Ultrasonic liquid level measurement

Density measurements are closely akin to liquid level measurements because both are often required simultaneously to establish the mass contents of a tank, and the same physical principle may often be used for either measurement. Thus, the methods of density determination include the techniques of direct weighing, buoyancy, differential pressure, capacitance, optical, acoustic, ultrasonic, momentum, rotating paddle, transverse momentum, nuclear radiation attenuation, and nuclear magnetic resonance. Each of the principles involved will be discussed along with its relative merits and shortcomings. 

Liquid Level. The most widely used devices for measuring Hquid levels involve detecting the buoyant force on an object or the pressure differential created by the height of Hquid between two taps on the vessel. Consequently, care is required in locating the tap. Other less widely used techniques utilize concepts such as the attenuation of radiation changes in electrical properties, eg, capacitance and impedance and ultrasonic wave attenuation. 

In each case, the volumetric rate of flow can be determined by measuring the liquid levels in the appropriate place. This is often achieved using an ultrasonic measuring system (Fig. 6.7d) in which the time taken for an ultrasonic wave to be reflected from the liquid surface is measured (see also Section 6.5.5). Accuracies of 2.5 mm/m distance between sensor and liquid surface are not uncommon. Standard designs of open channel restrictions can be found in BS 3680(l4). 

The determination of the thickness of the layers of fat and lean tissue in animal flesh is the most popular use of ultrasound in the food industry at present [5,6]. In fact there are over a hundred references pertaining to this application of ultrasound in the Food Science and Technology Abstracts (1969-1993). In contrast to most other applications of ultrasound in the food industry, which have rarely developed further than use in the laboratory, there are a number of commercial instruments available for grading meat quality [6, 30-32]. This application is based on measurement of time intervals between ultrasonic pulses reflected from boundaries between layers of fat, lean tissue and bone. Ultrasonic techniques have the advantage that they are fairly cheap, easy to operate and give predictions of meat quality of live animals. Other examples of thickness determinations include liquid levels in cans or tanks, thickness of coatings on confectioneries, egg shell thickness. 

Sonic (up to 9,500 Hz) and ultrasonic (10-70 kHz) level sensors operate either by the absorption (attenuation) of acoustic energy as it travels from source to receiver, or by generating an ultrasonic pulse and measuring the time it takes for the echo to return. If the transmitter is mounted at the top of the tank, the pulse travels in the vapor space above the tank contents, and if it is mounted on the bottom, the time of travel reflects the depth of liquid in the tank. In water, at ambient temperature, the ultrasonic pulse travels at 1,505 m/s (4,936 ft/s). 

Ultrasonic level measurement depends on a transducer sending an ultrasonic pulse to the liquid surface which is reflected back to the transducer. Electronics convert the ultrasonic lag time into a distance (D) corresponding to depth. Ultrasonic level measurement is based on the equation... 

In areas where general corrosion is the expected form, a simple ultrasonic thickness gage can be utilized to determine the extent of corrosion, based on baseline readings made at installation or previous inspections. The entire unit need not be examined. Attention can be focused on those areas most likely to corrode, such as liquid levels, mixing zones, or areas of high turbulence. Corrosion probes, which can be placed in process equipment or pipelines, can monitor corrosion conditions by measuring an actual corrosion current, or other process parameters known to be related to general corrosion rates. These data can be constantly monitored and recorded to predict equipment wear, or as an alert to upset conditions. 

In addition to reaction chambers and delivery systems, a number of supervising and sensor systems are of utmost importance for control and safety reasons. Sensors in automated workstations include measurement of temperature (thermocouple, thermistor, semiconductor), pressure, liquid flow and gas or liquid level. To monitor the presence or absence of vessels or devices, systems like capacitance, inductivity, ultrasonic monitors, magnetic sensors or optical sensors (reflective, beam interruption, color) can be integrated in automated workstations. 

