Inventron offers the largest selection of standard and special ultrasonic transducers in the industry. The ultrasonic transducer transmits an ultrasonic pulse which bounces off the material being measured and returns to the transducer.
The elapsed time for this echo to return is converted into an analog or digital signal proportional to the level in the tank, This output may be visually displayed, fed to control devices or used to activate integral alarm or control relays.
Every ultrasonic transducer that is manufactured by Inventron, employs some means of temperature compensation. This will be accomplished through the coax cable and not require separate wires. There will be exceptions to this rule as stated below:
At times it may be desirable to use a sunshade if the transducer contains the temperature compensation circuit and it is exposed to direct sunlight. The transducer could rise above ambient temperature causing an error in the temperature compensation for sound velocity. Alternatively a remote temperature probe (RTP) might be utilized.
We can compare the transmit signal of an ultrasonic sensor to the ringing of a bell. There is a certain decay time for the sensor to stop vibrating after it is pulsed, causing a deadzone. It is only after the ultrasonic wave has decayed, that the sensor can pick up the return echo. The return echo causes the sensor to again vibrate and this is detected by the controller.
The minimum distance between the transducer housing and product
being measured is called the Deadzone. The Deadzone for single sensor
assemblies will vary from 1-3 ft. (See chart).
The transducer model number must be carefully chosen for each specific application and mounting configuration. The choice of transducer depends upon the following:
Low frequency transducers are used for long range sensing and higher frequency transducers are used for shorter range sensing. The following chart can be used to determine the recommended transducer based on sensing range.
Beam angle is an important consideration in transducer selection. It defines how much the beam will spread with distance. A wide beam angle could cause reflections from the side of the vessel or from structural members in the vessel. The illustrations suggest the potential problems. The illustration below -right shows a transducer with a narrow beam angle. Note that the sound waves easily clear the side of the vessel. The illustration on the left shows a transducer with a much wider beam angle. If there is a welding bead inside the vessel or a build-up of solids on the vessel wall, or another obstruction such as an overflow pipe, reflections may be generated which can be interpreted by the controller as valid echoes.
Beam angle is largely determined by the frequency of the sound
waves. A high frequency transducer produces a narrow' beam. A low
frequency produces a wider beam. Beam angle can be shaped to some extent
in the transducer. The horns incorporated into Inventron transducer
housings are designed to minimize beam angle.
It should be noted that the horn of the transducer must be kept free of foreign material once installed. If product material splashes up into the horn and coats the horn (and possibly the sensor) false readings can occur. In some cases it may be wise to include an air or liquid purge in the transducer housing so that the horn and sensor can be cleaned periodically without being removed. INVENTRON transducers can be supplied with an air or liquid fitting and an internal channel through the transducer housing. The customer must provide an air or liquid line to the transducer. The standard fitting is 1/4" NPT.
Note -- The air purge option is not designed to provide cooling of the transducer assembly. It is designed for applications that may have an occasional build-up of material in the horn assembly, which would be eliminated by a controlled burst of air. For applications that exhibit a high temperature environment, you should specify the "i-IT" (high temperature) option.
A distinction must be made between the sensor material and the housing material since they are often different. Both must be considered for purposes of corrosion resistance. Thus, the glass filled phenolic housing may be suitable for a given application, but the SS 304 sensor may not (or vice versa).
Optimum operation is achieved when the sensor is mounted sufficiently high so that the material level will never go within the transducer deadzone. It is not harmful to the sensor for material to be closer than this minimum range unless this results in build-up of material on the element and/or horn assembly. for materials where such build-up is likely to occur, the sensor should always be mounted high enough above the highest material level so that coating cannot occur. Otherwise some program of maintenance to clean the transducer must be provided.
The sensor should preferably be mounted flush with the top of the bin, or on a bracket inside of the bin or silo. In applications where it is desirable to utilize existing small diameter nozzles on tanks, a sensor with the flange on the back of the housing can be provided. Extension tubes are available (see special options, PET & MET) that allow the end of the sensor to be flush with the top of the bin or silo.
If a tank is filled to the top, the transducer must be mounted above the tank in a manway or on a nozzle. Care must be taken to insure that the nozzle has a sufficient diameter for its height.
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