April 11th, 2016 Taipei
What can be seen on our OIML Approval Certificates
Some customers asked us: -
Why Fidelity Measurement indicator OIML approvals are all 10,000e, while most of the others’ are lower?
Why we always choose the most difficult approval target for class III approval (i.e. 10,000e)?
Well, we look at this issue the other way round… the design/components which we apply/use for our indicators are qualified for 10,000e approval. In such a case and if we applied for any lower approval accuracy, that means that we waste our and customers’ money.
On the other hand, there is one important point about our approvals was not asked by these customers, it is the very low span gain per e… i.e. 0.75μV/e.
Span gain per e (e= verification interval) represents the minimum amount of signal needed for each e. It tells also how well and sensitivity a weighing instrument can detect and react to a tiny amount of weight change.
If span gain per e requirement is high, then sensitivity, discrimination and sometimes repeatability could then be problem. That is why some weighing scales do not response well to small weight changes.
Weighing instruments which require high span gain per e may not be suitable for: -
High dead load and/or low load cell utilization applications (e.g. concrete weighbridge, silo and tank), and
High precision weighing application (e.g. precision balance) because load cells for this kind of application usually have low signal output… say 0.9 ~ 1.0mV/V.
Thus, it is usually true that the lower the span gain per e, the better the weighing instrument.
But low span gain per e may increase difficulty in handling EMS (electromagnetic susceptibility). It is because when span gain per e is low, even an extremely small interference from outside may cause excessive errors during approval tests.
So engineers must pay very special attention to PCB layout, how to divert circuits, components selection and combine all issues together properly to make sure the EMC performance of the instrument is good no matter the interference will come from front, left, right or rear side.
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