GHM Delta OHM HD404 Series, Operating Manual, Page: 10 / 24

HD404…
- 10 -
V1.2
The instruments can be mounted in any position, but typically they are fixed on a ver-
tical wall with the pressure inputs facing down.
The deviation of the zero due to the mounting position can be corrected by using the
CAL ZERO button. In the models with AZ option, an auto-zero circuit periodically au-
tomatically equalizes the differential pressure at the input of the sensor and corrects
the offset due to the mounting position or sensor aging.
To manually calibrate the zero, disconnect both the tubes from the + and – pressure
inputs, then proceed as follows:
•
press CAL ZERO until the red LED starts flashing;
•
when the red LED turns off, the zeroing procedure is completed: reconnect the
tubes to the pressure inputs.
In the models without AZ option, it is recommended to follow the auto zero procedure
at least once a year under normal operating conditions.
In models with AZ option, the zeroing is performed hourly. During the zeroing, which
takes about 4 seconds, the output and the display remain frozen at the last measured
value.
3.1 E
LECTRICAL CONNECTIONS
Current analog output Voltage analog output
RS485 connection
In the RS485 connection, the instruments are connected in a sequence through a
shielded cable with twisted pair for signals and a third wire for the common. Line ter-
mination must be set at the two network ends.
The maximum number of devices that can be connected to the RS485 line (Bus) de-
pends on the load characteristics of the devices to be connected. The RS485 standard
requires that the total load does not exceed 32 unit loads. The load of an HD404ST…
transmitter is equal to ¼ of unit load. If the total load is greater than 32 unit loads,
Power supply Power supply
Power supply
Other sensors with
RS485 output
Termination
Termination
PLC, data logger or
RS485/USB or RS485/RS232
converter for PC