Unit 4 — Calibration Procedure and Calculations

TM-CAL-005 — Open Handout TM Chapters: Chapter 5, Appendix A ELOs: Execute calibration procedure; compute error and determine pass/fail Estimated time: 30 minutes

Step 1: Read the TM

Open TM-CAL-005. Read Chapter 5 and Appendix A completely.

Then come back here.

Chapter 5 — Calibration Procedure

5-1. OSCILLOSCOPE TIMEBASE

  1. Connect AC mains reference output to oscilloscope channel 1.
  2. Set timebase to 5 ms/div (50 ms full screen for 10-div).
  3. The 60 Hz waveform should show exactly 3 full cycles in 50 ms (period = 16.667 ms).
  4. Measure period with oscilloscope cursors: place cursor 1 at a positive zero crossing, cursor 2 at the next positive zero crossing.
  5. Displayed period should be 16.67 ms ±0.5%.
  6. For better accuracy, measure 10 cycles: 10 × 16.667 ms = 166.67 ms.
  7. Error% = (measured_ms − 166.67) / 166.67 × 100%.

5-2. FREQUENCY COUNTER

  1. Connect reference to frequency counter input via 50 Ω coax.
  2. Set gate time to 60 seconds for best accuracy.
  3. Expected reading: 60.000 Hz ±0.5 Hz instantaneous, 60.000 Hz ±0.02 Hz over 60-second gate.
  4. If counter reads 59.995 Hz with 60 s gate, short-term grid deviation is present; repeat measurement and average 5 readings.

Appendix A — Formulas

Period from frequency
T = 1/f = 1/60 = 0.016667 s = 16.667 ms

Time error from frequency error
terror/day = (ferror/fnominal) × 86400 s

Example: 1 Hz error at 60 Hz
time error = (1/60) × 86400 = 1440 s/day = 24 min/day fast or slow

Key Formulas Summary

  • T = 1/f = 1/60 = 0.016667 s = 16.667 ms
  • terror/day = (ferror/fnominal) × 86400 s
  • time error = (1/60) × 86400 = 1440 s/day = 24 min/day fast or slow

The Calibration Procedure

Chapter 5 specifies 11 calibration steps.

Calibration is a comparison: you apply a known reference value to the instrument under test and record what the instrument reads. The difference is the error. You then either: - Adjust the instrument until the error is within the acceptance criterion, or - Record the error as a correction factor to apply to future readings

An error criterion found in Chapter 5: 0.5%. Confirm the exact criterion in the TM.

Practice Problems

Work these before checking answers.

P4-1. The reference value is 10.000 V. Your instrument reads 10.043 V. (a) What is the error in volts? (b) What is the error as a percentage of the reference value? Show your work.

P4-2. Using the formula for % error: error% = (measured − reference) / reference × 100 Apply it to: reference = 100.0 kHz, measured = 99,985 Hz. (a) Error in Hz. (b) Error in %. (c) Error in ppm.

P4-3. The acceptance criterion in the TM is ±1%. Your measurement gives an error of +0.8%. Does it pass? State your reasoning.

Practice Problem Answers

P4-1. (a) 10.043 − 10.000 = +0.043 V (b) 0.043 / 10.000 × 100 = +0.43%

P4-2. (a) 99,985 − 100,000 = −15 Hz (b) −15 / 100,000 × 100 = −0.015% (c) −15 / 100,000 × 1,000,000 = −150 ppm

P4-3. +0.8% is within ±1%. PASS. State: "error = +0.8%; criterion = ±1%; result = PASS." Always cite the TM section for the criterion.

Self-Check Questions

SC4-1. How many steps does Chapter 5 specify for the calibration procedure?

SC4-2. What reference value(s) does Chapter 5 apply to the instrument under test?

SC4-3. State the calibration acceptance criterion from the TM. Cite the section.

SC4-4. Write the error formula from Appendix A. Include units.

SC4-5. If the instrument reads 2.3% high, is the error positive or negative? What does a positive error indicate?

Answer Key

SC4-1. Count the numbered steps in Chapter 5. (TM Ch. 5)

SC4-2. See Chapter 5 for the reference values applied. These are the known-good inputs used to check the instrument. (TM §5-1)

SC4-3. See TM Chapter 5 or Chapter 7. Copy the criterion exactly with units and section number.

SC4-4. See Appendix A. Write it exactly as shown.

SC4-5. Positive (instrument reads higher than reference). A positive error means the instrument over-reads — it reports a value higher than the true value. (TM App. A)

Checkpoint

Before proceeding, you must be able to: - State the calibration acceptance criterion without looking - Write the error formula from memory - Work a % error and ppm calculation correctly

→ Proceed to Unit 5