Errors in Measurement

And how to avoid damage as a result

Measurement errors come from several sources:

• Lack of Accuracy
• Lack of Precision
• Mis-Interpretation
• Math errors.

For example, let's say you ask me to measure something, and I say "Oh, about 9" and then later you measure it and you say, "umm... this is only 6 inches". That's a lack of accuracy. That is an inaccurate measurement.

Now, if I had said "It's 6 inches" and you measure it and find that it's actually 6.2 inches, that is an error of precision. My statement was accurate (it's about 6 inches) but not precise. I could have said that it's 9.223, and that would be precise, but not accurate. So that is the difference between accuracy and precision.

And then there is interpretation, right? Sometimes people think you are talking about one thing when you are actually talking about something else entirely. Sometimes, what you are saying is misinterpreted.

One of the most common and damaging misinterpretations in engineering is using the wrong units. For example, in the space shuttle disaster, where the o-ring blew out, that happened because one group was working in inches, and another group was working in centimeters.

So I might say "oh! Did you think my measurement of 9 was in inches? It's actually 9 cm". So that would be misinterpretation. So when making a measurement, it's important to put the units into the recording of the measurement.

And then often people just make errors in doing the math. For example, 9 cm is not 6 inches; it's about 3.5 inches. That would be a math units conversion error.

Summary: You need to be accurate, and on top of that, you must be precise, and on top of that, you must be clear so you are not misinterpreted, and on top of that, you must hope no errors are made in the math. There are many sources of errors in measurement. The common proverb is "measure twice, cut once". Usually, I measure 3 times, cut once, find that it doesn't fit, curse, throw it away, and start again.

Whenever you are doing something in engineering, whether it's building something, an electronic circuit, a structure, or whatever there are a few ways to ameliorate the effect of measurement errors.

1. Confirm. Measure two different ways. You might measure once with a tape measure, and then again with a stick. Or measure once with calipers, then mark it and hold the part up to the structure to see if the mark looks like it's in the right place. Or measure the voltage, then measure the current and calculate the voltage based on known resistance. Or something like that. Make critical measurements two different ways.

2. Cut long. If possible, cut in such a way that you can cut again. So if you cut it too short, your done. But if you cut it too long, you can still trim it. Especially in any sort of mechanical construction. When building an electronic circuit, don't drop the component right down on the board, so if it's a through hole component, don't put it all the way down, instead leave a little lead length on the component side. E.g. for an LED, leave about 1/4 inch between the LED and the PCB. What that allows you to do is, if you find you have soldered in the wrong place, or you soldering the wrong thing in, or you soldering the right thing in and then managed to burn it out, that makes it very easy to clip the leads, clear the solder holes, and still have enough lead left to move that component to a different place, or put the right component in it's place. In general, when you are installing components, try to do it in a way that allows you to move or replace that component in the future. Screws are better than nails. Clamps are better than glue.

3. Buy Spares. When you are buying parts, especially if the components aren't terribly expensive, but you must have those exact components to complete the project (e.g. there are no "close enough" components on hand) always order extras. Buy extra wood, nails, glue, resistors, etc...

4. Look Before You Leap. Don't make irrevocable decisions, or watch out for the irrevocable decisions. When building an electronic circuit, you will come to a point where you have to power it up. You must apply the power and hope it doesn't smoke. So at that point, stop, check everything over before applying power. Go back over it. Check it. Make sure it's right. Often you can power it up without the main components, the expensive components ,installed. For example, if it has a microcontroller, power it up without that uC installed, and measure the pins to verify you have power on the Vcc pin, and ground on the ground pin, and that your outputs from the uC aren't being driven by the circuit (more than they should be). If it's an output with a pull up, can I ground it without arcing and sparking? Make those checks first.

See also:

• Beginning Engineers Checklist
• Electronic Construction - Tips, Tricks, Gens, Traps, and Snares
• Sensors

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