Is It the Sensor or the Process?

Who this is for and what it does: maintenance techs, plant engineers, and controls people staring at a reading that looks wrong. This guide helps you decide whether the instrument is lying or the process really is doing what the number says, before you start changing setpoints or swapping parts.

How to use this Work top to bottom. Find the symptom that matches what you're seeing, run the checks in order, and write down what you find as you go. The cheap checks come first on purpose. Most of the time you can settle the sensor-versus-process question without a single spare part, just by making the reading disagree or agree with a second source of truth. One habit worth building: before you touch anything, note the current reading, the time, and what the process is actually doing (running, idle, filling, heating). Half of these calls get solved by comparing the number against something you already know to be true.

The core question A reading is a claim: the level is 40%, the flow is 12 gallons per minute (GPM), the temperature is 340 degrees. Your job is to find one independent thing that agrees or disagrees with that claim. When two sources that should move together don't, one of them is wrong, and now you have a real problem to chase instead of a hunch. Independent means the second source does not share the failure. A second transmitter on the same power supply, cable tray, and ground is not fully independent. A tape measure, a sight glass, the motor's current draw, a delivery ticket, or a hand-held meter usually is.