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Process Current Measurements Using the DI-710 Data Logger

By Shawn MacDonald
DATAQ Instruments, Inc.

Download this application note.

For those looking to make several 4-20mA (1-5V) process control measurements, the DI-710 Data Logger is easy to use and cost effective. In the past, making several 4-20mA process control measurements could be a costly endeavor. Because each process control loop may have its own ground, the possibility exists that the sum of your input signal and any common mode voltage (CMV) that may be present (Figure 1) could damage your equipment. Eliminating this risk requires isolation, which used to mean costly additional signal conditioning. That is no longer the case.

Figure 1 — A common mode voltage may exist between the current loop and the data acquisition device.

The DI-710 makes multiple process control measurements affordable by providing built in input-to-output isolation (Figure 2).

Figure 2 — The DI-710 features Input-to-Output isolation.

Process Current Loops

Due to the presence of electrical noise in most industrial settings, measuring a sensor/transducer output as a voltage signal can produce inaccurate results. The solution is to transmit sensor/transducer outputs as current signals.

Process current loops are widely used in industry to measure and record values such as pressure, load, flow, temperature or torque. A basic process control current loop consists of a sensor/transducer and a power supply. In order to make process current measurements, a resistor is added (Figure 3).

Figure 3 — A process current loop with a resistor inserted to make measurements.

Current ( I ) in the loop (Figure 3) is proportional to the change in physical quantity being measured. The power supply is typically 28V and the value of the resistor is typically 250 ohms. Using Ohm's law (V = I × R):

If I = 4mA
4mA × 250ohms = 1V

If I = 20mA
20mA × 250ohms = 5V

Typically, 4mA (1V) represents zero-level while 20mA (5V) represents the full scale output of the sensor/transducer.

The theory behind a process control current loop is that all current leaving the negative side of a power supply must return to the positive side. The current is therefore not affected by outside noise, ensuring accurate results.

No More Common Mode Voltage Issues

When monitoring process control current it is important to note that each loop may have its own power supply and therefore, its own ground. As a result, a common mode voltage could be present, sufficient to damage or destroy your equipment. It is the isolation, built into the DI-710 that makes it an ideal tool for measuring several process control signals. By isolating the DI-710 from your PC, common mode voltage becomes a non-issue.

Connect and Configure your Process Control Signals

Connection configurations

When multiple grounds are present a differential connection is used to measure the potential difference across the current loop resistor in order to determine current. Up to 8 differential connections can be made.

Figure 4 — Differential voltage measurement.

Assuming a common ground, up to 16 single-ended connections can be made (Figure 5).

Figure 5 — With a common ground, up to 16 single ended connections can be made.

DI-710 Connections

Note the "DI-710 Screw Terminal Access" sticker on top of the DI-710 (Figure 6). This sticker diagram corresponds to the 32 pins on the front of the DI-710.

Figure 6 — DI-710 Screw Terminal Access sticker.

To connect your process control signal in a differential configuration on channel 1:

• Connect "+Diff." to channel "1" in "Analog Channels" under "Positive Differential."
• Connect "-Diff." to channel "9" in "Analog Channels" above "Negative Differential."

Channel "1" is now connected as a differential input. Note: Software must also be configured for differential input.

To connect your process signal in single-ended configuration on channel 1:

• Connect the positive (+) side of the resistor to channel "1" in "Analog Channels" under "Positive Differential."
• Connect the negative (-) side of the resistor to one of the two "AG" (analog ground) pins.

Conclusion

If you're looking to measure and/or record several process control signals you can't go wrong with the DI-710. From the built in isolation to the ease with which process control signals can be connected and configured, the DI-710 is tough to beat.