Unit 1 — Theory of Operation
TM-CAL-008 — 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-008. Read Chapter 2 — Theory of Operation completely.
Then come back here.
Chapter 2 Content
2-1. RESISTOR TYPES
For precision standards, use bulk metal foil (BMF) or metal film resistors. Wire-wound resistors are excellent for DC but introduce inductance at AC. Carbon film and carbon composition types are inadequate for calibration use due to high TC and poor long-term stability.
| Type | Tolerance | TC (ppm/°C) | Stability | Notes |
|---|---|---|---|---|
| Bulk metal foil (BMF) | 0.01% | 0.2–2 | Excellent | Vishay Z201, best choice |
| Metal film | 0.1% | 10–25 | Good | Yageo MFR, Multicomp |
| Wirewound | 0.1% | 5–10 | Excellent DC | Inductive at AC |
| Thick film (SMD) | 1% | 100–200 | Fair | Not suitable |
| Carbon film | 5% | 200–500 | Poor | Do not use |
2-2. 4-WIRE KELVIN CONNECTIONS
For resistances below 100 Ω, contact resistance in connectors and switch contacts introduces significant error. Use 4-wire (Kelvin) connections: separate current-force and voltage-sense terminals for each resistor position. See TM-CAL-010 for full Kelvin procedure.
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 PRECISION RESISTOR NETWORK (DECADE BOX) 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 PRECISION RESISTOR NETWORK (DECADE BOX) - The primary error source(s) - At least one key specification with its value
→ Proceed to Unit 2