Version: TCM_0450_E80_M_EN_150209_E007
34
Manual Operation
Elapsed time
"Remaining Error
(% of the step)"
1 *
t
30
2 *
t
10
3 *
t
3
4 *
t
1
A linear filter as it is realized in the TCE 8000 electronics just delays the flow reading and consequently the
TOTAL value. Independent of the slope (fast or slow) of the rising and falling flow, the error of the internally
calculated TOTAL and at the frequency output are canceled out, if the flow rises from zero (or any other value)
and later goes back to the starting value. For getting a correct TOTAL via the display or the frequency output, it
is just necessary to wait long enough after the flow is switched off.
For best results the TCE 8000 electronics provides two filters.
The FLOW FILTER filters the mass flow before calculating the TOTAL or the frequency and current outputs. For
normal applications a moderate filtering with
t
= 1 s is recommended.
The DISPLAY FILTER filters the flow display additionally to the FLOW FILTER. It does not affect any other
parameter or any of the outputs. The default setting is
t
= 1 s.
If the flow is changing fast or sometimes makes a jump and the outputs have to react as fast as possible, set
FLOW FILTER to
t
< 1 s. If nevertheless the flow display has to be stable for better readability, the DISPLAY
FILTER can be increased.
For setting up the FLOW FILTER refer to chapter 4.5.7.1, for the DISPLAY FILTER to chapter 4.5.4.2.
4.2.3.
CUT OFF
As mentioned above, a mass flow meter has no natural zero and the raw data is noisy. Consequently, with now
flow, a meter would indicate and give out continuously a small fluctuating flow.
The parameter CUT OFF is used to provide a clear zero. If the calculated and filtered flow is below CUT OFF,
the meter indicates zero, the TOTAL values remain unchanged and the outputs show zero flow as well.
The value for CUT OFF must be above the noise floor in the given application and well below the minimum flow
to be measured. As a good compromise the default value for CUT OFF is 0.3% of the full scale range of the
meter.
For setting CUT OFF please refer to chapter 4.5.6.1.
4.2.4.
Interaction of the Parameters
As each of the two parameters affects the calculation of the flow in a different way, a bad combination of
different parameters can lead to systematical errors.
FLOW FILTER and CUT OFF
If the filter constant is set to a high value, the calculated flow is delayed compared to the actual flow. In ON-OFF
operation this leads to the fact that it takes a long time until the calculated flow settles to the ON or OFF value.
The TOTAL value remains correct if the TCM measures long enough after the flow got switched off. If CUT OFF
is set to a high value, the meters stops measuring too early and consequently the calculated TOTAL is too low.
Also the number of pulses at the frequency output is too low. The error is systematic.
NOTE:
In ON-OFF operation high values for the flow filter combined with high values for CUT OFF must be avoided!
Jumps of the flow not going down to zero are not affected by CUT OFF.
