# 2 significant figures

Discussion in 'General Electronics Chat' started by KL7AJ, Sep 4, 2016.

1. ### KL7AJ Thread Starter Senior Member

Nov 4, 2008
2,013
273
When I was in college and calculators were first beginning to replace slide rules, one of my math professors said, "A calculator will give you the wrong answer to 15 decimal places."

After a few years of trying to teach electronics students back-of-the-envelope calculations of electronics problems, to little avail....I began confiscating calculators on the first day of class...only allowing them AFTER they'd demonstrated they could perform simple calculations.

In addition, I emphasize the point that unless you're building instrumentation, resistor, capacitor, and inductor values are only to 2 significant figures....even the 5 percent ones. It makes NO sense to work out calculations to 5 decimal places, when no components are accurate to more than 2.

When a student asks...."Should we use 3.14159 or 3.1416 for PI?" I answer....How about 3??!! That's as close as you'll need to pass ANY FCC exam....as well as create a functioning circuit.

We need to start teaching students again some general "feel" type of problems before we try plucking eyebrows on a termite. An electronics student should be able to look at a coil and tell if it's 1 henry or 1 microhenry.

Stay tuned

Bernard likes this.
2. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,546
834
Agreed, and same notion applies to areas other than electronics. If you don't understand the problem, significant figures don't matter. The classic rope around the Earth problem is just one example.

John

3. ### joeyd999 AAC Fanatic!

Jun 6, 2011
2,605
2,467
My work usually involves 5 or more decimal places. When you get into precision digital instrumentation, π=3 doesn't cut it.

4. ### #12 Expert

Nov 30, 2010
16,023
6,537
The only 2 significant figures in my life were my wives.

But seriously, I do 1% analog designs. 3 digits are almost enough.

5. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
1,084
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In some cases, 3e18 and 6e18 is essentially the same number. Zero doesn't exist sometimes.

6. ### wayneh Expert

Sep 9, 2010
11,892
2,845
Since back of the napkin calculations may now be done in a spreadsheet, I found value in keeping precision until the end. For instance it gives a way to check if two alternative ways to calculate the same thing actually do give the identical result. No worries about potential rounding errors causing an apparent discrepancy.

I also used precise values for constants because I could look at the work years later and know if it was mine. A 3.14 was a sure sign it wasn't my work.