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5.4.2
G
AIN AND VOLTAGE AMPLITUDE
As explained in the measuring principle in the introduction, the level of a product is converted from a return signal (the product reflection) received by the instrument:
this signal has taken a certain amount of time to return to the instrument and it has a certain strength / size measured in millivolts (dependant on the dielectric constant
r
of the product).
All pulse signals returning to the instrument electronics block (including flange, obstruction and the product surface reflections) are converted to voltage amplitudes.
The instrument’s microprocessor looks for part of the largest signal that is over a set voltage amplitude, called the “threshold”, and identifies this as the product being
measured. For this signal to be usable by the instrument, the microprocessor will amplify the signal by increasing the gain. Once the signal is within a set “working”
range, the instrument follows this signal. The instrument registers any changes in time for this part of the signal to return to the converter and translate this into a
displayed level or volume.
Gain is a function of voltage amplitude.
This defines the default threshold value when the instrument is searching for the product level.
A strong return signal will be given a low gain (i.e. Gain 0 or a small amplification).
However, if the signal is very weak, then a Gain of 3 (i.e. high signal amplification) is given.
Example of signal amplification:
Useable signal zone
Useable signal zone
Gain 2
Useable signal zone
Unusable signal...
increase gain
Unusable signal...
increase gain
Signal within zone
satisfies operating
constraints further
amplification using
Gain 3 for weakest
signals
(
)
unnecessary...
= normally not less
than 0.4-0.5 V
Peak
Threshold
Microprocessor
Gain 0
Gain 1
Level pulse
Gain
Threshold
G0 G1 G2 G3
t0
t1
t2 t3
where Gain
and level signal
e
= f A
A =
amplitud
t0
A0
t
A
1
1
t
A
2
2
er
ősebb jeleknél
Amplifier
Level measurement: Level pulse amplitude and threshold
After connection to a power supply, the VF04 2-wire will:
1.
Measure reflection pulses in terms of voltage amplitude by cycling through a set of gains.
2. Identify the highest amplitude as being the product level.
This screenshot of the HyView Oscilloscope function while measuring a typical one product application identifies:
1 Initial pulse
2 Flange reflection (except coaxial probe)
3 Non-product reflection (e.g. parasite: agitator)
4 Product level reflection
5 Level threshold (with two-metre steps).
Set in Measurement optimization screen
6 Offset
7 Distance measured as a function of time