Ultrasonic devices for monitoring liquid level and activating the opening and closing of switch contacts at predetermined liquid level thresholds.
A ultrasonic liquid level switch is an instrument for monitoring the height of a liquid and tripping a relay contact switch or transmitting a solid state relay switch output when a maximum, minimum or intermediate level has been reached. The liquid level switch output can then be utilised by other instrumentation to open a valve, illuminate a warning lamp, activate an audible alarm, or switch on a pump.
Ultrasonic liquid level switches are contactless measurement devices which transmit and receive high frequency acoustic waves to measure the distance to the surface of a liquid based substance.
Ultrasound liquid level switch technology transmits a high frequency sound wave pulse and receives the wave echo reflected off a liquid surface. The time taken for the acoustic pulse to traverse the distance between the transmitter and target surface, and back again to the receiver is measured.
The distance (d) between the ultrasonic liquid level switch and the liquid surface can be calculated by dividing the time taken (t) by two, and multiplying that by the speed of sound (c) in air, i.e. d = c ∙ t / 2. The liquid level in a tank (LT) can then be determined by subtracting distance (d) from the height of the ultrasonic level switch (HT), i.e. LT = HT – d.
The performance of a ultrasonic level switch is mainly affected by changes in the speed of sound and disturbances on or above the surface of the liquid.
The speed of sound will change if the air density changes. Since air density changes in the Earth’s atmosphere are directly related to changes in air temperature (measured in Kelvins), an ultrasonic fluid level switch is compensated by incorporating an air temperature sensor.
Surface effects can be reduced by experimenting with the position of the device, installing within a stilling well or wave guide, and some ultrasonic devices include features to minimise the effects of foaming, bubbles, vapour and turbulence.
Since there is no contact with the fluid to be measured, ultrasound liquid level measurement technology can be used in applications where switches requiring contact are otherwise prohibited.
Ultrasonic liquid level switches are typically housed in a single assembly combining the acoustic transmitter and receiver in one device.
Since ultrasonic liquid level switches do not make contact with the liquid media to be measured, they are mounted above the surface of the liquid in the top of a tank or from a platform spanning the top of an open tank.
- No media contact for vertically mounted type, so flexibility for use on different corrosive media, thick fluids, slurries, wastewater
- High temperature fluid compatibility for vertically mounted types, since no sensitive components make contact with the fluid
- High accuracy due to high frequency pulse wave and precise transit time time measurement
- Multiple switch contacts in one device
- Operates with conductive or non-conductive fluids
- Sealed pressurised vessel will not affect performance
- Foaming, splashing, vapour and humidity can affect measurement performance
- High cost compared to other technologies
- Top mounting only unless using point level type
- Minimal blocking distance limited operation when in proximity to liquid level
- Temperature compensation required to compensate for speed of sound variation with air temperature
- Sound waves cannot be transmitted through a vacuum