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newSchoolWorks.com › VoTechWorks › Building & Construction › Nuclear Power Plant Technician / Operator Prep › Instrumentation & Control for Nuclear Power Plant Operations › Lesson 9
BLD-NUCL84-04 · Lesson 9 · 30 clock hours · lab

Instrument Calibration: Procedure, Data Recording, and As-Found/As-Left Documentation

Objective. Students will perform a five-point calibration on a pressure transmitter following a written procedure, record as-found and as-left data, and determine whether the instrument passes its technical specification tolerance.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why Calibration Matters in a Nuclear Plant

Every safety system in a nuclear power plant depends on instruments that tell operators the true state of the reactor: coolant pressure, flow, temperature, and level. If a pressure transmitter reads 2% high, a relief valve may not open when it should, or an automatic safety signal may fire at the wrong setpoint. Calibration is the disciplined process of comparing an instrument's output to a known reference standard and correcting any deviation. Nuclear plants require documented calibration at scheduled intervals specified in their Technical Specifications (Tech Specs). A calibration performed without a complete, signed data sheet is not acceptable—it did not happen, from a regulatory standpoint.

The 4–20 mA Pressure Transmitter

Most nuclear plant process instruments transmit a 4–20 mA current signal. At 0% of calibrated span the transmitter outputs 4 mA; at 100% of span it outputs 20 mA. The 16 mA range maps linearly to the full process range (for example, 0–1000 psig). This two-wire loop signal is robust against electrical noise—important in an environment with pumps, motors, and radio-frequency sources. Your calibration bench recreates this loop: the precision pressure source drives the transmitter's sensing element, and a digital multimeter (DMM) or Fluke calibrator reads loop current in series.

Setting Up the Calibration Bench

Before applying any pressure:

1. Verify the procedure. Read the entire calibration procedure sheet before touching equipment. Confirm you have the correct procedure revision for the transmitter tag number.

2. Inspect equipment. Check all tubing, fittings, and electrical connections. Ensure the pressure source is within its rated range and that the reference standard (NIST-traceable) has a current calibration sticker.

3. Connect safely. Pressure connections must be tight and rated above the maximum test pressure. Do not exceed the transmitter's maximum allowable working pressure. If a deadweight tester is used, ensure the piston and weights are clean and the oil reservoir is filled.

4. Electrical safety. Verify loop voltage is within the transmitter's specification (commonly 24 VDC). Keep hands clear of energized terminals. Lockout/tagout (LOTO) does not apply during active calibration—the instrument must be energized—but confirm with your supervisor that the loop is in bypass or that any downstream automatic functions are defeated so a spurious signal does not trigger a plant response.

5. PPE. Safety glasses are required. If the calibration fluid is hydraulic oil or water at elevated pressure, wear appropriate gloves.

The Five-Point Calibration Procedure

A five-point calibration checks the transmitter at 0%, 25%, 50%, 75%, and 100% of calibrated span. This catches zero error, span error, and linearity error across the full range.

As-Found Check: Apply each pressure setpoint using the precision source while watching the reference standard. Record the transmitter's actual mA output on the calibration data sheet before making any adjustments. This is the as-found data—the honest snapshot of how the instrument arrived for calibration. Compare each reading to the ideal value and calculate percent-of-span error.

Adjustment: If any as-found error exceeds the acceptance tolerance stated in the procedure, adjust the transmitter's zero (offset) and span (gain) potentiometers or digital trim functions. Work zero first, then span, because span adjustment can shift zero.

As-Left Check: After adjustment, repeat the full five-point sweep. Record these readings as the as-left data. All five points must fall within the stated tolerance. The as-left data certifies the condition in which you returned the instrument to service.

Percent-of-Span Error Calculation

Error (% span) = [(Actual output − Ideal output) / (Span in mA)] × 100

For a 4–20 mA transmitter the span in mA is 16. If the ideal output at 50% is 12.00 mA and the transmitter reads 12.24 mA:

Error = [(12.24 − 12.00) / 16] × 100 = +1.5% of span

If the Tech Spec tolerance is ±1.0% of span, this as-found reading is out of tolerance and adjustment is required.

