Scale Readability vs Accuracy: Resolution Is Not Accuracy
Choosing a scale? Match readability to the job, then buy accuracy you can prove. The A&D EW-1500i (NTEP Class III, on sale $297.00) shows how multi-interval d changes with load — useful when you compare resolution to real tolerance.
Scale readability vs accuracy are not the same thing. Readability (resolution, often written as d) is the smallest increment the display can show. Accuracy is how close a reading is to the true mass — bounded by the instrument’s tolerance, calibration and uncertainty. A scale can read to 0.001 g and still be wrong by several milligrams if it is uncalibrated, off-level or used outside its class limits.
Definitions that hold up in metrology
| Term | Meaning | Typical symbol |
|---|---|---|
| Readability / resolution | Smallest difference between indicated values | d (scale division) |
| Verification scale interval | Division used for tolerance tests on legal-for-trade instruments | e (often e = d) |
| Accuracy (colloquial) | Closeness to the true value; in legal metrology, staying inside maximum permissible error | MPE / tolerance |
| Repeatability | Agreement of repeated indications under the same conditions | Often stated as σ |
| Linearity | How span error changes across the capacity | ± value in the datasheet |
Worked example: fine digits, coarse truth
A jeweler’s balance shows 10.004 g for a 10 g weight (d = 0.001 g). That does not prove 4 mg accuracy. If the balance’s Class II tolerance at that load is several milligrams, and the last calibration was months ago in a different room, the hundredths and thousandths can be noise relative to the real error. Conversely, a Class III shipping scale with d = 0.1 lb can be highly accurate for parcel work if it is NTEP, sealed and within maintenance tolerance — even though it will never show milligrams.
Multi-interval example: EW-1500i
The A&D EW-1500i is multi-interval: 0.1 g up to 300 g, 0.2 g up to 600 g, 0.5 g up to 1500 g. Readability gets coarser as load rises. That is by design under Handbook 44 multi-interval rules, not a loss of “accuracy class” by itself — the Class III certificate (CoC 04-019A3) sets the legal tolerances. Manual: EW-1500i manual explained.
How to choose readability without overbuying
- Process need: pick d about 1/5 to 1/10 of the smallest error you care about (rule of thumb, not a law).
- Weight class: calibration weights must support that d — see calibration weights.
- Environment: finer d needs better leveling, draft control and warm-up.
- Trade use: buy the NTEP class your commodity requires, not just more digits — NTEP guide.
Prove accuracy: calibrate and document
Accuracy is demonstrated, not assumed from the brochure. Use known test weights, record as-found/as-left, and recalibrate on a schedule. MNM Scales calibrates complete scales in Houston from $95 (≤2.2 kg in-house tier) with optional NIST-traceable certificates.
Related explainers
- How temperature, level and drafts affect scale accuracy
- Why do two scales show different readings?
- Scale readability vs accuracy
Frequently asked questions
What is the difference between accuracy and resolution on a scale?
Resolution (readability, d) is the smallest display step. Accuracy is closeness to the true value within the allowed tolerance after proper calibration and use.
Is a scale with more decimal places more accurate?
Not necessarily. Extra digits can display noise. Accuracy comes from design class, calibration, environment and tolerance — not from digit count alone.
What do d and e mean on a scale?
d is the scale division (display). e is the verification scale interval used for legal tolerances; on many instruments e equals d.