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Comment by walrus01 | Hacker News Reader

It's worth mentioning that if you need something basic for home DIY needs and things without very demanding precision, a not half bad digital caliper from China is under ten dollars on Amazon, accurate to 0.1 mm.

Digital Caliper, Adoric 0-6" Calipers Measuring Tool - Electronic Micrometer Caliper with Large LCD Screen, Auto-Off Feature, Inch and Millimeter Conversion https://a.co/d/1YBFS00

Of course you can go up to $50-60 for some more accurate all metal ones before taking the jump to serious $250+ precision instruments.

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No, it's _precise_ down to 0.1mm. There's no way I'm trusting it's _accurate_ down to 0.1mm.

Just the mere thermal expansion coefficient of abs, 100 * 10^-6 m/(m C), is enough to tell you that across a very conservative range of 50 degrees the full scale measurement of 10 cm will expand by L0 * alpha * DeltaT = 0.1m * 100 * 10^(-6) * (50 C) = 0.5mm. (Units don't check out, I just added them here and there so you know what's what). Even if you use fiber-glass reinforced ABS that's still 31% of that, 0.155mm > 0.1mm. And who knows if the digital scale is even printed so it's accurate at any point in time - for all you know they just took a random other inaccurate tool to check against. And this does not account for any mechanical slop or for deformations from using what's essentially a slightly colder bar of chocolate to clamp down on hard metal.

Accuracy: if you measure something with this tool, and then with a standard, it's going to come up with the same number, "down to 0.1mm of accuracy". As we've shown above, this thing is not accurate.

Precision: if you measure something with this tool, and then measure the same thing with this tool again later on, then it's going to shoe you the same number. And even here, this tool isn't really "precise", but we can assume that for all intents and purposes you won't be using it to measure things years apart (wear and tear can change its shape easily) or during massive temperature swings.

Moral of the story:

Accuracy /= Precision

Edit: at least tools made out of chocolate can be eaten https://www.thisiswhyimbroke.com/edible-chocolate-tools/

In your eagerness to show the world how well you know the difference between accuracy and precision, the point the OP was trying to make seems to have flown right over your head.

For DIY, you often need to make measurements that are a little bit more accurate than 1 mm, but not much, as well as measurements where 1 mm is plenty accurate but you need a reliable way to measure it, provided by the inner and outer jaws of the calipers. Even cheap calipers are excellent for this purpose, and the point is that people should not be scared off from them because they are not calibrated.

Also, no, 50C is not a very conservative range for that purpose, that would be ±5C around the steady 20C temperature inside the average house.

I agree that there are times when you don't need absolute accuracy. I have plastic calipers myself for crappy jobs. But I don't say that they're accurate to 0.1mm because they're not.

> Also, no, 50C is not a very conservative range for that purpose, that would be ±5C around the steady 20C temperature inside the average house.

That is true until you realize many people keep their tools in the garage or in a cellar or an attic, where the temperature can go anywhere between -20C and +45C. Then you measure a small nut or screw as often used in, exactly, home appliances, and between the thermal coefficient, slop, bad geometry, and an inaccurate digital potentiometer you arrive at 4.8mm instead of 4.1mm and you order a 5mm replacement for your 4mm screw and have to wait another week or two after you figure that out. I've had this exact story happen to me in the past, and my tools were stored in, as you say, "±5C around the steady 20C temperature inside the average house" and not out of plastic - just still not accurate enough.

For measurement of small nuts you apply an error calculated for 10cm. If the nut is small, the error will be also small, not 0.5mm, but something like 0.02-0.03. So if you measure 4.0-4.03 you won't make a mistake which nut it is. You can live with that as a DIY-er unless you need to measure 10cm+ nuts with high precision, I doubt that.

No, that's incorrect, because for small measurements the slop as well as the influence of bad geometry is going to be larger. Lack of accuracy is a problem that multiple issues contribute to.

I don't know where you got your numbers from. I doubt it's anything other than you thought the numbers just looked fine and illustrated your point.

If I was getting into the level of measuring things where the temperature coefficient mattered at all, other than a standard 20C room temperature, I sure as hell wouldn't be using a $9 plastic caliper from Amazon.

My point was that the ultra cheap caliper is suitable for measuring basic things where you can't get a good reading using just a steel ruler with 1mm increment measurements on it by eyeball, and maybe you might want to measure something a bit more precisely than 1mm increments. And it might be suitable for basic home DIY purposes. As I said, if I wanted a serious caliper I'd go spend $250 on one.

A cheap caliper like this, for example, might be suitable to tell me if a stripped copper wire's diameter really matches 23AWG or if it is 24AWG which is hard to see by eyeball.

$35 Husky digital caliper is metal with good battery life and lifetime warranty replacement at any Home Depot, https://news.ycombinator.com/item?id=32804727

What would it take to retrofit a digital mechanism onto vintage vernier calipers?

https://www.electroschematics.com/digital-caliper/

> There are several rectangular patterns etched onto a copper strip underneath the main scale (the part of the caliper along which the read head/slider moves). With the printed patterns on the bottom of the main circuit board, this forms a grid of capacitors. As the sliding jaw travels along the main scale, the capacitance of the printed patterns changes, the controller (chip-on-board) detects the resulting shifts in electrical charge, processes the signals and finally displays the readings on the LCD.

Would be nice to have open firmware for digital calipers.

> What would it take to retrofit a digital mechanism onto vintage vernier calipers?

Personally, analog calipers are more trustworthy to me, because you can never be in a state where you forgot to press the zeroing button - which happened to someone running a modern machine shop right in front of my eyes. If you care about the outcome of a process, eliminating every step that can fully destroy the result yields massive dividends. There's something to be learned here about every process, be it technology, customer retention, sales funnels, you name it. Having less steps is so extremely powerful that it can easily outweigh additional functionality of some sort.

I got given a fancy mitutoyo caliper once, it actually remembers the 0 setting. You don't need to set it every time like the cheap ones.

$50 iGaging clone of $130 Mitutoyo has absolute coords, https://www.amazon.com/iGaging-ABSOLUTE-Digital-Electronic-C...

If AR takes off, analog+AR could offer the best of both worlds.

You can get nice metal ones (digital) for much less than $50 from Harbor Freight.

The Chinese digital calipers are of questionable accuracy as the battery gets low. I much prefer a dial caliper at that price point.

A better battery may help, http://www.davehylands.com/Machinist/Caliper-Batteries/

> LR44 starting voltage is about 1.500 Volts. SR44 starting voltage is about 1.550 Volts. Both are rated as 150 mA hours, but the discharge curves give very different operational life depending on the required voltage for the application. The LR44 voltage drops over the duration of usage. The SR44 remains flat (and above 1.5V) for most of its useful life. The calipers need at least 1.25 to operate.

> The LR44 drops below 1.3 Volts after about 50% usage. The SR44 drops below 1.5 Volts after about 95% usage. The SR44 start at a slightly higher voltage, and their flat discharge curve makes them far superior for low power, long operational life applications, like a caliper.

got a fully manual caliper, it never runs out of battery and can't go wrong. Does the job for domestic and hobby use

http://www.longislandindicator.com/p11.html

> Before we start, let's state outright that the best, most reliable, and most useful caliper you can own is a vernier caliper. It isn't electronic and it doesn't have a dial ... Calipers should be frequently checked for accuracy using a gage block, or gage block combinations. To check for wear in the jaws do this: clean them and close them. Then hold them up to the light and if they're worn you'll see light shining through the gaps. At this point, measure with the unworn surfaces or have the calipers repaired. These jaws can be ground flat again.

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