Guided Wave Radar
Level Measurement: How to Make the Best of GWR
Different amplitude thresholds are used to filter out unwanted signals and to pick up the different pulses. The transmitter uses certain criteria to decide which type of pulse is detected. Some transmitters have a ‘Measure and Learn’ function, which creates an Amplitude Threshold Curve (ATC) that filters out all disturbing echoes. The ATC can also be manually edited (for example to remove random peaks) if further fine tuning is needed.
Overcoming Weak Signals
In some applications, for example those with a long measuring range or if the products have very poor reflectivity (low dielectric constant), the surface pulse is too weak to be detected. To overcome this problem, some GWR manufacturers have developed enhanced electronics that can carry out reliable measurements even in these circumstances.
For example, the Probe End Projection function in Emerson’s Rosemount 5300 transmitter is based on the principle that when the surface level is too weak to be detected, the echo from the probe end can be utilised because it (as displayed on the echo curve) will appear to be further away than if the tank were empty.
This is because the speed of the measurement signal through the product is less than the speed through air. The product surface level can be determined by comparing the actual probe end position, as given by the Probe Length value, with the apparent position of the Probe End pulse. The difference is related to the properties of the product, i.e. the Dielectric Constant, and the distance D travelled by the measurement signal through the product.