Documentation Requirements

The calibration data sheet is a quality record. In nuclear plants, quality records are controlled under the plant's 10 CFR 50 Appendix B quality assurance program. Every field must be filled in—no blanks, no white-out. Errors are crossed out with a single line, initialed, and corrected. The technician signs and dates the sheet. If a second verifier is required by the procedure, that person signs as well. An incomplete or unsigned data sheet means the calibration is not complete; the instrument cannot be returned to service.

Pass/Fail Decision

After the as-left check:

Key terms9

As-Found DataThe recorded output of an instrument at each calibration point before any adjustments are made; documents the instrument's condition upon receipt for calibration.
As-Left DataThe recorded output of an instrument at each calibration point after all adjustments are complete; certifies the condition in which the instrument is returned to service.
Calibrated SpanThe range of the process variable that the transmitter is configured to measure, from its lower range value (LRV) to its upper range value (URV).
Percent of Span ErrorThe difference between the actual output and the ideal output, divided by the total output span, expressed as a percentage; the standard metric for evaluating transmitter accuracy.
NIST-Traceable Reference StandardA measurement device whose accuracy is traceable through an unbroken chain of comparisons to national standards maintained by the National Institute of Standards and Technology.
Technical Specification (Tech Spec) ToleranceThe maximum allowable error, stated in the plant's NRC-approved Technical Specifications, that an instrument may exhibit and still be considered operable.
4–20 mA Current LoopA standard process instrumentation signal range in which 4 mA represents 0% of span and 20 mA represents 100% of span, providing a linear 16 mA working range.
Zero and Span AdjustmentThe two fundamental calibration corrections: zero shifts the entire output curve up or down (offset), while span stretches or compresses the slope (gain) of the output curve.
Quality RecordA completed document that provides objective evidence that a required activity was performed correctly; in nuclear plants, quality records are controlled under 10 CFR 50 Appendix B.

Worked examples4

1. A pressure transmitter is calibrated for 0–500 psig, outputting 4–20 mA. At the 25% calibration point (125 psig), the as-found output reads 8.64 mA. The ideal output at 25% is 8.00 mA. Calculate the as-found percent-of-span error.
  1. Identify the ideal output at 25% of span: 4 mA + (0.25 × 16 mA) = 4 + 4 = 8.00 mA.
  2. Identify the actual (as-found) output: 8.64 mA.
  3. Calculate the error in mA: 8.64 − 8.00 = +0.64 mA.
  4. Divide by the total span in mA: 0.64 / 16 = 0.04.
  5. Convert to percent: 0.04 × 100 = +4.0% of span.
Answer: As-found error at the 25% point is +4.0% of span. If the tolerance is ±1.0% of span, this point fails and adjustment is required.
2. After adjusting zero and span, the technician repeats the five-point check. The as-left outputs (mA) are: 0% → 4.08, 25% → 8.05, 50% → 12.03, 75% → 16.02, 100% → 20.07. The Tech Spec tolerance is ±0.5% of span (±0.08 mA). Determine whether each point passes.
  1. Calculate ideal outputs: 0% = 4.00, 25% = 8.00, 50% = 12.00, 75% = 16.00, 100% = 20.00 mA.
  2. Calculate errors in mA: 0% → +0.08, 25% → +0.05, 50% → +0.03, 75% → +0.02, 100% → +0.07 mA.
  3. Convert to % of span (divide by 16, multiply by 100): 0% → +0.50%, 25% → +0.31%, 50% → +0.19%, 75% → +0.13%, 100% → +0.44%.
  4. Compare each to ±0.50% of span tolerance: all five values are ≤ 0.50% in magnitude.
Answer: All five as-left points are within ±0.50% of span. The calibration passes. The technician may sign the data sheet and return the instrument to service.
3. A technician reads the as-left output at the 75% calibration point as 15.84 mA. The span is 0–800 psig (4–20 mA). The stated tolerance is ±1.0% of span. Does this point pass or fail, and what is the error in engineering units (psig)?
  1. Ideal output at 75%: 4 + (0.75 × 16) = 4 + 12 = 16.00 mA.
  2. Error in mA: 15.84 − 16.00 = −0.16 mA.
  3. Error in % of span: (−0.16 / 16) × 100 = −1.0% of span.
  4. Compare to tolerance: −1.0% equals the limit of ±1.0%; this is exactly at the boundary. Most nuclear procedures treat 'within tolerance' as strictly less than or equal to the limit, so this point passes (borderline)—confirm with the specific site procedure.
  5. Convert error to psig: 1.0% of 800 psig = 0.01 × 800 = 8.0 psig low.
Answer: The error is −1.0% of span (−8.0 psig). This is at the exact tolerance boundary. Whether it passes depends on the site procedure's wording (≤ vs. <). Document as borderline and consult the supervisor before signing off.
4. A transmitter has as-found outputs recorded at all five points. The largest deviation is at the 50% point: actual = 11.60 mA, ideal = 12.00 mA. Express this error in percent of span and identify whether it represents a zero error, a span error, or a linearity error.
  1. Error in mA at 50%: 11.60 − 12.00 = −0.40 mA.
  2. Error in % of span: (−0.40 / 16) × 100 = −2.5% of span.
  3. Check if a pure zero shift would cause this: a zero error shifts all points by the same mA amount. If the 0% point reads correctly at 4.00 mA but the 50% point is low by 0.40 mA, the error grows with the applied signal—suggesting a span (gain) error or a combined error.
  4. A technician would examine all five points: if errors increase proportionally with applied pressure, the problem is span. If all points are shifted equally, it is zero. If errors are non-uniform, a linearity issue exists. In this case, further data from the other four points is needed for a complete diagnosis.
Answer: The 50% point error is −2.5% of span. Based on the 0% point being correct, this is most consistent with a span error (output gain too low). Zero adjustment alone will not correct it; span must be increased.

