How to apply tolerances

Many test specifications, especially where power supplies and signal voltage levels are included, will give a tolerance as well as an expected test result. It is important to be clear on how to apply these tolerances, so that equipment is not rejected unnecessarily - or declared serviceable when it is not.

There are generally two ways of writing a tolerance - as a percentage or as a value, with respect to a nominal value. In each case the tolerance will indicate an upper limit, a lower limit, or both. For example, taking a nominal value of 10vdc:

10vdc + 1v means that the acceptable range is between 9 and 11 volts.

This can be written as 10vdc + 10% - i.e. 10% (or 1 volt) above and below 10

There may only be a tolerance above or below the nominal value:

10vdc + 1vdc giving an acceptable range between 10 and 11 volts

Or there may be different tolerances above and below the nominal value:

10vdc +10%, -20% giving an acceptable range between 8 and 11 volts