OPERATION ST51A/ST75A/ST75AV Mass Flow Meters
32 Fluid Components International LLC
4-20 mA Configuration
Flow/Temperature Assignment
Use the “V” menu to assign 4-20 mA current loop channels 1 and 2 to one of four flow/temperature configurations as required (see Figure
26). The default assignment is: Ch. 1 = Flow, Ch. 2 = Temp. Once the assignment is set or confirmed the “V” menu continues to NAMUR
mode configuration.
: The 4-20mA #1 parameter must remain at its default assignment of flow to comply with the
HART protocol. Do not change this assignment when using HART.
NAMUR Mode
The NAMUR feature globally drives the current loop to a predetermined state when a sensor fault is detected. NAMUR can be turned OFF
(disabled, default), set Low (current loop driven to 3.6 mA on sensor fault) or set High (current loop driven to 21 mA on sensor fault). Once
NAMUR is set or confirmed the “V” menu continues to HART Damping configuration.
HART Damping Value
The HART damping value configures the response time of the HART 4-20 mA output. The range is 0.1 second (no damping with update
every 0.1 s, default) to 100 seconds (max. damping with update every 100 s). HART damping prevents rapid flow changes from affecting
HART communications. HART damping can also be set via the CLI DV command (see CLI Commands, page 34). For most applications,
the factory-set HART damping value can be left unchanged. Once HART Damping is set or confirmed the “V” menu continues to Digital
Output Configuration.
The HART damping configuration menu always displays whether or not the HART option is present. The damping
value can be safely ignored on non-HART instruments since the setting has no effect on the output.
Source/Sink Output Configuration
ST51A/ST75A/ST75AV instruments provide a source output and a sink output.
● Source: The instrument supplies the DC voltage and current for the level output/pulse (22 V, 25 mA max.)
● Sink: The instrument accepts an external customer-supplied DC power source for the level output/pulse (40 V, 150 mA max.)
The source and sink outputs can either be a pulse signal or alarm (level) signal. For alarm configurations, the source output is dedicated to
Alarm0 and the sink output is dedicated to Alarm1.
Source/Sink Output Functions
Use the source/sink outputs for the following functions.
● Alarm: A level signal is generated (from High to Low or Low to High, depending on the Sink/Source state configuration) when the flow
crosses the corresponding SWITCH_point0 (for Alarm0) or SWITCH_point1 (for Alarm1) threshold (setpoint).
● Pulse: For pulse mode the user specifies a Pulse Factor and a Sample Period (0.5 to 5.0 seconds). Basically, each pulse represents
1 Pulse_Factor amount of flow in the current units. The pulses are clamped to 500 per second. The pulse output is well suited to
being connected to a pulse counter where the counted pulses could be converted to display information similar to a gas station pump
display. The length of the pulses is adjusted so that they fill a sample period, thus approximating a frequency output. There are,
however, no distinct pulse mode types (counter/frequency/etc.) as only one mode is provided.
Source/Sink Output Programming Parameters
Use the “V” menu to set up the following source/sink output parameters:
● Pulse factor: Number of pulses per selected engineering unit. Default =1
Example in NCMH:
1 = 1 pulse per NCM
0.1 = 1 pulse per 0.1 NCM (10 pulses per 1 NCM)
10 = 1 pulse per 10 NCM
Range pulse factor 0.001 – 1000
● Sample time: Time in seconds before calculating the next number of pulses.
● Sink/Source state: Sets the pulse/level output polarity: High to Low or Low to High.
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