2.2. Errors of Digital Instruments

Many modern laboratory instruments use digital displays. For example, you can measure weights using an electronic scale. It has the advantage that you don't have to judge what is the central value of your measurement. It will give you the central value. All you have to do is write it down.

Now, what is the error of this seemingly "precise" measurement? All instruments, no matter how sophisticated, have a limit to their precision. Usually, the manufacturer of the instrument will specify the error. (Most of the time it is written on the back of the instrument.) However, if neither you nor your laboratory instructor can find this information, use the following rule of thumb: The error of an electronic device is usually half of the last precision digit. The following example should make it clear. Suppose you measured your weight with the electronic device pictured above. The result is 138.2 lbs. What is the error? If you can't find the manufacturer information about the error, you notice that the resolution of your measurement is 0.1 lb. In other words, the device claims that it can give you a more precise value than 138 lbs, but it doesn't tell you whether your weight is 138.15 lbs or 138.24 lbs. Thus we can assume that the error of our measurement is 0.05 lb. And our answer is 138.2 lbs ± 0.05 lb.