Introduction
We can use ultrasonic level instruments for both continuous and point level measurement. The point detectors or level switches can be grouped into damped sensors or on/off transmitter types.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.
Working principle
A damped sensor-type level switch vibrates at its resonant frequency when no process fluid is present. When process material is present, vibration is dampened out. Most units use a piezoelectric crystal to obtain the vibration in the tip of the device. These instruments contain electronic circuits that detect the change in vibration and convert it into a dry-contact switch closure. These level switches are normally limited to liquid service because the damping effect of solids is insufficient in most cases. On/off transmitter-type level switches contain transmitter and receiver units. The transmitter generates pulses in the ultrasonic range, which the receiver detects. You can locate the transmitter and receiver in the same probe or on opposite sides of the tank, as shown in Figure. In the latter design, the signal is transmitted in air, and the level switch will be actuated when the sound beam is interrupted by the rising process material. This type of switch is effective for both solid and liquid material applications. The sensor design in which the transmitter and receiver are mounted in the same unit is generally used only for liquids.
Limitations
- The media must be liquid
- Process temperature between -40 and 250 degree F (-40 to 121 degree C)
- The media must have less than 5% suspended solids
- No aeration in fluids with a viscosity of 100cP (30W motor oil) or greater
