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
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, 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
The SR44 drops below 1.5 Volts after about 95% 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:
>*Finally* found these buttons at yer "better" 99c stores!!
>Along w/ *16 packs* of AA and AAAs--yeah, 99c--plus NYC .0825.
>Only thing is, you gotta scrape off the contacts on a wire wheel. No
>foolin--they say so on the package.

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,
>on the web.

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 years.
The self discharge of the battery will probably make it half this duration.

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 years.
The self discharge of the battery will probably not be significant

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.

Philip Freidin

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.

http://data.energizer.com/

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:

1. The nominal voltage is 1.4V which is insufficient head room for a caliper. In my first email I said that "The calipers need at least 1.25V to operate." . I have at least a few that just sit there blinking at 1.38V If you look at the second page of the Zinc/air manual you will see the discharge curve shows the voltage is below 1.3V for almost all of its life. I would guess that the reality is somewhere between 1.2V and 1.4V, but may change batch to batch, and is almost certainly load dependent.

2. The major bite though is this paragraph on the next page:

   "The activated (tab off) air cell batteries have an expected fresh capacity maintenance, depending on cell size, of 50% after 3-12 weeks at 20oC (68oF). Beyond 20 weeks, at 20oC (68oF), fresh capacity maintenance drops to 0-10%. It is therefore very important to keep the tab seal in place until usage."

   So these cells have a really crappy life after you take the tab off. This is OK for the hearing aid market where currents are several hundred microamps up to a few milliamps, and you expect to change the battery every few weeks. Not so good if you have a Chinese caliper drawing 18uA, and a real bad choice for the Mitutoyo calipers.

Answer: This I am less sure about. Here is my understanding:

1. There are many chemistries, beyond the LR44 and SR44. How many are available in this package I don't know, but you could just go through all the chemistries at the above URL, and see if there is a 11.6 mm diameter and 5.4 mm high part. I found 4, Manganese Dioxide/Zinc, Silver Oxide/zinc, Zinc/air, and mercury.

2. Each manufacturer uses their own number system for different chemistries and package size, and for different grades (life or mAHour) within the same chemistry and package combination. There are standard names from ANSI and IEC which most manufacturers will cross reference their part numbers to. It is common in the electronic industry that if you specify that your product can replace a competitors product, your product must be equal or better in specifications, and must be a drop in replacement (mechanical and electrical).

   For example, the SR44 has the following names from the following vendors:

   Renata 303
   UCAR 303
   Energizer 303
   Energizer 357
   Eveready 303
   Maxell SR44SW
   Panasonic SR44SW
   Sony SR44SW
   Toshiba SR44SW
   Varta V303
   Rayovac 303
   Timex A
   Citizen 280-08
   Seiko SB-A9
   I.E.C. SR44
   I.E.C. SR1154

   All of these at a minimum have silver oxide chemistry, and are 11.6 mm diameter and 5.4 mm high

   Energizer lists the following as cells that their 357 can replace: 303, A76, AG13, EPX76, LR1154, LR44, SR44SW, and SR44W

   Notice that LR44 (Manganese Dioxide chemistry) is on the list. What this says is that they believe that in all respects the 357 (SR44) can be used to replace any manufacturers LR44 cell.

   Hopefully they do not claim on their A76 (Energizer's LR44 product) that it is a suitable replacement for the SR44, as my previous first post showed that that is not the case.

   So just because one vendor says that a product can replace another does not mean that the converse is true.

3. While packages may look identical, it is clear that the internal chemistry can radically affect the voltage, discharge curve, and usage life. See the above exposé on the ac675 versus the SR44

4. When equipment manufacturers recommend a specific cell (Mitutoyo recommends SR44), there is a good chance that they know what they are saying. It may well mean that the device was designed to specifically make best use of the battery's characteristics.

5. When a known vendor says their cell is a replacement for xxxx, I tend to trust them. If I am designing equipment that is going to be battery operated, I will download the data sheets for 4 or more vendors of the battery class I am planning to use. I then do a spec for spec compare of EVERY characteristic of the cells. I will find diffences. I then create a master spec that is the lowest (or highest, as appropriate) value for each spec. I then design to this, knowing that any of the cells will work. For example, operating temp:

                 Vendor   Min temp   Max temp
                 1        -30         +80
                 2        -40         +75
                 3        -20        +125
   
                 My spec: -20         +75
                 

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.