Level. Level sensors that depend on a pressure difference between two points, one submerged and one in the vapor, are subject to the same installation considerations described for pressure measurement. The inferred level measurement is more accurate when determined by a differential pressure transducer using remote diaphragms than when calculated from the difference between two absolute pressure transducers due to errors from calibration and transducer drift. In either case, the measurement is affected by changes in the weight percent solids in the solution an increase in the liquid density would be interpreted as an increase in the level. Ultrasonic level sensors are not affected by the slurry density but are sensitive to fouling. 

Ultrasonic sensor The reflection time of a high-frequency sound pulse can be measured to give an accurate estimate of distance from an object or from a liquid level. In the latter case, an ultrasonic sensor is a noncontact alternative to a float transducer. 

The theory of sonic-electronic level measurement is fundamentally based on a sound wave emission source from a transmitter, and a reflection of the sonic wave pulse to a receiver. Measurement of the transit time of this sound pulse and its correlation with electrical impulses provide a means for liquid level detection. Two basic designs operating on this principle use the vapor phase and the liquid phase methods. As most of the attention is currently devoted to the latter type system, this discussion deals exclusively with ultrasonic gaging over a liquid path. 

A transducer sends pulses of ultrasonic sound to the surface of the liquid to he measured. The liquid surface reflects these pulses and the distance from the transducer to the liquid level is calculated. This calculation is based on the speed of the signal and the time elapsed between the sending and receiving of the ultrasonic sound signal (Figure 2.9). 

Other methods are available to measure liquid level. One of them employs an ultrasonic transducer located at the bottom of the tank which generates pulses and receives them back after a delay proportional to the liquid level by reflection from the interface between the liquid level and the air space above. 

Ultrasonic Doppler velocimetry is a nonintrusive technique that has been developed into a very useful technique for opaque liquid flows [3]. This technique provides good measurement of velocity new high-frequency techniques give a space resolution on the millimeter level, and even the large turbulent scales can be resolved. 

Another interesting alternative is ultrasonic velocimetry. Sound travels more quickly in solids than liquids, and it is possible to measure the solids content of an oil sample by measuring the time needed for an ultrasonic pulse to move through a fixed pathlength of oil as a function of temperature (McClements and Povey, 1987). Comparisons of ultrasonic with NMR methods have shown the former perform at least as well as the latter, and perform better in the case of low levels of solids (McClements and Povey, 1988). However, because this technique has not been widely adopted or received detailed review by the AOCS or other professional organizations, it is not given the status of a recommended method here. 

Trophic level Position in the food chain, e.g., herbivore, carnivore, bottom-feeder. Ultrasonic cleaner Lab equipment using ultrasound in liquid bath for cleaning. Variance A single measure of spread or range in ratio data. 

Atmospheric vessel The product to be stored in vessel 1 will be fed via a supply hne (RLOOl) which is led into the vessel and ends with a 45 elbow in the tank. The elbow will prevent the liquid from plashing into the vessel from great altitude, but induce it to run down the vessel wall. Thus, interferences of the analogue level control which is partly sensitive to uneven surfaces of liquids (e.g. ultrasonic measuring) can be avoided. 

A more reliable technique, and one that can be used where there are larger surface fluctuations, is to use an ultrasonic radar probe (Machon et al., 1991). Such probes are commercially available, from, for example, Endress and Hauser. The probe is mounted above a representative part of the fluctuating surface and measures the distance to the surface. Care must be taken in the calibration (especially if any foam is present) to ensure that the true surface is detected and that an adequate range of fill levels is covered. Some foams will not be penetrated by the ultrasonic beam. Calibration with a moving liquid surface is recommended. 

The ultrasonic pulse echo (UPE) method is used to measure the wall thickness of vessels. UPE can be adapted to distinguish chemical munitions from conventional types and to determine the filling level of shells. Differentiation between munition types is based on the fact that, besides the normal reflexion signal from the front wall, the detection of a rear-wall or burster-tube echo clearly indicates the presence of a liquid as filling, whereas with conventional munitions the original pulse is reflected only from the front wall. 

---