Theoretical Knowledge about ultrasonic Reflection

Ultrasonic Turbidity Measurement 

Per definition is Turbidity an optical Impression.
Turbidity describes the characteristic of a transparent product, to scatter or absorb light. A focused light beam will be attenuated and scattered in hazy products, so that this product can become practically opaque in bigger layers. Turbidity is caused by particles in transparent products. A particle is defined as something with a different refractive index as the carrier liquid. Some examples of particles are minerals, yeast cells, metals, oil drops in water, milk in water, gas bubbles and aerosoles.

Ultrasonic Reflection
The ultrasonic particle measurement is used to detect non-dissolved (suspended) particles in a liquid, similar to a turbidimeter. Turbidity is an optical effect. Therefore the acoustical method is typically named as particle or concentration measurement. Different as at the optical method here a particle is defined as something with a different speed of sound as the carrier liquid. Equal to a sonar system, the acoustic probe will transfer ultrasonic pulses into the measurement sample. When the acoustic pulses hit particles inside these sample, a part of this ultrasonic energy will be reflected as an echo. The quantity and intensity of these echoes will be detected, evaluated and shown as measurement values.

TRANSMITTER MODEL AT3

PROBE MODEL AS3

The sensor model AS3 is designed as insertion probe. Installation can be done in open channel or via insertion adapter to any process pipe with inner diameter of 50mm (2”) or larger.   Installation features like flanges, swivel clamps, etc. are available as option.

OVERVIEW

The AS3/AT3 particle monitor is a microprocessor based, real-time, in-line instrument used to monitor liquid-borne particle concentration and oil contaminants in various process liquids. Use of the microprocessor and graphics display provides a user friendly interface for setup and calibration. The instrument measures independent of the liquid’s color, opacity, density, flow rate or photosensitivity, because the measurement principle is based on ultrasonic reflection. The sensor is relatively immune to the effects of coatings that would blind an optical sensor. Sensors mount directly into process piping, thus eliminating the handling error and dead time associated with grab sampling. The unit’s dot matrix LCD display can present any user defined engineering unit (i.e., PPM, mg/l, etc.) along with the ability to plot calibration curves. The raw data is displayed as ^“counts”.

The instrument detects suspended solids as well as oil in water through the microprocessor establishing a correlation between ^“counts” and the known concentration (PPM, mg/L, %, etc.) can be determined, and the display will read out in engineering units.

Installation

The sensor model AS3 can be easily installed to manifold applications, due to its probe technology.

DESIGN CRITERIA

The AS3/AT3 is designed as an on-line, continuous monitor for the detection of particles and Oil in liquids. The more conventional optical methods of particle detection, (i.e. light scattering, light absorbance) were not satisfactory and did not meet the present design and functional criteria of the AS3/AT3. The use of focused ultrasound as the
detection principle, allows the following functional characteristics:

(a)       The system detects particles and bubbles below 1 micron.

(b)       The system causes a very low maintenance requirement

(c)       The system is self cleaning by ultra sonic pulses.

(d)       The system does not have wearing parts

(e)       The system has extreme extended calibration intervals

(f)        The system measures in opaque- or photosensitive fluids

Figure 1, shows a view of a pipe with installed AS3 probe, the ultrasonic beam field is generated, by the spherical lens of the probe.

 Figure 1,

Principle of measurement

The transmitter model AT3 sends electrical pulses to a piezoelectric crystal inside the AS3 probe. The crystal converts the signal into ultrasonic pulses and transmits it into the flowing liquid. The focusing of the pulses, as depicted by the dash lines (Figure1), causes a high sound density in the focal region. Particles in the focal region reflect the sound energy and the pipe wall reflects this energy as well. The transducer crystal in the probe tip now functions as a device which reconverts the reflected sound energy into electrical energy. The ultrasonic pulses travel at a high speed (1500m/sec for water) across the diameter of the pipe. The reflections (echoes) caused by particles in the focal region will returned earlier in time as the reflections from the wall of the pipe.

                                                                                                                      Figure 2,

Figure 2, shows a typical pulse diagram of the oscilloscope screen of AT3 transmitter.

The measurement window can be programmed in position (time), width and intensity.

Only particle echoes in the window region, above the programmed intensity (threshold) will be evaluated and calculated to a concentration signal. At 128 times a second the AT3 sends out a burst of energy into the process liquid, turns around and listens for echoes that return to the transducer, and then analyzes the data to determine a measurement.

Background Knowledge Ultrasonic Reflection 

Acoustic Lenses:

Acoustic lenses have been used since 1852. Acoustic lens theory parallels optical theory such that Snell's law.

C₁sinθ₁ = C₂sinθ₂

Where C₁ is the speed of sound in a solid lens, and C₂ is the speed of sound in a liquid medium for purposes of this technical note θ₁ and θ₂ are the refraction angles of the solid and liquid mediums respectively. The speed of sound for various liquids and solids measured with model AS3/AT3, are given in the following table. The sensitivity of the system is the better, the larger the difference in speed of sound between particles and carrier liquid.

Acoustic Absorption

The acoustic absorption of the liquid is important in determining the size particle which the system can detect. The more absorptive the fluid, the less sensitive the system is to the detection of small particles. The table lists some common absorption for various fluids that are monitored by AS3/AT3.

*The Neper (unit symbol Np) is a logarithmic unit for ratios of measurements of physical field and power quantities, such as gain and loss of electronic signals. The unit's name is derived from the name of John Napier, the inventor of logarithms. As is the case for the Decibel and **Bel, the neper is not a unit in the International System of Units (SI), but it is accepted for use alongside the SI.

** The Bel (B) is named according to Alexander Graham Bell. An auxiliary unit for the identification of levels and dimensions (logarithmic size).