5
time-constant (averaging filter) in the user selectable ran-
ge of 0.0 to 100.0 secs. When the damping is set to ma-
ximum, it takes more than 2 minutes to reach the final
value for a full scale pressure jump.
In many cases fluctuating pressure readings do not cau-
se a problem, except when the plant / equipment is at
zero (differential) pressure condition and readings fluc-
tuate near about zero value. The set-up parameter
NP
is
meant to take care of this. Its value defines the number
range across zero (similar to the zero offset correction
number), within which the measured value is forced to
zero. If a value of 8 is set for
NP
any pressure measure-
ment in the range -0.08 to +0.08 mbar (or -8 to +8 Pa)
displayed as zero. Only when the actual pressure is out-
side this range will the display indicate a non-zero value.
The actual and displayed pressures will agree starting
from double the value of the
NP
setting (in the given ex-
ample: 0.16 mbar or 16 Pa).
5.3.5. Output Signal Setting
The transmitter output signal depends primarily on the
measured pressure. However, this signal can be adapted
to meet users' application requirements precisely. The
basic pressure range (as marked on the product identifi-
cation label) and the type of output signal (voltage or cur-
rent) always remain unchanged for a particular
instrument unit.
The set-up parameters
MA
(measuring range starting
point) and
ME
(measuring range end point) specify the
pressure values between which the measurements are
expected to be. Both values can selected anywhere wi-
thin the specified measuring range of the instrument
(e.g., 400 Pa). This user-programmed pressure range
will correspond to the output signal (current or voltage)
range, which will be as specified on the product identifi-
cation plate: i.e., 0 - 10 V or 4 - 20 mA. If
MA
is lower than
ME
, the signal is said to have a positive slope: i.e., the
output signal increases as the pressure increases. If
ME
is lower than
MA
, the output signal has a negative slope:
i.e., the output signal decreases as the pressure increa-
ses.
The difference between the values of
MA
and
ME
must be
at least 25% of the specified measuring range of the in-
strument (100 Pa for 400 Pa instrument example men-
tioned above). The software does not permit a smaller
pressure span to be entered (the instrument will not allow
storing of, nor exit from an invalid span).
Note: If you change
MA
and/or
ME
the
look-up table (s .5.3.7., 5.3.8.) that exi-
sted up to that instant is deleted!
5.3.6. Output Signal Limiting (Namur)
The three set-up parameters
oG1
,
oG2
and
oEr
specify
the limits of the signal output current or voltage that are
not to be exceeded, irrespective of the actual pressure.
These limit values have higher priority than the
MA
and
ME
pressure span settings.
These settings serve mainly to prevent control systems
from interpreting brief pressure excursions outside the
measuring range as error / fault events.
oG1
sets the out-
put signal minimum value, and is useful only for a
4 - 20 mA current signal, because sometimes a value be-
low 3.8 mA is defined as a sensor fault condition.
oG2
sets the upper signal limit and is valid for either current or
voltage signal (e.g., voltage signal output can be limited
at 10.2 V).
An instrument fault condition can be transmitted as an
output signal value set as parameter
oEr
. However, it
should be understood that not all instrument fault and er-
ror conditions can be detected and signaled by the self-
diagnostic functions of the instrument!
5.3.7. Signal Conversion and Transfer Functions
In certain cases other process variables are derived from
primary pressure measurement. Examples are flow rate
derived from differential pressure across an orifice plate,
and liquid level derived from hydrostatic pressure of li-
quid measured at the bottom of the tank. Such derivati-
ons often involve non-linear transfer functions, while it is
necessary for the output signal to be linearly proportional
to the derived variable (e.g. liquid volume in a tank in m
3
,
or flow rate in cm
3
/sec).
The set-up parameter
F
allows the user to select the ap-
propriate signal conversion function from those available:
F
= 0: Linear characteristic (default)
F
= 1: Square root extraction
F
= 2: Horizontal cylindrical tank
F
= 3.....30: Look-up table with 3 to 30 pairs of values
The tables generated by functions
F
= 0,
F
= 1 and
F
= 2
are not visible. For these functions, internal values are
used for table computation. These values cannot be mo-
dified by the user.
For all conversion functions, when the actual pressure is
equal to the
MA
value, the output signal will be at the lo-
west end of its range (0 V, 0 mA or 4 mA). When the ac-
tual pressure is equal to the
ME
value, the output signal
will be at the highest end of its range (10 V or 20 mA).
The user can enter only the 1...28 intermediate values of
the look-up table function
F
= 3....30. The parameters
MA
and
ME
relate to the start and end values of the look-up
table. A change in either of these parameter values cau-
ses the conversion function to be re-set to
F
= 0.
Whenever the value of
F
is changed,
the instrument internally generates a
new look-up table. All previous table
values are deleted and replaced by
new linear progression values.
Contact: EK Engineering (India)
Web: www.ekeindia.com
Email: [email protected]
