Why would you use a live bottom (4-20 mA) rather than a true bottom (0-20 mA) in a loop?

• A. Using a live bottom would fail to indicate a healthy transmitter.
• B. With a true bottom at the lowest output there is no way to tell your transmitter is working properly as it will always read 0 mA
• C. Live bottom provides better fault detection.
• D. There is no practical difference.

Answer: (B)

The key idea is fault visibility in a current loop. In a 4-20 mA loop, the current not only carries the measurement but also signals that the transmitter is alive and powered. If the bottom of the range is zero, the smallest current you can see could reflect the actual minimum signal or a fault (no current at all). Because you can’t distinguish those two cases from the reading alone, you can’t reliably tell whether the transmitter is functioning properly when you’re at the bottom of the scale. In contrast, a live bottom (a non-zero minimum like 4 mA) guarantees a baseline that proves the transmitter is powered and capable of signaling; any deviation from that baseline indicates a fault. So using a true bottom makes health status ambiguous, which is why it’s not desirable.