Unit 1 — Theory of Operation
TM-CAL-009 — Open Handout TM Chapter: Chapter 2 ELOs: Understand how the reference standard generates or maintains its reference value; identify key specifications Estimated time: 20 minutes
Step 1: Read the TM
Open TM-CAL-009. Read Chapter 2 — Theory of Operation completely.
Then come back here.
Chapter 2 Content
2-1. BRIDGE BALANCE CONDITION
The Wheatstone bridge consists of four resistors in a diamond configuration with a battery across one diagonal and a null detector across the other.
Bridge balance (null condition)
Rx = Rs × Rb / Ra
At null: Rx is the unknown, Rs is the standard (decade box), Ra and Rb are the ratio arms (precision fixed resistors). No current flows through the detector at balance; its internal resistance does not affect accuracy.
| Ratio Rb/Ra | Rx Range | Notes |
|---|---|---|
| 1:1 (equal arms) | 1 Ω–10k Ω | Highest accuracy |
| 10:1 | 10–100k Ω | Extends range |
| 100:1 | 100 Ω–1M Ω | Higher ratio, lower sensitivity |
Why Theory Matters for Calibration
You cannot calibrate what you do not understand. Theory tells you: - What the instrument or standard is supposed to do — so you recognize when it is not doing it - What the sources of error are — so you know which ones your calibration procedure addresses - What the limits of the calibration are — so you know when to stop and call it good
If a calibration measurement produces a surprising result, theory is where you look first.
Self-Check Questions
SC1-1. In one sentence, state the operating principle of the WHEATSTONE BRIDGE NULL METHOD as described in Chapter 2.
SC1-2. What does Chapter 2 identify as the primary source(s) of measurement error or uncertainty?
SC1-3. What key specification(s) (accuracy, range, resolution) does the TM state for this standard?
SC1-4. What safety precaution does §1-3 specify? State it exactly.
SC1-5. What references does §1-2 cite? List at least two.
Answer Key
SC1-1. See TM §2-1. Compare your sentence to the first substantive paragraph of Chapter 2.
SC1-2. See Chapter 2. Look for language about error sources, uncertainty, drift, or limiting factors.
SC1-3. See Chapter 2. Look for numbers with units: %, ppm, Hz, Ω, dB.
SC1-4. See TM §1-3. Copy the safety text exactly.
SC1-5. See TM §1-2. The references list is there.
Checkpoint
Before proceeding, state without looking: - The operating principle of the WHEATSTONE BRIDGE NULL METHOD - The primary error source(s) - At least one key specification with its value
→ Proceed to Unit 2