SMAR LD291, Operation And Maintenance Instruction Manual

Page: 32 / 84

LD291- Operation and Maintenance, Instruction manual
3.6
This calibration procedure is also recommended for zero suppression or elevation in those cases where
the instrument installation results in a residual measurement in relation to a certain reference. This is
the specific case of the wetted tap.
NOTE
In most applications with wetted taps, indication is usually expressed as a percentage. Should
readout in engineering units with zero suppression be required, it is recommended to use the User
Unit feature for such conversion.
Engineering Unit Selection
Transmitter LD291 includes a selection of engineering units to be used in measurement indication.
For pressure measurements, the LD291 includes an option list with the most common units. The
internal reference unit is inH
2
O @ 20
o
C; should the desired unit be other than this one, it will be
automatically converted using conversion factors included in Table 3.1.
As the LD291 uses a 4 ½ digit display, the largest indication will be 19999. Therefore, when selecting a
unit, make sure that it will not require readouts greater than this limit. For User reference, Table 3.1
presents a list of recommended sensor ranges for each available unit.
CONVERSION FACTOR NEW UNITS RECOMMEND RANGE
1.00000 Inches H
2
O at 20
o
C 1, 2,3 & 4
0.0734241 Inches Hg at 0
o
C all
0.0833333 Feet H
2
O at 20
o
C all
25.4000 Millimeters H
2
O at 20
o
C 1 & 2
1.86497 Millimeters Hg at 0
o
C 1, 2, 3 & 4
0.0360625 Pound/square inch - psi 2, 3, 4, 5 & 6
0.00248642 Bar 3, 4, 5 & 6
2.48642 Millibar 1, 2, 3 & 4
2.53545 Gram/square centimeter 1, 2, 3 & 4
0.00253545 kilogram/square centimeter 3, 4, 5 & 6
248.642 Pascal 1
0.248642 KiloPascal 1, 2, 3 & 4
1.86497 Torr at 0
o
C 1, 2, 3 & 4
0.00245391 Atmosphere 3, 4, 5 & 6
0.000248642 MegaPascal 4, 5 & 6
0.998205 Inches of water at 4
o
C 1, 2, 3 & 4
25.3545 Millimeters of water at 4
o
C 1 & 2
Table 3.1 - Available Pressure Units
In applications where the LD291 will be used to measure variables other than pressure or in the cases
where a relative adjustment has been selected, the new unit may be displayed by means of the User
Unit feature. This is the case of measurements such as level, volume, and flow rate or mass flow
obtained indirectly from pressure measurements.
The User Unit is calculated taking the working range limits as a reference, which is, defining a value
corresponding to 0% and another corresponding to 100% of the measurement:

0% - Desired readout when the pressure is equal to the Lower Value (PV% = 0%, or transmitter
mode output equal to 4 mA).

100% - Desired readout when the pressure is equal to the Upper Value (PV% = 100%, or
transmitter mode output equal to 20 mA).
The user unit may be selected from a list of options included in the LD291 . Table 3.2 makes it possible
to associate the new measurement to the new unit so that all supervisory systems fitted with HART

protocol can access the special unit included in this table. The user will be responsible for the
consistency of such information. The
LD291 cannot verify if the values corresponding to 0% and 100%
included by the user are compatible with the selected unit.