sensors. Conclusion:-1.The Water level controller can be used in hotels, factories, homes, commercial complexes, drainage, etc. 2.Fuel level indicator in vehicles

3.liquid level indicator in huge containers in most big companies