Walt — Simple Man Takeaway
A bad analog signal can lie politely. Do not believe a smooth number until the process agrees.
Plain-English Summary
Use this flow when an analog reading drifts, jumps, floats, drops out, scales incorrectly, or gives a believable value that does not match the real process. Analog signals can lie smoothly. A wrong analog signal may look reasonable unless compared to the real process condition.
R.E.A.L. firstProve process truthCapture before changingCompare field state to logic stateHumans remain authoritative
Sensor / Instrument First-Check Flow
Field Checks
- Signal type and range
- Field instrument vs. PLC raw value vs. scaled value vs. HMI display
- Scaling range, engineering units, decimal position, offset
- Wiring, shield, grounding, signal common, terminals, loop power
- Noise, drift, clipping, saturation, dropout, floating signal
- Routing near VFDs, contactors, welders, motors, noisy devices
- Transmitter range, damping, configuration, process connection
- Known reference or calibrated meter where qualified
- Real process condition
Watch Out For
- A smooth number that is wrong
- Scaling mismatch after sensor or program change
- Ground loops or shield problems
- Signal changing when a drive starts or a valve shifts
- Transmitter dampening hiding fast events
- AI or HMI reporting a value without source truth
Reverse-Trace / Ghost Busting™ Decision
if an analog permissive, high/low limit, compare, or process-ready condition is blocking a sequence.
if the analog value spikes, dips, drifts, or crosses a threshold briefly and recovers before diagnosis.
Recipe / Health Log
Record the good, the bad, and the in-between: real-world condition, sensor state, PLC input state, HMI display, logic use, product/material, method, atmosphere, utility condition, timing, symptom, corrective action, and result.
Related Calculators / S.W.A.T. Screens
Related Handbook / Flows
Walt says STOP! - Safety First
Make these checks prior to proceeding.
Stop and follow site procedures when the analog signal controls heat, pressure, speed, chemical feed, level, or motion; when forcing or simulating a signal could cause unsafe operation; when live electrical work is required; or when process pressure, temperature, chemicals, or lockout/tagout requirements apply. Analog signals may command real-world energy.