⚠️ Disclaimer: These tasks are guidelines only. They do not include lockout/tagout (LOTO), energy isolation, or other safety requirements. Review and verify suitability for your specific equipment and application. Add all required safety procedures per your company's policies and regulatory requirements before use. You are responsible for the safe and appropriate execution of all maintenance activities.
Capacitance and conductive level sensors don't fail dramatically. No smoke. No obvious noise. No seized shaft. They drift, foul, or lose calibration — and the control system keeps reading whatever the sensor last told it was true. By the time someone notices the tank is empty or the vessel is full when it shouldn't be, the sensor has been lying for a while.
The failures that matter most here are the ones that don't trip an alarm. A sensor that fails hard — goes to zero, goes to full-scale, drops signal — your system probably catches that. A sensor that reads two inches off, or whose trip point has shifted because the probe fouled, or whose sensitivity setting was right for last year's process fluid but not this year's? That one runs until a batch is ruined or a pump runs dry.
This checklist covers the PM tasks that catch those failures before they become production problems. For a broader look at how sensor PM fits into a complete instrumentation program, start with industrial sensor and instrumentation preventive maintenance.
How to Use This Checklist
Record your findings with specificity — not "checked" or "OK," but what you actually observed. "Probe clean, no buildup" is useful. "OK" is a checkbox answer that helps no one when you're trying to figure out what changed between the last PM and the failure that just happened.
Trend your readings over time. A calibration that's 1% off this quarter and 2% off next quarter is telling you something. A sensor whose output matches the DCS reading perfectly every PM for two years, then suddenly reads 4% off, is also telling you something. Neither signal matters if nobody's writing it down.
Good finding: "Output reads 11.8 mA at mid-span; DCS shows 11.6 mA. Within tolerance. No adjustment made. Logged."
Bad finding: "Signal OK."
Field Checklist — Critical Tasks
Visual Inspection Tasks
| Task | Freq | Type |
|---|---|---|
| Inspect sensor body, housing, and mounting hardware for physical damage, corrosion, or loose fittings. Tighten or flag for replacement as needed. | Every PM | MEC |
| Clean the sensor probe/electrode and mounting area of any process buildup, scale, or coating using a damp cloth or appropriate solvent per material compatibility. Ensure sensing surface is clean and unobstructed. | Every PM | MEC |
| Verify the sensor cable and conduit connections are intact, undamaged, and properly secured. Look for chafing, pinched insulation, or loose fittings. | Every PM | ELE |
| Inspect the process connection (fitting, flange, or threaded entry) for leaks, seepage, or process material migration. Tighten or replace seal as needed. | Every PM | MEC |
Electrical Inspection Tasks
| Task | Freq | Type |
|---|---|---|
| Check all electrical terminations at the sensor junction box or terminal strip for tightness, corrosion, and heat discoloration. Tighten and clean as needed. | Quarterly | ELE |
| Inspect sensor grounding connection. Verify ground wire is present, secure, and free of corrosion at both the sensor and panel ends. | Quarterly | ELE |
Operational Checks
| Task | Freq | Type |
|---|---|---|
| Perform a functional test: simulate a high or low level condition (or use the sensor's test function if available) and verify the output signal or indicator responds correctly. | Monthly | ALL |
| Confirm sensor output signal (analog or discrete) matches the control system reading. Compare sensor indication to actual level or a calibrated reference and note any discrepancy. | Monthly | ALL |
Reference Checklist — Full Task Library
Visual Inspection Tasks
| Task | Freq | Type |
|---|---|---|
| Inspect sensor body, housing, and mounting hardware for physical damage, corrosion, coating degradation, or loose fittings. Document condition. | Every PM | MEC |
| Clean the sensor probe/electrode and sensing area of process buildup, scale, foam residue, or coating using appropriate solvent or cleaning method per material compatibility data sheet. | Every PM | MEC |
| Inspect sensor cable, conduit, and all associated wiring for chafing, pinched insulation, UV degradation (outdoor units), and secure routing. Repair or replace as needed. | Every PM | ELE |
| Inspect the process connection (flange, NPT fitting, or compression fitting) for leaks, material migration, or thread damage. Replace seals/gaskets on schedule or at any sign of seepage. | Every PM | MEC |
