The following question was asked in a usenet news group, and what follows is my answer.
Question (about calipers): Anyone get the sense that they burn thru batteries like wildfire? Mitutoyo's seem to last 1/2 of ferever, but these goddamm cheapies... Which is surprising, cuz they have an auto-off feature, that my Mits don't have!!
Answer (Philip): OOOhhh, OOOhhh, OOOhhh,
(visual: me at the back of the class room, jumping up and down because at long last there is a question I know the answer to :-)
So it turns out that for the last 3 months, almost all my free time has been spent studying the power consumption characteristics of electronic calipers and micrometers.
First off, here is a surprise for you. Many calipers continue to operate even when the display is off. Calipers that do this can be recognized as follows: Set the jaws to anything other than closed, and you read the normal measurement. Turn the caliper off. Turn it back on. If it still reads the same value, then only the display was turned off. If it doesn't read the correct value, and needs to be reset every time you turn it on, then it really was off (but may still use some power in the off state). I don't like calipers that forget zero.
On a side note, I have a 1" micrometer that forgets the 0.0000 position, and has to be closed and reset every time it is turned on. Mix that in with an auto-off function, and you will really hate it when you were measuring something 0.75123" and you put it down on the desk, and it turns off. Next time you need it you have to wind it all the way closed to reset the zero position. Really hate it if I am measuring another 0.75xxx part. When shopping for digital measurement tools, check whether it remembers zero across being turned off and back on.
For those calipers that do remember the zero position, they also keep doing measurements, even if the display is blank. For the Chinese calipers, they typically draw the same power as when they are on. The Mitutoyo draws about half as much power as when it is on. Don't believe me??? Try this: Turn on your Chinese caliper, close the jaws, press the zero button, open it to 2 inches, turn it off, press the zero button, move it to 3 inches and turn it back on.
It was making measurements, and taking notice of the zero button while it was "off". Actally, all it was doing was disabling the display, which does not save any significant power. Even the SPC output is active when it is "off".
The Mitutoyo caliper draws about 1/4 the power of the chinese caliper when on, and about 1/8 the power when off.
> ... bunch of question about battery numbers ...
The two common cells for calipers are the LR44 and the SR44. Calipers are usually shipped with the cheaper LR44.
LR44 is a Zinc + Manganese Dioxide chemistry SR44 is a Zinc + Silver Oxide chemistry
LR44 starting voltage is about 1.500 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, and then drops sharply at the end of life. The calipers need at least 1.25 to operate.
The LR44 drops below 1.3 Volts after about 50% usage
So you only get about 50% of the 150 mA hours from an LR44.
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.
>Jes tryna get more g-d bang fer my miserable buck.
Pick a Mitutoyo caliper, and run it with SR44 :-) You get what you pay for.
>Oh, oh, speaking of bang and bucks, I'm in 7th heaven:
All of these el-cheapo batteries come from manufacturers you have never heard of ("sun-moon batteries", "rising star batteries", ...) and although they say they are replacements for SR44 and LR44, they are poor quality LR44 chemistry.
>You can get the button cells for about 50c ea, mebbe less, in
q's of 20-100,
The good news is that since you will be changing batteries often, you will get to use them all.
Here is the data I collected. properly
========================================== All measurements are in microamps Off On Mitutoyo 500-171 2.0 4.0 Chinese Vendor 1 17.5 18.0 Chinese Vendor 2 17.5 18.5 Chinese Vendor 3 7.0 7.5 Chinese Vendor 4, #1 15.5 16.0 Chinese Vendor 4, #2 18.0 18.0 Chinese Vendor 4, #3 19.0 19.0 Chinese Vendor 4, #4 17.0 18.0 Chinese Vendor 4, #5 19.5 19.5 Chinese Vendor 4, #6 17.5 18.0 Chinese Vendor 4, #7 19.5 19.5 Chinese Vendor 4, #8 17.5 18.5 Chinese Vendor 4, #9 17.5 18.5 Chinese Vendor 4, #10 19.5 20.0 ==========================================
Using an SR44:
So, for a Mitutoyo that is on for 1 hour per day the calculation is: (1 hour * 4uA) + (23 hours * 2uA) = 50uAHours per day
SR44 (150 mA Hour) * 90% = 135mA Hours = 135000uA Hours
Battery life is therefore 135000/50 = 2700 days = 7.39
Here it is for the Chinese calipers
(1 hour * 18uA) + (23 hours * 17.5uA) = 420.5uA Hours per day
SR44 (150 mA Hour) * 90% = 135mA Hours = 135000uA Hours
Battery life is therefore 135000/420.5 = 321 days = .87
You can do the math your self for the LR44 batteries. Remember to use 50% rather than 90%.
For sale: 10 chinese calipers (vendor 4), $25 each + S&H . Comes with a LR44 battery.
Question: What do you think of Steve's reference to the zinc-air battery, and it's whopping ma-hrs? I wonder what the $$ is for that.
check out the specs on the ac675 (zinc air) vs. the standard 357 (silver oxide miniature) which most calipers take.
Answer: Looks great at first, but reality is not so gentle.
The LR44 and SR44 are both rated as 150mA Hour, ac675 is the same package but has the very impressive 635mA Hour rating. If it costs less than 4 times as much you may think this is a great deal. Not so. You need to read the "Zinc Air Application Manual" at the above URL carefully. Here's where you get bitten twice:
Answer: This I am less sure about. Here is my understanding:
Question: I'm wondering if you did any accuracy and repeatability checks on all of those calipers. If so, what were the results?
Answer: Sure I did.
All calipers, and one micrometer were all compared against a 1.00000 that was supplied with my 2" Mitutoyo micrometer. PN 167-141
Mitutoyo 293-761-30 1.00005 Mitutoyo 500-171 1.0000 Chinese Vendor 1 0.9995 Chinese Vendor 2 0.9990 Chinese Vendor 3 1.0010 Chinese Vendor 4, #1 0.9995 Chinese Vendor 4, #2 0.9995 Chinese Vendor 4, #3 0.9995 Chinese Vendor 4, #4 1.0000 Chinese Vendor 4, #5 0.9995 Chinese Vendor 4, #6 0.9995 Chinese Vendor 4, #7 0.9995 Chinese Vendor 4, #8 1.0000 Chinese Vendor 4, #9 1.0000 Chinese Vendor 4, #10 0.9995
All measurements were made 3 times, and were repeatable (mostly spot on, a few were +/- .0005. Since the display is digital, and the last digit can only be 0 or 5 ( x .0001) , this is not considered an error because when digital systems resolve an analog signal, they discrete-ize the value. Since the analog value (a continuous function) could be right at the point where the least significant digit changes, the change in displayed value could be the result of a change much smaller than the delta of the two displayed values.)
All the calipers have a resolution of .0005"
From the data I would say that the accuracy is .001"
Because of the way the calipers make their measurement, the single value that I measured is of no use in estimating what the reading would be at 2" or 5.333" etc. The resolution is .0005" over the full travel, but the accuracy can be no better than .001" , and may be worse. Accuracy errors may not be accumulative. i.e. if 1.0000 read 1.0010, and 2.0000 read 2.0010, you would be incorrect to assume that 3.0000 would read 3.0010
There is insufficient data above to say whether a 6" Mitutoyo caliper is more accurate that the Chinese ones, plus the measurement depends on the pressure used in closing the caliper on the standard. Typically the caliper is closed with the little thumb roll wheel. If excessive pressure is used, a low reading results.
I find I get more consistent results by closing the caliper on the work piece by pressing on the outside of the jaws inline with the work piece measurement axis.