Viscometer accuracy: what “percent of full scale” means

An accuracy figure needs its basis. “±2% of full scale” and “±2% of reading” are different statements. Comparing the percentages without that distinction can lead to an unsuitable instrument or an unrealistic acceptance limit.

Brookfield's calibration guidance uses the full-scale range of the spindle/speed setup when calculating its stated instrument contribution, and treats the reference-fluid contribution separately. Its numerical specifications apply to its own equipment; the example below is an independent illustration of the arithmetic. [1]

Calculate an illustrative full-scale allowance

Assume a hypothetical setup with a 10,000 mPa·s full scale and a specification of ±2% of full scale:

• Instrument allowance: 10,000 × 0.02 = ±200 mPa·s.
• At a reading of 8,000 mPa·s, 200 ÷ 8,000 × 100 = 2.5% of that reading.
• At a reading of 1,000 mPa·s, 200 ÷ 1,000 × 100 = 20% of that reading.

The ±200 mPa·s amount stays the same in this example because the assumed full scale stays the same. Its size relative to the reading changes. These values illustrate the definition; they do not identify a tested spindle/speed configuration or predict the complete uncertainty of a real measurement.

Read the SNB specification accurately

The current SNB-1, SNB-2 and SNB-3 listings state accuracy of ±2% of full scale and repeatability of ±1% of full scale. They also retain the condition to confirm the applicable procedure and configuration.

For an inquiry, provide your expected working values and any allowable measurement error. Ask which spindle/speed combination is proposed and what full-scale value applies there. Do not insert the model's largest catalog viscosity into a worksheet unless it is actually the full scale of the selected setup.

Distinguish a check from the complete uncertainty

A reference-fluid check is useful only when its conditions and acceptance rule are specified. Read the fluid's certificate and the instrument procedure together; the instrument term is not the only consideration in interpreting the check. [1]

Our suggested record includes the fluid identity and certificate, certified value and temperature, instrument ID, configuration, observed conditions and result. Keep the arithmetic used for the acceptance decision with that record. If your organization requires a formal uncertainty evaluation or accredited calibration, state that requirement separately when requesting documentation.

Ask for a configuration, not just a percentage

Before choosing between models, ask these questions:

• Is the quoted percentage based on full scale, reading or another definition?
• What is the full scale of the proposed spindle/speed combination?
• Which operating conditions and accessories apply to that statement?
• What procedure and documentation will be used to verify the setup?

Answering these questions gives your purchasing and laboratory teams the same basis for comparison. It also prevents a small headline percentage from being mistaken for a guarantee at every point across a broad catalog range.