What is the difference between '% reading' and '% full scale reading'? (FAQ - Pressure)
The measurement uncertainties achievable with pressure gauges, particularly traditional circular 'dial' gauges, are often expressed in one of two ways - as a percentage of reading or as a percentage of full-scale reading and the differences can be very significant, particularly when working at pressures much lower than an instrument's full-scale.
The dominant measurement uncertainties in an instrument are often constant - a specified number of pascals for example that does not change as the pressure changes. Expressing such a 'fixed' pressure uncertainty as a proportion of the pressure value (which is what many users want to know), however, creates some very large numbers; indeed at zero pressure the uncertainty expressed as percentage of reading is literally infinite. Manufacturers' specification sheets sometimes show measurement uncertainties that are expressed as a proportion of full-scale pressure, rather than as a fixed term, and this can misleadingly imply better performance.
The table below shows the uncertainties in the measurement of pressure, first given as 1% of reading and second expressed as 1% of full-scale reading to illustrate the difference; in the region marked with asterisks, the device performing to 1% of full-scale reading is unlikely to make a meaningful measurement.
| Percent of reading | Percent of full-scale reading | |||
| Instrument reading in pressure units (eg pascal) |
Uncertainty in pressure units (eg pascal) |
Equivalent ‘percentage of full-scale reading’ |
Uncertainty in pressure units (eg pascal) |
Equivalent ‘percentage of reading’ |
| 1000 | 10 | 1% | 10 | 1% |
| 500 | 5 | 0.5% | 10 | 2% |
| 100 | 1 | 0.1% | 10 | 10% |
| 50 | 0.5 | 0.05% | 10 | 20% |
| 10 | 0.1 | 0.01% | 10 | 100%* |
| 5 | 0.05 | 0.005% | 10 | 200%* |
| 0 | 0.00 | - | 10 | infinite* |
Example comparison of two common methods of expressing uncertainty showing different meanings of '1% uncertainty'
