htk4014a600p_04
61/68
1 Initial
pulse
2 Flange
reflection
3 Level
signal
4 Interference
signal
5 Threshold
In the diagram above the inconstancy of the threshold level is visible:
400 mV at 1 m (3.3 ft), and only 250 mV at 10 m (33 ft). No attenuation is required at a probe length
≤
3 m (10 ft).
The form of the threshold is depending on the attenuation and is automatically adjusted by the device over the measured length.
Setting the level threshold
If the level threshold is set too high, i.e. it is greater than the amplitude of the level reflection, the device will not find any levels even with maximum amplification.
If the level threshold is set too low, i.e. it is below the amplitude of some of the interference signals, the device will identify and indicate one of these interference signals
as a level reflection even if the tank is empty.
Precise setting of the level threshold is especially important when the dielectric constant
r
is low.
In order to perform the setting level (amplitude of the reflection) must be known. A level of 500 mm (20") is ideal. The level threshold should be half-way between the
invalid interference signals and the level reflection signal. The reflection from the probe tip, which is clearly identifiable at a low
r
value, does not need to fall below
the level threshold.
Observe at all the reflections over the entire probe length and change the level threshold and/or the amplification factor upon this observation in the “Oscilloscope
diagram” tab 5.1.2.4 menu.