Ultrasonic point level switches are a cost-effective solution for your applications. Installation requires mounting the sensor (threaded or flanged) to the vessel, connecting the power and control wires, and applying power. There is no additional set-up or calibration required. Since it is an electronic instrument with no moving parts, preventive maintenance is limited to an annual visual inspection. The only recommended spare part is the “board” at a quantity of one board for every 10 units. A technician with basic electrical skills (wiring) can service the instrument. Why use an ultrasonic switch over other level technologies? Depending upon your application, there may be three or four technologies equally suited for your application; however; only one will be the best choice when considering its features and benefits.
Ultrasonic Measurement Working Principle
An ultrasonic switch is a device that uses inaudible high-frequency sound (ultrasound) to detect the presence or absence of a liquid at a designated point. The device consists of an electronic control unit and a sensor. Ultrasonic level switches use the properties of sound transmission in vapor and liquids to detect liquid level. When sound travels in air, it loses a great deal of signal strength. When traveling in liquid, sound retains almost all of its signal strength. To detect liquid level, we must determine if there is a liquid or gas (air) in the gap. Since liquids have a higher density than gasses, it is easier to transmit sound through them. One side of the sensor gap transmits sound, the other side detects it. When liquid is present, a high amount of sound is received at the detection side. When gas (air) is present, a small amount of sound is received. The electronics detect this difference and switch a relay accordingly.
Ultrasonic switch sensors contain two piezoelectric crystals, one transmits sound and one receives sound. Each crystal is mounted on one side of a gap in the metal sensor. The transmit crystal generates high frequency sound (1MHz to 3 MHz) that is directed across the gap to the receiver crystal. The receiver crystal converts the sound energy received into an electric signal, which is processed by the electronics to determine if the gap has liquid or air in it. The drawing below shows the basic construction of an ultrasonic level switch sensor. An electrical signal is sent to the “transmit” crystal, which causes it to vibrate and produce high frequency sound. The “receive” crystal converts the high frequency sound that strikes it to another electrical signal, which is sent back to the electronics for processing. The sound energy that makes it across the sensor gap is very weak in air, and becomes very strong in liquid.
Restrictions of Ultrasonic
- The media must be liquid
- Process temperature between -40 and 250 deg F (-40 to 121 deg C)
- The media must have less than 5% suspended solids
- No aeration in fluids with a viscosity of 100cP (30W motor oil) or greater
Model Numbers 701, 711, 721, 712, 722 (Electronics and Housing)
Model Numbers 371, 372 (Ultrasonic Probes)
All Ultrasonic Level Switches SOR offers consists of two parts - the electronics/housing and the probe. The different electronics and housings serve different application needs while the different probes are used in different process medias. For more information on the probes, please see page 19 of the Ultrasonic Level Switches (Form CAT1145).
701 Single-Point Ultrasonic Switch
The 701 tip-sensitive ultrasonic switch is a single-point device designed for economical detection of clean liquids. There are no moving parts and no calibration. The 701 is available either integral or remote mounted. An optional time delay can be used to eliminate false alarms due to turbulence in the process. An optional field selectable fail-safe switch is also available. The standard unit is set to High-level Failsafe (HLFS)
711 Single-Point Ultrasonic Switch
The 711 ultrasonic switch is a versatile single-point device designed for the detection of clean liquids. The 711 is available with a variety of sensor types to meet most process conditions. It is available integral or remote mounted and comes standard with field-selectable failsafe.
721 Single-Point Ultrasonic Switch
The 721 ultrasonic switch is an advanced single-point device designed to meet your needs for overfill protection. In addition to the features of the 711, the 721 includes a “Sensor Monitor.” This circuit continuously tests the functionality of the electronics and sensor. The “Sensor Monitor Relay” de-energizes if a fault is detected. The 721 is available with a variety of sensor types to meet most process conditions. It is available integral or remote mounted and comes standard with field-selectable failsafe.
712 Dual-Point Ultrasonic Switch
The 712 ultrasonic switch is a versatile dual-point designed for the detection of clean liquids. The dual-output can be used as independent alarm points or used in tandem to provide pump control. It is available integral or remote mounted and comes standard with field-selectable failsafe.
722 Dual-Point Ultrasonic Switch with Self Test
The 722 ultrasonic switch is an advanced dual-point device designed to meet your needs for sump/ pump control. In addition to the features of the 712, the 722 includes a “Sensor Monitor.” This circuit continuously tests the functionality of the electronics and sensor (both gaps). The “Sensor Monitor Relay” de-energizes if a fault is detected. It is available integral or remote mounted and comes standard with field-selectable failsafe and time delay.