Can pressure instruments be calibrated without shutting down?

Usually yes, if the instrument can be isolated at its manifold and vented safely. That is a routine operation on a properly fitted point. Where there is no isolation valve, the calibration has to wait for a shutdown — and fitting a manifold is worth doing at that point so future calibrations are simple.

Do you test safety relief valves?

No. We calibrate pressure measurement instruments — gauges, transmitters and switches. Relief-valve set-pressure testing and certification is a separate discipline with its own equipment and approvals, and we will say so rather than stretch our scope.

Why do you test both rising and falling pressure?

Because mechanical instruments do not retrace their own curve. Hysteresis and stiction mean a gauge can read correctly on the way up and be out on the way down. A single upward pass will not show it, and the interlock might trip on a falling pressure.

My DP transmitter is calibrated but the level is still wrong. Why?

Almost always the impulse lines or the assumed density, not the transmitter. A blocked leg, trapped gas or a wet leg that has partly evaporated all shift the reading while the instrument itself is perfectly accurate. That is why we assess the installation as part of the calibration instead of only certifying the box.

What accuracy can you achieve on site?

It depends on the reference and the range, and we state the uncertainty on every certificate rather than quoting a single headline figure. As a general rule the reference should be several times more accurate than the tolerance you need to prove, and we select it on that basis.

Is the calibration traceable?

Yes. Our reference gauges and comparators hold their own accredited calibration certificates, and each report names the reference and its traceability chain. Our procedures are built to ISO/IEC 17025 requirements, and our ESYD accreditation is in progress.

Pressure measurement

Pressure & Differential Pressure Calibration

Pressure instruments carry more weight than their price suggests: they protect equipment, they infer level and flow, and they trigger interlocks. They also fail predictably — zero shift after over-pressure, blocked impulse lines, wet legs that have dried out, diaphragm damage. SEMAC calibrates pressure instrumentation on site against a traceable reference and documents what the loop actually does.

What we calibrate

Pressure gauges and indicators

Bourdon and diaphragm gauges, including the effect of over-pressure events and pulsation on zero and span.

Pressure transmitters

Gauge, absolute and sealed types — zero, span, linearity and the 4–20 mA / HART output against applied reference pressure.

Differential pressure transmitters

The most error-prone family in the plant, because most DP faults live in the impulse lines rather than the instrument.

DP level and density applications

Level inferred from DP depends on an assumed density and a healthy wet leg. We verify the calculation as well as the instrument.

Pressure switches

The actual switching and reset points under rising and falling pressure, which is what an interlock depends on.

Vacuum and low-pressure instruments

Negative-gauge and low-range applications, where leaks and reference selection dominate the error budget.

How we do it

Comparison against a reference gaugeA hand pump or comparator applies pressure to the instrument and a traceable reference simultaneously, at several rising and falling points.
Rising and falling sequenceTesting both directions exposes hysteresis and stiction — the errors a single upward pass hides completely.
Loop and output verificationThe applied pressure is compared against the transmitter output and the value the control system records, so a scaling error cannot hide.
Impulse line and manifold assessmentBlockage, trapped air, dried wet legs and leaking manifolds are checked as part of the work, because they cause errors no recalibration can fix.
Switch set-point testingActual trip and reset pressures are measured and documented against the intended set point.

What you receive

Calibration certificate per instrument or loop
As-found and as-left deviation at each pressure point
Rising and falling points, so hysteresis is visible
Zero and span error reported separately
Reference instrument used, with its traceability chain
Stated measurement uncertainty per point
Impulse line, manifold and installation findings
For switches: measured trip and reset points versus set point

When you need it

Equipment and personnel protection

Any pressure instrument that feeds an alarm or interlock needs documented proof that it still reads correctly.

After an over-pressure event

A single excursion beyond range can permanently shift zero. The instrument keeps reporting confidently while being wrong.

Level or flow inferred from DP

When level and flow are derived from pressure, a DP error propagates straight into inventory and balance figures.

Audit and compliance

Quality-system and safety audits expect a calibration record for every critical pressure point.

Unexplained control instability

Hunting loops and drifting setpoints frequently trace back to a sticking or hysteretic pressure instrument.

Calibration intervals

Twelve months is a reasonable default for a general process pressure point. Instruments feeding safety functions, custody transfer or inventory calculations are usually checked every six months, and any instrument that has seen an over-pressure event should be checked immediately regardless of when it is due. As always, your own as-found history is the best evidence for lengthening or shortening the interval.

Calibration procedures follow ISO/IEC 17025 methodology; ESYD accreditation is in progress.

Industries served

Where we operate

Common questions

Tell us about the measurement point you need checked

Describe the instrument and your plant, and we will tell you what can be verified in-situ, what needs a workshop calibration, and what it will take.

Keep your instrumentation performing at its best

Speak with SEMAC's service team to discuss your maintenance needs and plan your verification & calibration service.

Request calibration support Explore services