Is this StopWatch Accuracy correct? [closed]

Question

This question would be easy for anyone who has some tech experience with stopwatchs.

I'm going to make some measurements. and I need a 1ms accuracy stopwatch.

I found one that could do the work with the following specificacions : Accuracy (time): ±30 seconds per month (stopwatch): 99.9988% Based on the document stopwatch and timer calibrations 2009 (National inst. of standars and tech) http://tf.nist.gov/general/pdf/2281.pdf , I used thier formula to try to find out the accuracy in milliseconds.

x1/day * x2 Where x1 is the original resolution in seconds and x2 is the new interval to compute in seconds. In the particular of the stopwatch i want to buy: x1/month * 1/(2592000/x1) month 30/86400 = 0,000347222222 sec.

This is my problem: The document suggest to have caution using the formula for less periods of time than the tech specifications shows. so In this case I used 30 seconds intervals instead of 1 month. (my measurements will be done in 30 seconds intervals).

Now 0,000347 obviously is not the resolution.

But How can i know that the stopwatch has a resolution of 1ms or better?

Seems like I'm stock and I can't find out a way to solve this problem.

UPDATE: I'd like to keep the focus of the question in how to know the actual accuracy of the device, the use of the formula provided by the document mentioned abovea and it's limitations. thanks

Answer 1

A stopwatch accuracy specified as 30 seconds per month is equivalent to 1 second per day. Since there are 86400 seconds in a day, the maximum interval that can be timed with an accuracy of 1 millisecond is 86.4 seconds. However since the readout of the stopwatch is limited to 1 millisecond, the uncertainty of the measurement is also 1 millisecond. Thus if one is measuring a 30 second interval (for which the accuracy is theoretically 30/86.4 milliseconds or 0.35 milliseconds), the best accuracy attainable is limited to +/- 1 millisecond. In addition, since this stopwatch can only be activated by pushing a button, human reaction time must be considered. According to an experiment conducted by NIST personnel and reported in NIST 960-12 (Stopwatch and Timer Calibration), human reaction time averages about 100 milliseconds. One might assume that the reaction time on pushing the button at the start of the interval is somewhat equal to that at the end of the interval thus reducing the effect of reaction time. Even then, unless the reaction times agree to much less than 1%, the uncertainly just due to reaction time is going to exceed the overall requirement of 1 millisecond accuracy. It does not seem feasible to use a manually operated stopwatch to measure to that degree of accuracy. One would need a timing means that is entirely electronic or mechanical to achieve such an accuracy.,

Answer 2

Is this StopWatch Accuracy correct?
I'd like to keep the focus of the question in how to know the actual accuracy of the device, the use of the formula provided by the document mentioned above and it's limitations.

This is an electronic design site and none of your focus error are on topic here, unfortunately.

If you go to this page, pass the minor IQ test required to find out how to join the PICList, and join and ask your question there, you may get many good answers if you explain your question well.

There or here you really should explain what you are trying to achieve and to time, how the timer will be started and stopped and as much else as is relevant.

As Barry noted, the claimed accuracy is equivalent to about 0.4 milliseconds in 30 seconds. Even if the 30s/month spec was met there is no guarantee that the short term drift rates are not higher than this and no requirement for the maker to make it so. They are interested in making a useful timing device for everyday purposes and your application is not one.

A relatively ordinary metal bodied watch worn always on my wrist and adjusted to apparently 0 drift during setup can achieve perhaps 1 second per week drift - this is in major part due to it sitting in my wrist, which acts as a temperature controlled "oven".

The 100 mS reaction times which people have cited are for an alert operator and a random event which occurs with no warning - such as a traffic light turning green, a balloon exploding while being inflated etc. IF the sequence which precedes your event can be observed you can get user post-event delay times that are much smaller than a00 ms. eg if I time from the moment a ball is hit by a batsmen or caught by a fielder I can anticipate the moment of impact and time my triggering action accordingly. With experience the ability to minimise response time improves.

eg with practice I can use a stopwatch to time the fall of a coin from the top of a doorway to the floor, provided that I release the coin, with enough accuracy over 10 trials to usefully [tm] measure doorway height based on gravitational acceleration and time of fall. Here "usefully" is not as good as using a tape measure but probably better than guessing by eye.