Setting Pressure Set Points; Figure 4.8 Pressure Set Point Set - Honeywell PPT User Manual

The user compensated pressure output can be expressed in terms of X= and Y=
where m=0 to ±120. The offset value, Z=, can be expressed as 'b' where b=0 to ±120.
Pressure Output = [(1 + m x 0.00005) x Pressure Reading] + [(b x 0.00005) x (full scale)]
The F= command is used in conjunction with the X=, Y=, and Z= command to customize the full
scale range and user compensation of the PPT. The F= command allows the user to reduce the full
scale range of the PPT as much as one-half the factory FS value. The F= command value can have up
to 5 significant digits with a decimal point. Enter an
return to the factory default (M=) full scale value. Using the F= command, the maximum allowable
full scale pressure is the M= value and the minimum allowable FS pressure is (0.5 x the M= value).
The new F= value becomes the standard FS number used for other commands and range calcula-
tions. Note that the accuracy specification is always referenced to the factory (M=) full scale value.
For example, to customize the full scale range of a 20 psig (554 in. water column—inwc) device to 300
inwc, first select the desired display units using the DU command. Enter the command
followed by a *ddDU=INWC. Then enter a *ddWE followed by a *ddF=300. The full scale pressure
for this unit is now 300 inwc.
4.7 SETTING ANALOG PRESSURE SET POINT
Another feature of the PPT is the ability to program an analog pressure set point that triggers the
analog output from the low voltage setting (L=) to the high voltage setting (H=) using the W= and O=
commands. These outputs can be used to activate alarms or lights based on pressure conditions. This
feature can operate in an analog only mode once the set point has been configured and stored to
EEPROM. The set point can be changed in the application through the digital RS-232 connection.
Analog Out (V)
5
Figure 4.8—Pressure Set Point Set
If a set point from a 20 psig unit was desired whenever the pressure equals, or exceeds 12 psi, then set
O=60 (12 psi = 60% of 20 psi FS) and W=S (for setpoint). Figure 4.8 illustrates the pressure vs. voltage
response curve. The output will change to 5 volts at 12 psi. The output levels can be changed by using
the L= and H= commands. If the unit was a 20 psid (differential), then the commands O=80 (32 psi =
80% of 40 psi FS) and W=S are used. The difference is that the offset (O=) is referenced to the mini-
mum pressure value, which is -20 psi for the differential unit. If the pressure signal has some noise in
it, then a deadband should be added to the set point. For a 20psig unit, setting DS=60 will put a ±0.06
psi deadband around the 12 psi set point. That is, the output will switch from 0 to 5V at 12.06 psi for
increasing pressures and switch from 5 to 0V at 11.94 psi for decreasing pressures. The I= command
can be used to filter, by increasing the integration time, any noise on the pressure signal.
12 20 Pressure (psi)
F=0 command to disable this function and
Analog Out (V)
5
Dead Band
12
Figure 4.9—Deadband on Set Point
1 8
slope values 'm'
*ddWE
20 Pressure (psi)