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The level signal can be optimized by way of two factors:
Amplification factor
The amplitude of the signals is proportional to the dielectric constant
r
of the product. At low amplitudes the signal should be amplified.
The amplification factor is dependent on the dielectric constant
r
and on the probe type. The device sets the gain automatically.
The following factors apply to the set gain:
Gain
Amplification factor
0
1.05
1
2.10
2
4.37
3
8.93
Level threshold
The level threshold suppresses interference signals so that only the reflections from the product surface (level signal) are shown. The factory setting of the level
threshold is suitable for standard applications.
The threshold will need to be adjusted in the case of very low dielectric constants
r
, multiple interference reflections or unfavourable installation conditions.
The illustrations below show interference signals when the oscilloscope function is used.
These reflections can be due to various causes, e.g. tank internals or multiple reflections within the measuring range.
Even though the interference signals are very weak, the threshold should nevertheless
be set to a value above these signals.
1 Initial pulse
2 Flange reflection
3 Level signal
4 Interference signal
1 Initial pulse
2 Flange reflection
3 Level signal
4 Interference signal
5 Threshold
In the diagram above it can be seen that the level of the threshold is not constant:
400 mV at 1000 mm or 3.3 ft, and only 250 mV at 10000 mm or 33 ft. No attenuation is required at a probe length ≤ 3000 mm or 10 ft.
The form of the threshold is dependent on 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 level 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 only if the tank is empty.
Precise setting of the level threshold is especially important when the dielectric constant
r
is low.
To set, the level (amplitude of the reflection) must be known. A level of 500 mm or 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 lie below the level threshold.
Take a look at all the reflections over the entire probe length and then change the level threshold and/or the amplification factor in the Measurement optimization
screen.
