Dynasonics TFX Ultra Transit Time Flow Meters - Manual (Page 28)

INPUTS/OUTPUTS
General
The flow metering system is available in two general configurations. There is the standard flow meter model that is equipped
with a 4-20 mA output, two open collector outputs, a rate frequency output, and RS485 communications using the Modbus
RTU command set.
The energy version of the flow metering family has inputs for two 1000 Ohm RTD sensors in place of the rate frequency and
alarm outputs. This version allows the measurement of pipe input and output temperatures so energy usage calculations can
be performed.
4-20 mA Output
The 4-20 mA output interfaces with most recording and logging systems by transmitting an analog current signal that is
proportional to system flow rate. The 4-20 mA output is internally powered (current sourcing) and can span negative to
positive flow/energy rates.
For AC powered units, the 4-20 mA output is driven from a 15V DC source located within the meter. The source is isolated
from earth ground connections within the flow meter. The AC powered model can accommodate loop loads up to 400 Ohms.
DC powered meters utilize the DC power supply voltage to drive the current loop. The current loop is not isolated from DC
ground or power. Figure 23 shows graphically the allowable loads for various input voltages. The combination of input voltage
and loop load must stay within the shaded area of Figure 23 .
200
100
300
400
500
600
700
800
900
1000
1100
10 12 14 16 18 20 22 24 26 28
Supply Voltage (VDC)
Lo
op L
oad (
O
hms)
Operate in the
Shaded Regions
Supply Voltage - 7 VDC
0.02
= Maximum Loop Resistance
Figure 23: Allowable loop resistance (DC powered units)
INPUTS/OUTPUTS
Page 28 March 2014