Skogsgurra
Electrical
- Mar 31, 2003
- 11,815
A dominating business magazine (electroics) sent me a link to a book so I could brush up on measurement techniques. The book is sponsored by another company, known for modular instrumentation with a “no programming” programming language – which means that it is graphical instead of text based.
I consider myself experienced in many measurement techniques. With a lifetime of field work in process industry where I have been commissioning, calibrating and finding out and putting right old analogue system to the latest DSP based ones and ranging from medicine to steel production I think that it would be sensational if I didn’t know the basics of everyday measurements. But, humble as I am, I thought that there might be something for me in the book and set one hour aside to study it.
I didn’t like what I read. The author was very particular about measurement uncertainty – although he never used that word – and there were definitions like:
“Precision–The quality of being sharply or exactly defined. It is the resolution with which a quantity is measured.
¨¨¨
Accuracy is a number that indicates how close a measurement is to its true value. Accuracy is the ratio of the actual value to the true value expressed as a percentage. Accuracy = (Measured value/True value) x 100% If the measured value is greater than the true value, use this variation. Accuracy = (True value/Measured value) x 100%
……
Error is a number that indicates the amount by which the result of measurement differs from the true or correct value. It is a measure of inaccuracy. If you know the accuracy value you can calculate the error like this: %Error = 100 - %Accuracy Otherwise, you can calculate error like this: Error = [(True value – Measured value)/True value] x 100%
……
Parts per Million–Parts per million (ppm) is a term sometimes used in very precise measurements. It is used to express accuracy or inaccuracy”.
I don’t know about you. But to me, this is next to incomprehensible. But, I read on. There must be some explanation – I am probably too dumb, after all… So, I continued reading. Hoping for some useful information. But when I tried to decipher the DC&AC voltage measurement chapter. I gave up. What about this (yes, the author is still wrestling with accuracy, error and such things):
“Accuracy is typically ± 0.01% to ±0.1% range. Accuracy is sometimes expressed as the error. Where 100% is perfect accuracy, an actual value might be 99.005% or expressed as the amount of error, 100 - 99.005 = 0.995 or .00995%. Accuracy may also be expressed in ppm, a range of 10 to 100 ppm is typical”?
So? Accuracy is sometimes expressed as an error – and 0.995% is equal to .00995% - and if 0.01% to 0.1% is the typical range – why then giving an example where the error (or is it accuracy?) is 1% ? Not to mention the 10 to 100 ppm, which (in my obviously limited world) equals 0.001 to 0.01%
I gave up here. But leafed through a few more pages. Just to find out that the author didn’t even care to borrow a picture of a sine-wave. Instead he uses a triangle wave with rounded crests. THAT IS NOT A SINE! Also, he defines period time as the distance between the peaks of that “sine” instead of between the zero crossings. Amateurish and showing no respect for hundreds of thousands of hard-working and knowledgeable EE:s.
Before throwing the “book” in the waste bin, I couldn’t help noticing that the author doesn’t have a clue how to measure current with a clamp-on current probe. What he shows is something that seems to measure the cable diameter (yes, jaws open) and the important soft-iron area is thinned out where the jaws meet. Like a tool used to measure diameters in a lathe.
SHAME, SHAME, SHAME!
Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
I consider myself experienced in many measurement techniques. With a lifetime of field work in process industry where I have been commissioning, calibrating and finding out and putting right old analogue system to the latest DSP based ones and ranging from medicine to steel production I think that it would be sensational if I didn’t know the basics of everyday measurements. But, humble as I am, I thought that there might be something for me in the book and set one hour aside to study it.
I didn’t like what I read. The author was very particular about measurement uncertainty – although he never used that word – and there were definitions like:
“Precision–The quality of being sharply or exactly defined. It is the resolution with which a quantity is measured.
¨¨¨
Accuracy is a number that indicates how close a measurement is to its true value. Accuracy is the ratio of the actual value to the true value expressed as a percentage. Accuracy = (Measured value/True value) x 100% If the measured value is greater than the true value, use this variation. Accuracy = (True value/Measured value) x 100%
……
Error is a number that indicates the amount by which the result of measurement differs from the true or correct value. It is a measure of inaccuracy. If you know the accuracy value you can calculate the error like this: %Error = 100 - %Accuracy Otherwise, you can calculate error like this: Error = [(True value – Measured value)/True value] x 100%
……
Parts per Million–Parts per million (ppm) is a term sometimes used in very precise measurements. It is used to express accuracy or inaccuracy”.
I don’t know about you. But to me, this is next to incomprehensible. But, I read on. There must be some explanation – I am probably too dumb, after all… So, I continued reading. Hoping for some useful information. But when I tried to decipher the DC&AC voltage measurement chapter. I gave up. What about this (yes, the author is still wrestling with accuracy, error and such things):
“Accuracy is typically ± 0.01% to ±0.1% range. Accuracy is sometimes expressed as the error. Where 100% is perfect accuracy, an actual value might be 99.005% or expressed as the amount of error, 100 - 99.005 = 0.995 or .00995%. Accuracy may also be expressed in ppm, a range of 10 to 100 ppm is typical”?
So? Accuracy is sometimes expressed as an error – and 0.995% is equal to .00995% - and if 0.01% to 0.1% is the typical range – why then giving an example where the error (or is it accuracy?) is 1% ? Not to mention the 10 to 100 ppm, which (in my obviously limited world) equals 0.001 to 0.01%
I gave up here. But leafed through a few more pages. Just to find out that the author didn’t even care to borrow a picture of a sine-wave. Instead he uses a triangle wave with rounded crests. THAT IS NOT A SINE! Also, he defines period time as the distance between the peaks of that “sine” instead of between the zero crossings. Amateurish and showing no respect for hundreds of thousands of hard-working and knowledgeable EE:s.
Before throwing the “book” in the waste bin, I couldn’t help noticing that the author doesn’t have a clue how to measure current with a clamp-on current probe. What he shows is something that seems to measure the cable diameter (yes, jaws open) and the important soft-iron area is thinned out where the jaws meet. Like a tool used to measure diameters in a lathe.
SHAME, SHAME, SHAME!
Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
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