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OPERATION
Explanation of Non-Zero Based and Zero Based Calibration
Non-Zero Based Calibration
In a non-zero based calibration, the low-limit output signal (4 mA) is equal to the minimum calibrated flow. The minimum calibrated flow is a
value greater than zero. The flow meter's output signal indicates the low-limit flow signal (4 mA) from zero flow up to the low-flow limit
parameter. Use a non-zero based calibration when the minimum flow rate does not approach zero and the turndown ratios are small.
At no-flow the output signal indicates the minimum calibrated flow (4 mA).
Zero Based Calibration
In a zero-based calibration, the slope of the output signal is shifted so the low-limit flow output signal (4 mA) is equal to zero flow. See
Figure 36
below for details. Flow meters cannot measure zero flow accurately. The flow meter reads out the low-limit signal (4 mA) from
zero flow up to the minimum calibrated flow at which point the output signal steps up to the proper signal value of the mass flow.
OUTPUT
(mA)
The output signal is easier to interpolate when milliamp output is interfaced with control room 0 to 100% gauges; 50% of the signal will
correspond to 50% of the maximum flow rate.
Zero-based flow meters have less signal to resolve over full scale. A zero-based flow meter with a turndown ratio of 10:1 will have 10%
less output range to resolve flow (5.6 mA to 20 mA instead of 4 mA to 20 mA in a non-zero-based flow meter).
Zero-based calibration is the factory default.
Delta R Table
The Delta R sheet included with the instrument contains simulated sensor data for the flow meter. The Delta R portion of this sheet relates
calibration measurement points of the differences (delta) between the reference and active RTDs at certain flow rates.
These parameters are used at the factory to determine the linearization coefficients (parameters) over the flow range in question to correct
for non-linearity. The corresponding current and voltage output readings are also shown (again at factory default settings) changes to
range and zero will only affect the output signals. Flow versus Delta R relationship is fixed for a given set of flow elements and the
coefficients should not be changed except in special circumstances as the overall accuracy of the system is tied to these numbers. Call
Customer Service if there is a problem in this area.
The Calibration Table is a printout (at factory default settings for zero and offset) of the relationship of displayed mass flow readings related
to the current output when calculated over the entire flow range using the fitting equation and its respective coefficients.
42
20
16
ZERO
12
8.0
4.0
0.0
0
10
20
MIN
FLOW
FLOW (SFPS)
Figure 36 – Zero Based Calibration
50% FULL SCALE
50
30
40
60
MT100 Multipoint Flow Meter
ZERO BASED
NON ZERO BASED
C00114-3-2
Fluid Components International LLC
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Troubleshooting
