primary standards in national laboratories indicate the uncertainty of their pressure references with 70
to 90 ppM or close to 0.01%. Commercial deadweight testers used to calibrate the transmitters indicate
an uncertainty or accuracy of 0.025 %. Below these levels, expression “Precision” is the ability of a
pressure transmitter to be at each pressure point within 0.01 %FS relative to these commercial standards.
C1.3 POLYNOMIAL COMPENSATION
This uses a mathematical model to derive the precise pressure value (P) from the signals measured by
the pressure sensor (S) and the temperature sensor (T). The microprocessor in the transmitter calculates
P using the following polynomial:
P(S,T) = A(T).S0 + B(T).S1 + C(T).S2 + D(T).S3
With the following coefficients A(T)…D(T) depending on the temperature:
A(T) = A0 . T0 + A1 . T1 + A2 . T2 + A3 . T3
B(T) = B0 . T0 + B1 . T1 + B2 . T2 + B3 . T3
C(T) = C0 . T0 + C1 . T1 + C2 . T2 + C3 . T3
D(T) = D0 . T0 + D1 . T1 + D2 . T2 + D3 . T3
The transmitter is factory-tested at various levels of pressure and temperature. The corresponding
measured values of S, together with the exact pressure and temperature values, allow the coefficients
A0...D3 to be calculated. These are written into the EEPROM of the microprocessor. When the pressure
transmitter is in service, the microprocessor measures the signals (S) and (T), calculates the coefficients
according to the temperature and produces the exact pressure value by solving the P(S,T) equation.
Calculations and conversions are performed at least 400 times per second.
C1.4 CONFIGURATION
Through the probe set-up, it is possible to configure the Highly Precise Sensor by adding or modifying
an existing sensor and associating the logical code 0 to this sensor and associating the multiplexer input
252 special code.
C1.5 SENSOR CALIBRATION
Calibration is carried out as described in the Operator’s Manual for the standard pressure sensor.
However, the following calibration coefficients must be configured:
Coefficient (a) = 0.0
Coefficient (b) = 10.0
Coefficient (c) = 0.0
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