7.3 Analog output Calibration of GFM2 Mass Flow Meters
GFM2 series Mass Flow Meters are equipped with
calibrated 0 to 5 Vdc and 4 to 20 mA output signals. The set
of the jumpers (J7A, J7B, J7C) on the printed circuit board
is used to switch between 0 to 5 Vdc and 4 to 20 mA output
signals (Figure C-1 “GFM2 configuration jumpers).
Figure C-1 GFM2 Analog Output Configuration
Jumpers
The GFM2 analog output calibration involves calculation
and storing of the offset and span variables in the
EEPROM for each available output. The 0 to 5 VDC output
has only scale variable and 20 mA output has offset and
scale variables. The following is a list of the Gas
independent variables used for analog output computation:
Index
Name
Description
25
AoutScaleV -
DAC 0 to 5 VDC Analog Output Scale
27
AoutScale_mA -
DAC 4 to 20 mA Analog Output Scale
28
AoutOffset_mA -
DAC 4 to 20 mA Analog Output Offset
7.3.1 Initial Setup
Power up the Mass Flow Meter for at least 15 minutes prior to commencing the
calibration procedure. Make sure absolutely no flow takes place through the
meter. Establish digital RS-485/RS-232 communication between PC
(communication terminal) and GFM2. The commands provided below assume that
calibration will be performed manually (w/o Dwyer supplied calibration and
maintenance software) and the device has RS-485 address 11. If Dwyer supplied
calibration and maintenance software is used, skip the next section and follow the
software prompts.
Enter Backdoor mode by typing: !11,MW,1000,1[CR]
Unit will respond with:
!11,BackDoorEnabled: Y
Disable DAC update by typing:
!11,WRITE,4,D[CR]
Unit will respond with:
!11,DisableUpdate: D
7.3.2 Gas Flow 0 to 5 VDC Analog Output Calibration
1. Install jumpers J7A, J7B and J7C on the PC board for 0-5 Vdc output (see
Table VI).
2. Connect a certified high sensitivity multi meter set for the voltage measurement
to the pins 2 (+) and 1 (-) of the 15 pins D connector.
3. Write 4000 counts to the DAC channel 1: !11,WRITE,1,4000[CR]
4. Read voltage with the meter and calculate:
5. Save FlowOutScaleV in to the EEPROM: !11,MW,25,X[CR]
Where: X – the calculated AoutScaleV value.
7.3.3 Gas Flow 4 to 20 mA Analog Output Calibration
1. Install jumpers J7A, J7B and J7C on the PC board for 4 to 20 mA output (see
Table VI).
2. Connect a certified high sensitivity multimeter set for the current measurement
to pins 2 (+) and 1 (-) of the 15 pins D connector.
3. Write 4000 counts to the DAC channel 1: !11,WRITE,1,4000[CR]
4. Read current with the meter and calculate:
5. Write zero counts to the DAC channel 1: !11,WRITE,1,0CR]
6. Read offset current with the meter and calculate:
7. Save AoutScale_mA in to the EEPROM:
!11,MW,27,Y[CR]
Save AoutOffset_mA in to the EEPROM: !11,MW,28,Z[CR]
Where: Y – the calculated AoutScale_mA value
Z – the calculated AoutOffset_mA value
Enable DAC update by typing:
!11,WRITE,4,N[CR]
Unit will respond with:
!11,DisableUpdate: N
Close BackDoor access by typing: !11,MW,1000,0[CR]
Unit will respond with:
!11,BackDoorEnabled: N
8. RS-485/RS-232 SOFTWARE INTERFACE COMMANDS
8.1 General
The standard GFM2 comes with an RS-485 interface. For the optional RS-232
interface, the start character (!) and two hexadecimal characters for the address
must be omitted. The protocol described below allows for communications with
the unit using either a custom software program or a “dumb terminal.” All values
are sent as printable ASCII characters. For RS-485 interface, the start character
is always (!). The command string is terminated with a carriage return (line feeds
are automatically stripped out by the GFM2). See section 2.2.3 for information
regarding communication parameters and cable connections.
8.2 Commands Structure
The structure of the command string:
!<Addr>,<Cmd>,Arg1,Arg2,Arg3,Arg4<CR>
Where:
!
Start character.
Addr
RS485 device address in the ASCII representation of
hexadecimal (00 through FF are valid).
Cmd
The one or two character command from the table below.
Arg1 to Arg4
The command arguments from the table below.
Multiple arguments are comma delimited.
CR Carriage
Return character.
Several examples of commands follow. All assume that the GFM2 has been
configured for address 18 (12 hex) on the RS485 bus:
1.
To get current calibration tables:
!12,G<CR>
The GFM2 will reply:
!12,G 0 AIR<CR>
(Assuming Current Gas table is #0, calibrated for AIR )
2.
To get current Alarm status:
!12,A,R<CR>
The GFM2 will reply:
!12,N<CR>
(Assuming no alarm conditions)
3.
To get a flow reading:
!12,F<CR>
The GFM2 will reply:
!12,50.0<CR>
(Assuming the flow is at 50% FS)
4.
Set the high alarm limit to 85% of FS flow rate:
!12,A,H,85.0<CR>
The GFM2 will reply:
!12,AH85.0<CR>
Analog
Output
RS485
Terminal Resistor
Function
0 to 5 VDC
4 to 20 mA
OFF
ON
J7A
5-9
1-5
J7B
6-10
2-6
J7C
7-11
3-7
JCD
8-12
4-8
The analog output available on the GFM2 Digital Mass Flow
Meter was calibrated at the factory for the specified gas and
full scale flow range (see the device’s front label). There is no need to perform
analog output calibration unless the EEPROM IC was replaced or offset/span
adjustment is needed. Any alteration of the analog output scaling variables in the
Gas independent table will VOID calibration warranty supplied with instrument.
NOTICE
It is recommended to use the Dwyer supplied calibration and
maintenance software for analog output calibration. This
software includes an automated calibration procedure which may radically
simplify calculation of the offsets and spans variables and, the reading and writing
for the EEPROM table.It is recommended to use the Dwyer supplied calibration
and maintenance software for analog output calibration. This software includes an
automated calibration procedure which may radically simplify calculation of the
offsets and spans variables and, the reading and writing for the EEPROM table.
NOTICE
AoutScaleV =
20000
Reading[V]
AoutScale_mA =
4000
Reading[mA]
AoutOffset_mA = -FlowOutScale_mA*Offset_Reading[mA]
When done with the analog output calibration make sure the
DAC update is enabled and the BackDoor is closed (see
command below).
NOTICE
Default address for all units is 11. Do not submit start
character and two character hexadecimal device address for
RS-232 option.
NOTICE
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