FMCW: Frequency Modulated Continuous Wave

A radar signal is emitted via an antenna, reflected on the product surface and received after a time t. The radar principle used is FMCW (Frequency Modulated Continuous Wave). The FMCW radar emits a high frequency signal whose frequency increases linearly during the measurement phase (called the frequency sweep). The signal is emitted, reflected from the measuring surface and received with a time delay, t. Delay time, t=2d/c, where d is the distance to the product surface and c is the speed of light in the gas above the product. For further signal processing the difference Δf is calculated from the actual transmit frequency and the receive frequency. The difference is directly proportional to the distance. A large frequency difference corresponds to a large distance and vice versa. The frequency difference Δf is transformed via a Fourier transformation (FFT) into a frequency spectrum and then the distance is calculated from the spectrum. The level results from the difference between tank height and measuring distance.

Ultrasonic

Short ultrasonic pulses in the range of 18 to 70 kHz are sent from the signal transducer to the product to be measured, reflected from the surface of the fill goods and received by the signal transducer. The pulses propagate at the speed of sound, where the time between the sending and receiving of the signals depends on the level in the tank. The latest microprocessor technology and the tried and tested analysis software ensure that you will be able to reliably determine the level echo even when interference reflections are present and to calculate the exact distance to the surface of the fill goods. To compensate for the duration of the acoustic signal, an integrated sensor detects the temperature in the tank. A level-proportional signal is formed from the distance by simply inputting the tank dimensions. It is not necessary to fill the tank for the adjustment.

TDR: Time Domain Reflectometry

The guided radar (TDR) level transmitter has been developed from a tried and tested technology called
Time Domain Reflectometry (TDR). The device emits low-intensity electromagnetic pulses of
approximately one nanosecond width which are guided along a rigid or flexible conductor. These pulses move at the speed of light. When the pulses reach the surface of the product to be measured, they are reflected with an intensity that depends on the dielectric constant, εr, of the product (e.g., water has a high dielectric constant and the pulse is reflected back to the transmitter at 80 % of its original intensity). The device measures the time from when the pulse is transmitted to when it is received: half of this time is equivalent to the distance from the reference point of
the device (the flange facing) to the surface of the product. The time value is converted into an output
current of 4 to 20 mA and/or a digital signal. Dust, foam, vapor, agitated surfaces, boiling surfaces,
changes in pressure, temperature and density do not have an effect on device performance.

Float

The magnetic bypass level indicator (MLI) operates on the principle of communicating vessels. The measuring chamber is connected adjacent to the tank so that the same conditions are obtained in the chamber as those in the tank. The float is equipped with a system of permanent magnets to transmit measured values to the local indicator. The magnet system of the float activates either the magnetic flaps according to the liquid level, or a movable follower magnet in the indicating section of the indicator depending on the method of indication chosen. The column of reversed yellow magnetic flaps, or the vertical position of the follower magnet, indicates the liquid level.

Displacer

The BW 25 level indicator works according to the displacer principle. In this principle, the length of the displacement element rod corresponds to the measuring range. The body, which is suspended on a measuring spring, is immersed in the liquid where it determines the lifting force that is proportional to the displaced mass of the liquid (Archimedean principle). Any change to the weight of the rod corresponds to a change in the length of the spring and is therefore a measure of the level. The extension of the length of the spring, and thereby the measuring stroke, are transmitted to a display from the measuring room.

Potentiometric

The BM 500 level transmitter works according to the potentiometric measuring principle and can only be used with a minimum conductivity of 50 µS/cm for all electrically conductive media (e.g. pure water). The level probe (sensor) consists of a low-resistance measuring tube, which is immersed in an electrically conductive liquid. An AC generator runs a higher frequency current through the measuring tube. A voltage is taken from between the probe and the tank wall and sent to an amplifier. In homogeneous conditions in the medium, this is proportional to the level.The potentiometric measuring method is particularly suitable for measuring levels in small vessels containing tough, pasty or strongly adhesive media. The electronic evaluation unit is integrated in the signal converter and supplies a level-proportional output signal of 4 to 20 mA.

Hydrostatic pressure level measurement in open vessels

In an open vessel, the contents are connected to the atmosphere. Any change of ambient pressure causes a change of pressure of the fluid in the vessel. In order to measure the change of the fluid column in the vessel, gauge pressure transmitters or differential pressure transmitters (with open low pressure side) are used to measure hydrostatic pressure in the vessel.

Hydrostatic pressure level measurement in closed/pressurised vessels

The pressure in a closed vessel can assume any value. In order to measure the true hydrostatic pressure of the volume in the tank, a differential pressure between the head pressure and the total pressure at the bottom of the vessel needs to be measured at the same time.
Therefore, the high pressure side H is connected to the tank bottom and the low pressure side L is connected to the top. This ensures that the differential pressure applied to the transmitter is proportional to the height of the liquid, regardless of the head pressure inside of the vessel.

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