Tolerances and Conformity

Understanding Instrument Tolerances and Statements of conformity

In the comments section of your Teltherm Calibration Certificate there are some statements that may seem difficult to understand to the uninitiated. This article will clarify what some of these statements actually mean.

At Teltherm we are IANZ accredited to the ISO 17025 Standard. This Standard states that we must inform our customers about certain decision rules we make as to whether your instrument passes or fails.IANZ small

Most instruments we test – both digital and analogue have a stated accuracy Class issued by the Manufacturer. For an instrument to ‘Pass’ any deviation found during testing must not exceed the stated manufacturer’s accuracy.

Determining the Instrument Accuracy Class

If the instrument is a Class 1 gauge with a range of 0 / 1000 kPa. Any correction on the certificate must not exceed ±10 kPa.3100-924

To calculate the maximum allowable error an instrument is allowed use this formula;

(Class of gauge divided by 100) multiplied by the range of the gauge.

A Teltherm 3100 pressure gauge has a range of 0 / 2500 kPa and a stated accuracy Class of 1.

The Maximum Allowable Error for this gauge equals

(1/100)*2500 = 25 kPa

The Tolerance and the stated Uncertainty

Once we have determined the tolerance we then ask ourselves will my instrument still comply if I take into account its uncertainty.

This article will not discuss how we determine the uncertainty of an instrument but it’s handy to know why we issue an uncertainty for your instrument. The uncertainty tells you how ‘good’ or how much confidence you can have in your measurements when you use your device.

When we calculate uncertainty we check

• how repeatable or precise your instrument is
• its resolution or readability
• the accuracy of the reference equipment used to test your instrument
• any other contributors

Again if we look at a 0 / 2500 kPa Teltherm 3100 pressure gauge – for example it may have a reported uncertainty of 6 kPa. We know the manufacturer’s maximum allowable error is 25 kPa but if we take into account the 6 kPa uncertainty as well our tolerance now becomes

25 – 6 = 19 kPa.

For this reason, the Teltherm lab always try to tweak and improve your instrument accuracy to not only meet the manufacturer’s specification but to also comply with the manufacturer’s specification AND include an allowance for the uncertainty – by doing this you can rest easy knowing that when you take a reading your measurement will be comfortably within the expected accuracy of your instrument.

Instrument Type

The above principles apply to temperature, humidity and gas detection instruments.

Pass or Fail or ‘We’re not certain’

Once we know the manufacturers maximum allowable error plus the uncertainty we can then check with any applicable Standards and determine if your instrument complies with both the manufacturer’s specification and or any recommendations stated in relevant Industry Standards. From this information we can then determine if your instrument or device is a pass, a fail or on the odd occasion a ‘we cannot be sure’.

This unsure category can also be described as a ‘Window of Uncertainty’. If your instrument is the type that can be repaired we try to avoid this scenario but in some cases instruments cannot be adjusted and for this reason you may see this noted on your certificate.

An example

Your gauge is a Class 1 accuracy but has a rolled bezel and cannot be adjusted. Its range is 0 / 2500 kPa and therefore maximum allowable error of ±25 kPa. Its uncertainty is reported as ±3 kPa.

The reported correction at 2000 kPa on your certificate is +24 kPa.

24 kPa is within manufacturer’s specification (±25 kPa) but if we include an uncertainty of ±3 kPa this would mean any measurements taken at 2000 kPa could lie between 21 kPa and 27 kPa. 27 kPa would take your instrument outside of tolerance but 21 kPa would not – we cannot be certain.

For this example we suggest you add the corrections shown on your certificate to improve the accuracy of your measurement and we will state that we cannot be certain.

Any Questions

If you require any further clarification, please do not hesitate to contact our Laboratory. lab@teltherm.co.nz