Optical Scientific Inc
OFS 2000CW User’s Guide
P/N 1910-905
Rev. 03/28/13
33
3.12 4-20 mA Current Loop Connections
The terminal block TB3 of the interface board connects to isolated 4-20 mA circuitry, diagnostic relay,
calibration relay and calibration check request interface. The pin designation is as follows:
Pin 1:
Current Loop 1 -
Velocity
Pin 2:
Current Loop 1 +
Pin 3:
Current Loop 2
–
Vol. Flow
Pin 4:
Current Loop 2 +
Pin 5:
Current Loop 3
–
Temperature
Factory
Option
Pin 6:
Current Loop 3 +
Pin 7:
Current Loop 4
–
Pressure
Factory
Option
Pin 8:
Current Loop 4 +
To set the full-scale range and other 4-20 mA
features. [See Section 4.2.4.
“Current Loop”]
Figure 3.26 TB3 Connections
These four pairs are for connection of the customer’s 4-20 current loop for velocity measurement. The
OFS system provides a cur
rent at all times while the system is powered (4mA representing “0” flow). In
common terminology, this is a “sourcing” 4-20ma loop, meaning the OFS controller provides the loop
power. Do not connect wires that have a voltage on them to these terminals. The maximum loop
resistance is ~600 ohms. This includes the cable resistance and the load resistor in the customer data
acquisition system. Loop 1 is standard on all models.
Current Loop 1 - Velocity sets the velocity measurement range. Default velocity range is 0 to 40 m/s
(meters per second). The user can opt for a lesser range by entering "Setup Mode" using the serial
connection [Section 4.24].
Current Loop 2 - Volume is the volumetric flow output range, The output (mA) is linear, with 4 mA = 0
SCMH minimum and 20 mA = Velocity
MAX
* Stack Area * 3600 SCMH
e.g.
Velocity 40 m/s and stack area 1 sq meter. F. S. Vol. Flow = 40 * 1 * 3600 = 144 k SCMH
Current Loop 3
– Temperature output is available as a factory-purchased option. The output (mA) is
linear, with 4 mA = -40°C (minimum) and 20 mA = 500° C (maximum).
Current Loop 4
– Pressure output is available as a factory-purchased option. The output (mA) is linear,
with 4 mA = 750 hPa (minimum) and 20 mA = 1150 hPa (maximum).
To convert the current loop readings to velocity, use the following formula:
Velocity (m/s) = (ma
– 4) X (F.S./16)
where ma is the measured loop current and F.S. is the full-scale velocity selected in m/s