The math in this lesson2

Learners compute as-found calibration error as a percent of the 16 mA span.
7.RP.3F-LE.1
Brush up in the math halls at newSchoolWorks.com:
In class: Set up a calibration bench: connect a precision pressure source, a reference standard, and a multimeter to a training-grade pressure transmitter; verify all connections before applying pressure. · Apply pressure at 0%, 25%, 50%, 75%, and 100% of calibrated span; record as-found output at each point, calculate error in percent of span, and compare to the acceptance tolerance stated in the procedure. · Adjust zero and span as needed; repeat the five-point check; record as-left data on the official calibration data sheet and sign the completed document.
Materials: Training-grade 4–20 mA pressure transmitter, precision pressure calibrator or deadweight tester, NIST-traceable reference standard, digital multimeter, calibration data sheet forms, procedure sheet, Fluke or equivalent calibrator
Check: Calibration data sheet reviewed by instructor: as-left errors must be within the stated tolerance at all five calibration points; data sheet must be complete, legible, and signed; any out-of-tolerance as-left result requires rework and re-check.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NUCP Uniform Curriculum — Instrumentation and Control
1. A technician records the output of a pressure transmitter before making any adjustments. This data is called:
NRC Non-Licensed Operator Prep — Instrumentation
2. For a 4–20 mA transmitter calibrated for 0–600 psig, what is the ideal output at the 75% calibration point?
NUCP Uniform Curriculum — Instrumentation and Control
3. An as-left output at the 50% point is 12.20 mA on a 4–20 mA transmitter. The acceptance tolerance is ±1.0% of span. Does this point pass?
NRC Non-Licensed Operator Prep — Instrumentation
4. When performing a five-point calibration, which adjustment should be made first?
NRC Reactor Operator (RO) License — Plant Safety Systems
5. Why must automatic safety functions be defeated or placed in bypass before calibrating a process transmitter while it is in service?
NUCP Uniform Curriculum — Quality Assurance
6. A calibration data sheet has an entry with white-out correction fluid covering an error. Under nuclear quality assurance requirements, this is:
NRRPT Radiation Protection Tech / NUCP — Measurement and Calibration
7. Which of the following best describes the purpose of using a NIST-traceable reference standard during calibration?
NRC Non-Licensed Operator Prep — Quality Assurance and Documentation
8. An instrument's as-left data shows all five calibration points within the Technical Specification tolerance, but the technician did not sign the completed data sheet. What is the correct next step?
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