Welcome to AAC

A point about measurement...

When measuring something, the instrument has two major properties:

1. Precision
2. Accuracy

Unfortunately, when you really want to rely on an instrument you discover that the error band created by the confluence of these three parameters is often big enough to swallow what you wanted to measure.

Precision is "easy" and it is often used to disguise a lack of accuracy in lesser instruments. A display that reads out to four decimal places may seem "accurate" but that's not accuracy. It's just like a ruler with more lines on it—they could be in the wrong spots so it makes no difference how many there are except in cases where you only care about relative results.

But if you are trying to see the outcome of theory in practice, you want absolute results. Accuracy is how closely to the standards an instrument measures. A meter stick needs to be a meter long, and have marks at millimeter spans. A stick that isn't a meter can still make relative measurements but can't tell you how the measured object relates to the standard.

In electronics, measurements are plagued by the fact that everything you use, including the instruments are, themselves, resistors, capacitors, and inductors. The leads of a multimeter have a non-trivial resistance and so if you are concerned with accurate resistance measurements you need to use a four wire method which cancels this out (for example). Every refinement like this improves the outcomes but also increases the cost.

The components you will measure each have a tolerance and will only match their nominal values within the percentage indicated by the manufacturer—if they are from a reputable source. If not, all bets are off. If you don't know the actual values, you have to take the stacked tolerances and add them up to work out what the error band for the practical circuit looks like.

But, you can do something to make things better: if you take your meter and measure each component, not relying on its markings, then do your calculations based on that measurement, then you are relying mostly on the third major property of an instrument: repeatability. That is, the ability to return the same result each time something of a certain value is measured.

You should still be closer, though. But, keep in mind the influence of temperature on both the DUT (Device Under Test) and the instrument testing. It can be substantial.

Sorry for the screed, good luck.

 

Welcome to AAC

A point about measurement...

When measuring something, the instrument has two major properties:

1. Precision
2. Accuracy

Unfortunately, when you really want to rely on an instrument you discover that the error band created by the confluence of these three parameters is often big enough to swallow what you wanted to measure.

Precision is "easy" and it is often used to disguise a lack of accuracy in lesser instruments. A display that reads out to four decimal places may seem "accurate" but that's not accuracy. It's just like a ruler with more lines on it—they could be in the wrong spots so it makes no difference how many there are except in cases where you only care about relative results.

But if you are trying to see the outcome of theory in practice, you want absolute results. Accuracy is how closely to the standards an instrument measures. A meter stick needs to be a meter long, and have marks at millimeter spans. A stick that isn't a meter can still make relative measurements but can't tell you how the measured object relates to the standard.

In electronics, measurements are plagued by the fact that everything you use, including the instruments are, themselves, resistors, capacitors, and inductors. The leads of a multimeter have a non-trivial resistance and so if you are concerned with accurate resistance measurements you need to use a four wire method which cancels this out (for example). Every refinement like this improves the outcomes but also increases the cost.

The components you will measure each have a tolerance and will only match their nominal values within the percentage indicated by the manufacturer—if they are from a reputable source. If not, all bets are off. If you don't know the actual values, you have to take the stacked tolerances and add them up to work out what the error band for the practical circuit looks like.

But, you can do something to make things better: if you take your meter and measure each component, not relying on its markings, then do your calculations based on that measurement, then you are relying mostly on the third major property of an instrument: repeatability. That is, the ability to return the same result each time something of a certain value is measured.

You should still be closer, though. But, keep in mind the influence of temperature on both the DUT (Device Under Test) and the instrument testing. It can be substantial.

Sorry for the screed, good luck.

Great Input- not sure what you mean about sorry about the screed though?
I feel nobody needs to be sorry about any comments, everyone has been pleasant and given lots of great insight and I appreciate all the patience and feedback.
Kinda reminds me of when I spent 6 hours letting a up and coming driver back up my super B trains. I knew I could do it well in seconds , but he had to figure it out on his own, I just helped him with the principles

 

this helps....alot. Lots to ponder here.

without diving too deep into it and some quick math, would this put my (res/reactance) at around 4081ish ohms giving me my 27 ma im reading on my fluke. Fun stuff

At its root it's simple trig when using polar/rectangular notation for complex numbers.

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-2/polar-rectangular-notation/


https://www.allaboutcircuits.com/worksheets/ac-phase/

 

Here is your first DC voltage measurement experiment.

Using any available battery V1, even a regular 1.5 V cell will do, assuming that you also have a proper battery holder, connect two resistors, R1 and R2, 1MΩ each as shown in the circuit diagram.

Using your DMM, take two readings, one between nodes A and C, and a second between nodes B and C.
Do it again with your other DMM. The expected reading between B and C is half that of the reading between A and C.
Verify this using Ohm's Law calculations.

Now, replace R1 and R2 with 10MΩ resistances. Repeat the experiment. Show your results and comment on the results.
What can you learn from this experiment?

 

.... not sure what you mean about sorry about the screed though?

An archaic use of screed: "a long speech or piece of writing, typically one regarded as tedious."

 

An archaic use of screed: "a long speech or piece of writing, typically one regarded as tedious."

And a modern use of the term is a a screed, a bar for leveling wet concrete.

 

And a modern use of the term is a a screed, a bar for leveling wet concrete.

View attachment 345727

I kinda thought it was the back of a paver and you knew something about when i was hauling one 20 feet wide down highway 16 ,25 years ago.....wondered how you would have known about that

 

Beware of the word"Precision" because what you really need is accuracy and resolution.
"ACCURACY" is the inverse of "uncertainty", while resolution is usually the number of digits in the description of a quantity.
Precision is a much broader term often implying greater accuracy and higher resolution.

 

Precision is about repeatability.

Accuracy is about correctness, on average.

Resolution beyond what is justified by the prior two is meaningless.

Most of the time, I would greatly prefer precision over accuracy, because I can usually calibrate to yield a commensurate degree of accuracy. But if I have accuracy without sufficient precision, then it is usually a much harder task, often involving taking many measurements and averaging them.

 

Then use the correct term! " Precision is about repeatability." So use the word "repeatability", instead of a much broader term!
In language, using the more narrowly defined words is a way to more closely provide the intended meaning, without much effort or cost.
Avoiding any ambiguous or inadequately defined terms was always a big part of writing sales letters, that portion of a technical proposal that described just what was to be provided in a system that was to be purchased by some client. That meant that every word needed to be the proper choice and have only one possible meaning.
Of course, this is in very direct conflict with the attitude "that words do not matter" that some folks suffer from.
So certainly it is fine to say "that one would make a precise measurement using precision equipment", but then describe the results as being accurate and repeatable, with suitable resolution.

 

Then should people use "correct" instead of "accurate"?

How about we use a word, one of whose broadly accepted definitions is, "the ability of a measurement to be repeated consistently"? (American Heritage Dictionary of the English Language)

Or how about one whose definition, in science, is, "Precision is also the level of agreement of a particular measurement with itself when it is repeated"? (Cambridge Academic Content Dictionary)

Or how about, "in Chemistry and Physics, the extent to which a given set of measurements of the same sample agree with their mean"?

Oh, wait, that word in each case is "precision".

https://sciencenotes.org/what-is-the-difference-between-accuracy-and-precision/

 

In all my years of science and engineering education, the definitions of the words, accuracy, resolution, and precision, have not changed. Why do we need to create a new vocabulary?