Control Valve Partial Stroke Testing (PST) for Safety Valves

-
Practical Procedure · Safety Systems Control Valve Partial Stroke Testing (PST) for Safety Valves Applies to: Emergency shutdown (ESD) valves and safety instrumented function (SIF) final elements  |  Equipment needed: PST-capable positioner/logic solver, position feedback monitoring, coordination with process/operations Full Stroke vs. Partial Stroke — Why PST Exists Tap a tab to see the difference between a full functional test and a partial stroke test, and why PST lets you test without shutting down. Full Stroke Test Partial Stroke Test (PST) ESD Valve — Process Running 0% → 100% → 0% process fully interrupted during test ESD Valve — Process Running ...
Post a Comment

Pressure Transmitter Calibration

-

Practical Procedure · Calibration

Pressure Transmitter Calibration — Step-by-Step Field Procedure

Applies to: Smart pressure transmitters (HART/analog output)  |  Equipment needed: Pressure calibrator/pump, HART communicator (if trimming digitally), multimeter or mA reference

Before You Start

  • Isolate the transmitter from the process safely — close upstream/downstream valves per your isolation procedure, and vent/bleed trapped pressure before disconnecting.
  • Note the transmitter's configured range (e.g. 0–10 bar = 4–20 mA) from the nameplate or existing configuration before touching anything.
  • Let the transmitter stabilize at ambient temperature if it's just been removed from a hot/cold process area — thermal effects can cause a false-looking error if you calibrate too soon after isolation.

Step 1 — As-Found Check

Before adjusting anything, apply pressure at 0%, 25%, 50%, 75%, and 100% of range and record the transmitter's output at each point as it currently is. This "as-found" data matters — it tells you how much the transmitter had actually drifted, which is often required for calibration records and trending drift over time.

Step 2 — Zero Check and Adjustment

Apply zero pressure (0%) and check the output reads correctly (4 mA for a standard 4–20 mA transmitter, or 0% on a HART digital value). If it's off, adjust the zero — either via the local zero screw/button on older transmitters, or digitally through a HART communicator on smart units.

Step 3 — Span Check and Adjustment

Apply full-scale pressure (100% of range) and check the output reads correctly (20 mA or 100%). Adjust span if needed. Note: always set zero before span — adjusting span first and then correcting zero will throw your span off again, since the two interact.

Step 4 — Mid-Range Points

Check 25%, 50%, and 75% points after zero and span are set. These confirm linearity — a transmitter can be perfect at 0% and 100% but still be non-linear in the middle if the sensor itself has drifted or is damaged. If mid-range points are consistently off in the same direction, that's usually a sensor problem, not something a zero/span adjustment can fix.

Step 5 — Digital Trim vs. Analog (Loop) Trim

On smart/HART transmitters, be clear on which trim you're doing:

  • Digital (sensor) trim — corrects the transmitter's internal sensor reading against your reference pressure. This is what you use if the sensor itself is reading incorrectly.
  • Analog (loop/output) trim — corrects the 4–20 mA output signal to match what the receiving system (PLC/DCS analog input card) actually sees. This is used when the sensor reading is correct but the output loop has drifted — often due to the receiving card, not the transmitter.

Doing a loop trim when the real problem is a sensor issue (or vice versa) won't fix the fault — it'll just mask it temporarily or introduce a new error elsewhere.

Step 6 — As-Left Check and Documentation

Once calibration is complete, run through 0%, 25%, 50%, 75%, and 100% one more time and record the "as-left" values. Compare against your as-found data, and log both in your calibration records along with the date, equipment used, and technician name.

Common Mistakes to Avoid

  • Adjusting span before zero, causing repeated back-and-forth adjustment.
  • Skipping mid-range points and missing a linearity/sensor problem.
  • Confusing digital trim with loop trim, and applying the wrong one.
  • Not letting the transmitter thermally stabilize before calibrating, especially right after removal from a hot process line.
  • Not recording as-found values, losing the ability to trend drift over time.

