Yokogawa EJX930A - Manual (Page 42)

<6. Explanation of Basic Items>
6-3
IM 01C25R03-01E
6.3.3 Functions Relating to Differential
Pressure
Reference to differential pressure value:
By accessing PRIMARY_VALUE, it is possible to
refer to the differential pressure value and status.
The update period of this value is 200 mseconds.
The pressure unit is selected by XD_SCALE.
unit of the AI block, in which PRIMARY_VALUE is
selected. The status is normally Good-Non Specific.
However, in the case of a sensor failure or out of
measurement range, it turns to Bad or Uncertain.
For details, refer to Table 8.9.
Setting of signal damping for differential
pressure:
When the differential pressure signal fluctuates
greatly, the fluctuation can be reduced by increasing
the time constant for signal damping. PRIMARY_
VALUE_FTIME is the parameter for setting the
signal damping for differential pressure. The unit of
signal damping is second.
Determination of the range limit of differential
pressure signal:
When the differential pressure signal deviates more
than 10% from the capsule measurement range
shown in PRIMARY_VALUE_RANGE, set the
status of PRIMARY_VALUE to Uncertain-Sensor
Conversion not Accurate.
Adjustment of pressure/differential pressure
signal:
EJX multivariable transmitter have been accurately
adjusted according to the specifications at factory
setting. Minimal errors can occur due to the
environment where the transmitter is installed and
the mounting position. To fine-tune any errors,
three zero/span adjustment functions can be used:
automatic and manual zero/span adjustment
by communication terminals, and zero-point
adjustment by means of the external adjustment
screw on the transmitter.
Automatic adjustment
The value of the pressure exerted on the point
where the adjustment is to be made is assigned
to parameters (CAL_POINT_LO, CAL_POINT_
HI). After this assignment is set, the amount of
adjustment is calculated by the transmitter and
automatically updated.
Manual adjustment
From the exerted pressure and the output of the
transmitter, calculate the amount of zero/span
adjustment manually, and assign the calculated
value to parameters (CAL_DEVIATION_LO,
CAL_DEVIATION_HI).
Zero-point adjustment by the external screw.
With pressure being exerted on the point where
the adjustment is to be made, zero adjustment
needs to be performed. To do this, adjust the
calculated value by turning the external Zero-point
adjustment screw, so that it agrees with the actual
input pressure (true value). The output value
increases when the screw is rotated to the right. It
decreases when the screw is rotated to the left. The
adjustment width is small when the rotation speed
is low and large when fast.
When performing zero-point adjustment by the
external screw, EXTERNAL_ZERO_TRIM needs to
be set to “Enable.”
Procedure for automatic adjustment
The procedure for automatic adjustment is as
follows:
F0603.ai
(1) Change the Block mode
Set MODE_BLK to O/S.
(2) Enter zero% pressure
Apply the actual zero% pressure to EJX multivariable
transmitter.
(6) Change the operation mode
Set MODE_BLK to AUTO for normal operation.
(3) Perform zero adjustment
When the value of the applied pressure is written to
CAL_POINT_LO, EJX calculates the amount of
adjustment for zero adjustment. The calculated amount
of adjustment is incorporated into CAL_DEVIATION_LO.
(5) Performance of span adjustment
When the value of the applied pressure is written to
CAL_POINT_HI, EJX multivariable transmitter calculates
the amount of adjustment for span adjustment. The
calculated amount of adjustment is incorporated into
CAL_DEVIATION_HI.
(4) Input pressure for span adjustment
For span adjustment, apply the actual 100% pressure to
EJX multivariable transmitter.