A 4-20 mA signal is by far the most common method of transferring instrument signals to a PLC or chart recorder. When scaled, the 4 mA is equivalent to the low end of the range and the 20 mA is equivalent to the full scale of the range. For example, let’s look at a 3-8850 conductivity transmitter that is monitoring a solution on a scale of 0 to 2000 μS. The 4mA set point would need to be set to 0 μS and the 20mA set point needs to set to 2000 μS. If the measured solution is 1000 μS (middle of the user’s scale), then the resulting current will be 12mA (middle of the 4-20mA scale).
Using this quick calculation can determine if the mA output of the instrument is correct:
• 4-20 mA scale has 16 mA (20mA – 4mA = 16mA)
• 16mA divided by 2000 μS (full scale) = .008mA per 1 μS
• Add the 4mA to the calculated value
• 1μS = 4.008mA
Signet instruments and some blind sensors offer two options when selecting a device with a 4-20mA output:
A passive loop or an active 4-20 mA loop.
A passive loop:
Requires the end user to provide a power source (typically 24 volts DC). The installer MUST pay attention to the wiring during installation or a reverse polarity signal may occur (a negative mA signal). Rule of thumb, the power supply should maintain its polarity as shown in figure 1, the positive terminal of the power supply is wired to the positive terminal of the Signet instrument, and the negative terminal of the power supply is wired to the negative terminal of the device that is to capture the mA signal.