This is a general field procedure — specifics can vary by transmitter brand and plant calibration standards. Adjust to your own SOP where needed.

📘 For complete books related to these topics: • Electrical Fundamentals for Beginners — 30-Lesson Animation-Driven Training Course • Industrial Instrumentation Fundamentals — 30-Lesson Practical Training Course • Intermediate/Advanced Electrical Theory — 31-Lesson Professional Training Course • LADDER LOGIC FOR PLANT TECHNICIANS • PLC Fundamentals for Plant Technicians

🎥 Watch It Explained on PLC Key Channel

Prefer video? I break down topics like this with animations on my YouTube channel.

▶ Watch the full playlist on PLC Key Channel →

🛠️ Tools I Actually Use in the Field

A few affiliate links to tools that come up constantly in instrumentation work — I only list things I'd recommend to my own team:

As an Amazon Associate I earn from qualifying purchases. It doesn't cost you anything extra.

Got a specific instrumentation problem you're stuck on? Drop it in the comments — I answer based on real plant troubleshooting, not textbook theory.

Comments

Post a Comment

Popular posts from this blog

Belt weigh feeder Zero and Span Drift

-
Image
Fault Log · Field Report Schenck Weigh Feeder — Zero and Span Drift, Two Different Root Causes Plant: Cement  |  System: Schenck weigh feeder, belt scale, hopper load cells  |  Downtime: None Symptom A Schenck weigh feeder started showing both zero drift and span drift at the same time. Two separate accuracy problems showing up together made this a two-part investigation from the start — zero and span errors don't always share the same root cause, even when they appear together. Investigation — Zero Drift Started with the load cell loop and mechanical side of the belt scale: Load cell loop — checked the full loop, cleaned the load cell area. Idlers/rollers — checked the rollers for wear or misalignment. Belt tracking — with the belt empty, noticed it was drifting to one side. Zero was being tared in that empty, drifted position. Once material loaded onto the belt, the belt tracked back closer to center — meaning the zero reference had been tak...
Read more

Load Cell Calibration for Weigh Feeders

-
Image
Practical Procedure · Calibration Load Cell Calibration for Weigh Feeders — Dead-Load, Test-Weight, and Why Shunt Calibration Isn't Enough Applies to: Belt weigh feeders, weigh hoppers with load cell-based measurement  |  Equipment needed: Certified test weights or a dead-load/material reference, shunt calibration resistor (if supported), digital indicator/weigh controller access Before You Start Confirm the feeder/hopper is mechanically free — check idlers, belt tension, and load cell mounting for anything that could add friction or false load before assuming a load cell problem. Check load cell cabling and junction box connections for corrosion or loose terminations, especially in dusty or humid plant environments. Note whether the system uses a single load cell or multiple load cells summed together — multi-cell systems need each cell checked individually, not just the combined output. Method 1 — Shunt Calibration Shunt calibration simulates a known load...
Read more

Zirconia Oxygen Analyzer Reading Maximum O2

-
Image
Fault Log · Field Report Zirconia Oxygen Analyzer Reading Maximum O2 — Not the Cell, a Rusted Outlet Plant: Cement  |  System: Zirconia oxygen analyzer, sample extraction system  |  Downtime: None Symptom The zirconia oxygen analyzer started reading maximum oxygen — pegged at the top of the range rather than tracking the actual process value. A reading stuck at maximum on a zirconia cell usually means the cell is seeing something close to ambient air instead of process gas, which points straight at a sample delivery problem before you even think about the analyzer itself. Investigation Started at the sample path, from the process side inward: Probe, extractor filter, sampling line, pump — checked the full sample path from the probe through the extraction filter and sampling line, including the sample pump. Performed a leakage test across this section. All found ok. Panel-side tubing and membrane filter — checked all tubing inside the panel along...
Read more