The series of HD404T transmitters is able to measure relative pressures with reference to
the atmosphere or differential in the range from 50 to 1000 Pa (0.2” H2O to 5” H2O). HD404T
transmitters use a silicon “micromachined” type sensor compensated in temperature that has
an excellent linearity, repeatability and stability over time. The output signal from the sensor
is amplified and converted into a standard analogical output in current (4…20mA) and in one
in voltage (0…10V), which, then, can be transmitted over long distances with a high immunity
In each model it’s possible to choose, via a dip switch, between two measurement ranges in
order to select the optimal scale for each application.
Usually the low pressure transmitters are susceptible to the guidance by which they are
mounted. In HD404T series there is available a special auto-zero circuit, which periodically
equalize the differential pressure at the input sensor and corrects the offset; the transmitters,
provided with this circuit, are insensitive to the mounting position. In addition, the circuit compensates
autozero aging and the deviation of the zero of the sensor to temperature changes:
virtually eliminates maintenance.
It’s available the (L) “display” option, in which the pressure is visualized on a display with 4
digits in the selected measurement unit.
The (SR) “square root” is especially useful if the transmitter is connected to a Pitot or Darcy
tube, as the output is directly proportional to the speed of airflow. The SR version with L option
also allows to display, in addition to the pressure measured, the speed of airflow calculated.
The SR version is configurable by the user by connecting the transmitter to a PC and sending
serial commands via a standard communication software. It is possible to set the coefficient of
the Pitot or Darcy tube used, the parameters for the calculation of the speed (airflow temperature,
barometric pressure, differential static pressure in the duct), the speed unit of measurement
(m/s or ft/s) and the speed full scale for the analog output.
The transmitters are ready to use and are supplied calibrated at 3 points by the manufacturer.
Typical applications for HD404T series are clean room monitoring, filters’ control, flow measures
(matched with the Pitot tube), the air conditioning control and the ventilation one.
In all the models, the sensor and the electronic are contained in a sturdy plastic case with an
IP67 protection degree. Opening the lid are available 3 mm diameter holes that let you set the
base of the transmitter directly to a panel or a wall.
HD404T can be mounted in any position, but typically on a vertical wall with the pressure inlets
facing downwards. The gap from zero due to the mounting position can be compensated by
using CAL ZERO button. The procedure to follow for manual calibration of zero is the following
• Make sure that the transmitter is powered by at least 1 hour;
• Disconnect both tubes from the + and – pressure inlets;
• Press CAL ZERO button until the red LED starts to flash;
• When the red led turns off, the zero procedure is completed and you can reconnect the
tubes to pressure fits.
We suggest you to perform the auto-zero procedure at least once a year under normal working
In models with auto-zero circuit (AZ option), this procedure is regularly performed every 15
minutes without disconnecting the hoses from pressure taps. During the reset, which takes
about 4 seconds, the analogical outputs and the display will remain frozen at the measured
value. Models with auto-zero have virtually no need for maintenance.
Setting the output range: the RANGE jumper allows you to choose one of this output ranges:
with LOW you choose the low range, with HIGH the extended range.
Response time setting: the FAST SLOW jumper lets you choose the response time of the
transmitter: in FAST position the measurement is integrated over 1 s, while in SLOW position
is integrated over 4 s. SLOW position is recommended if there are conditions of turbulence or
disruption of air fl ow.
Setting of the parameters in the SR versions: the transmitters are preset by the factory. To
change the settings, proceed as follows:
• Connect the transmitter COM AUX serial output to the PC RS232 (through the RS27 cable)
or USB (through the CP27 cable) port. If the CP27 cable is used, install in the PC the related
• In the PC, run a serial communication software (e.g. Hyperterminal), set the baud rate to
115200 and the communication parameters to 8N2.
• To change the display confi guration (commands O3E, O3D, O4E, O4D, O5E, O5D) it is necessary
to send the CAL START command to enter the confi guration mode. It is not necessary
to send the CAL START command to change the parameters related to the speed measurement
(tube coeffi cient, temperature, pressure, full scale for the analog output).
Models with L suffi x are provided with a LCD display with 4 digits.
Pressure display resolution:
50 – 100 – 250 – 500 Pa → 0.5 Pa
1000 Pa → 1 Pa
5 – 10 – 25 – 50 mmH2O → 0.05 mmH2O
100 mmH2O → 0.1 mmH2O
0.2 – 0.4 – 1 – 2 – 4 inchH2O → 0.002 inchH2O
Speed display resolution in the SR models: for all the ranges, the speed resolution can be
centesimal fi xed or with automatic change from decimal to centesimal as a function of the
measured value. The selection between the two options is made via the serial commands
O4E and O4D.
Undr → it appears if the measured value is smaller than the minimum value that
can be measured
OvEr → it appears if the measured value exceeds the maximum value that can be
CAL Error → it appears when the zero calibration is fi nished if the maximum offset
value that can be corrected is exceeded.