| Inspect sensor insertion length and orientation in vessel. Confirm probe is not fouled, bent, or repositioned from original installation. Verify measurement range is still appropriate for vessel geometry. | Semi-Annually | MEC |
Electrical Inspection Tasks
| Task | Freq | Type |
|---|---|---|
| Check all electrical terminations at the sensor, junction box, and panel for tightness, corrosion, and heat discoloration. Torque to spec and clean contacts as needed. | Quarterly | ELE |
| Verify sensor grounding at both the sensor head and control panel. Resistance to ground should be < 1 ohm. Document reading and correct if out of spec. | Quarterly | ELE |
| Inspect and verify sensor sensitivity or threshold setting (if adjustable). Confirm setting is appropriate for the current process fluid and vessel geometry. Adjust if process conditions have changed. | Semi-Annually | ELE |
| Inspect any temperature compensation or RFI/EMI shielding features on the sensor cable and head. Verify shielding is intact and drain wire is grounded at one end only. | Annually | ELE |
| Verify fail-safe output behavior: confirm sensor drives to the correct safe state on loss of power or broken wire (fail-high or fail-low as required by process safety design). | Annually | ELE |
Operational Checks
| Task | Freq | Type |
|---|---|---|
| Verify sensor output (4–20 mA, discrete, or digital) against the control system or DCS reading. Investigate and correct any offset or discrepancy greater than ±2% of span. | Monthly | ELE |
| Review the sensor's alarm and trip history in the control system over the past PM interval. Identify any nuisance trips, missed detections, or anomalies and investigate root causes. | Every PM | ALL |
| Perform sensor functional test by simulating a level condition (use certified calibration liquid, test vessel, or manufacturer test mode). Verify trip point accuracy and output response. | Quarterly | ELE |
| Check for process fluid or material changes that may affect sensor performance (conductivity changes, new fluid, coating agents, or temperature range shifts). Update sensitivity or recalibrate as needed. | Quarterly | ELE |
| For conductive sensors: verify minimum conductivity of the process fluid meets sensor specification (typically >20–50 µS/cm). If fluid conductivity has changed, flag for engineering review. | Semi-Annually | ELE |
| Calibrate or trim sensor zero and span per manufacturer procedure. Document pre- and post-calibration readings. Obtain sign-off if required by process control procedure. | Annually | ELE |
| Confirm spare sensor(s) of the correct model and range are stocked and accessible. Verify spare is within shelf life or storage requirements if applicable. | Annually | ALL |
Failure Modes This Checklist Targets
Probe fouling and coating buildup. Process material accumulates on the sensing surface and changes the apparent dielectric or conductivity reading. The sensor doesn't fail — it just lies. Capacitance sensors are especially vulnerable in viscous, scaling, or coating-prone processes.
Sensitivity drift from process fluid changes. A capacitance sensor calibrated for one fluid may read significantly off when the fluid changes — different viscosity, different dielectric constant, different temperature profile. The PM program that doesn't account for this is validating a sensor for a process that no longer exists.
Grounding faults. Both sensor types depend on reliable grounding for signal accuracy and noise rejection. A corroded ground wire or a single-point ground violation in the shielded cable introduces noise that reads as signal. This shows up as erratic output, nuisance trips, and readings that shift with nearby electrical activity.
Calibration and trip point drift. Zero and span drift over time, especially in high-temperature or high-vibration installations. A sensor whose trip point has drifted 3% may never trigger an alarm — it's still sending a signal, the DCS is still reading it, and nobody has compared the reading to actual level since commissioning.
Process connection failure. Leaking fittings and degraded seals let process material migrate into the conduit or junction box. By the time it's obvious, the wiring is already damaged. This one is caught by looking — which is why it's on every PM.
Fail-safe state misconfiguration. A sensor set to fail-high in a high-level alarm application will mask the alarm on failure. A sensor set to fail-low in a low-level cutout application will mask a dry-run condition. This gets verified at commissioning and then never checked again. Until it matters.