Again and again I read at Apug quite a nonsense relating to digital readouts of scales and thermometers.
This dramatic case hopefully makes you aware of the potential errors:
These days in Germany a healing practitioner is at criminal court for killing 3 of his patients by applying by negligence an overdose of a drug.
His aim was to apply by infusion 105mg of that substance and he had to weigh the substance before.
He employed a digital scale from a laboratory scale manufacturer (Kern PCB 200 at about 250€)
of
200g max load
and
-,-- g display
and
d= 0,01 designation
-) He did not realize that with any digital readout the last digit (10mg in this case) is insecure and can be +/- 1digit
-) He did not realize that, more important, a metering device by design may incorporate an error by far greater than that last digit, especially at the verge of its range.
A expert witness showed that the scale in question at 1160mg can have an error of 340mg. Thus 30%.
I looked up at the data sheet:
linearity: 20mg
minimum load: 20g
(Linearity here means the max. absolute error over the complete load range. In this case it is lower than the error shown by the expert. But that data sheet error only applies at the allowed load range, here thus above 20g.)
I hope this makes things more clear...
A pun saying amongst german electronic technicians is "Wer mißt, mißt Mist".
Which can be translated as "Every metering results in bullshit". This is applied on electronics metering where the effect of the circuit on the metering complicates things, but doubt on what one is doing should be applied on any metering.
This dramatic case hopefully makes you aware of the potential errors:
These days in Germany a healing practitioner is at criminal court for killing 3 of his patients by applying by negligence an overdose of a drug.
His aim was to apply by infusion 105mg of that substance and he had to weigh the substance before.
He employed a digital scale from a laboratory scale manufacturer (Kern PCB 200 at about 250€)
of
200g max load
and
-,-- g display
and
d= 0,01 designation
-) He did not realize that with any digital readout the last digit (10mg in this case) is insecure and can be +/- 1digit
-) He did not realize that, more important, a metering device by design may incorporate an error by far greater than that last digit, especially at the verge of its range.
A expert witness showed that the scale in question at 1160mg can have an error of 340mg. Thus 30%.
I looked up at the data sheet:
linearity: 20mg
minimum load: 20g
(Linearity here means the max. absolute error over the complete load range. In this case it is lower than the error shown by the expert. But that data sheet error only applies at the allowed load range, here thus above 20g.)
I hope this makes things more clear...
A pun saying amongst german electronic technicians is "Wer mißt, mißt Mist".
Which can be translated as "Every metering results in bullshit". This is applied on electronics metering where the effect of the circuit on the metering complicates things, but doubt on what one is doing should be applied on any metering.
Last edited: