🔍 Key Concepts

• Meaning of live zero in instrumentation loops • Difference between signal zero and absolute zero for 4–20 mA and 3–15 PSI signals • How span and range are defined for transmitters and controllers

💭 Think About

• Think about why industrial instruments do NOT usually use 0 mA or 0 PSI as the working zero point in a control loop. • Consider what current or pressure value should represent the minimum measurable process value (not “nothing connected”). • Ask yourself: which option correctly describes the signal level that still indicates the loop is alive, but the process is at its lower range value?

✅ Before You Answer

• Verify what value is commonly used as the lower range value in a standard 4–20 mA loop and a 3–15 PSI loop. • Check which choice clearly indicates a value that is above zero and used to show the loop is healthy even at minimum measurement. • Eliminate any option that suggests the live zero is an actual numerical zero output signal in the loop.