User Manual

Engineering
Implementing volume flow measurement
3
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3 Engineering
3.1 Implementing volume flow measurement
Implementing volume flow measurement In addition to normal (differential)
pressure measurement, the QBM3700.. provides volume flow measurement (i.e.
used with 3rd party controllers).
Flow is a volume over time. It is related to differential pressure according to the
following formula:
Flow measurement with QBM3700.. is set up in an application as follows:
1. Write the vendor specific k-factor for the measured device to register 4x0083
(sensor 1) and/or 4x0093 (sensor 2). Permitted range 0…1500.
2. Select requested unit in register 4x0084 (sensor 1) and/or 4x0094 (sensor 2).
Selectable are m
3
/h, m
3
/s or l/s.
 QBM3700.. calculates the flow value.
3. Read out the flow value from registers 4x0081/4x0082 (flow low/flow high of
sensor 1) resp. registers 4x0091/4x0092 (flow low/flow high of sensor 2).
The two registers exist for mathematical reasons: The QBM3700.. works with a
high 16 bit and a low 16 bit register.
● Value in register "flow high" = 12
● Value in register "flow low" = 35012
Calculation:
12*65536 = 786432
786432 + 35012 = 821444
With scaling (0.01) the end value = 8214.44m³/h.
3.2 Modbus registers
Supported function codes
03 (0x03) Read holding register
04 (0x04) Read input registers
06 (0x06) Write single register
16 (0x10) Write multiple registers
Any changes in settings are completed with setting 4x0768 (Bus config
command) to "1" (toggle bit).
Value in register
Example