Tuesday, August 5, 2008

A Friendly Tech Tip: Size Matters

How relevant is the capacity of a force or torque measuring instrument to the expected load during a test? More than many people think.

The accuracy of measuring instruments can be specified in two ways. In some cases, accuracy is specified as a percentage of actual reading, in other cases as a percentage of full scale. Mark-10 specifies accuracy as a percentage of full scale. What does that mean exactly?

Consider the following analogy: Suppose you are weighing yourself on a 300 lb capacity common bathroom scale that specifies accuracy of ±2% of full scale. 2% of 300 lb equals 6 lb. If you weigh yourself and the scale shows 150 lb, your true weight could be anywhere between 144 and 156 lb. Even though the accuracy specification is ±2% of full scale, the percentage reading error could be up to 6 lb / 150 lb = 4%. Suppose your weight is only 100 lb. The percentage reading error could be as high as 6%. The lower the load, the greater the percentage reading error.

The same principle applies to our force and torque measuring instruments. Since accuracy is typically specified as a percentage of full scale, there is a fixed numerical value by which the reading may be incorrect at any point in the range of the instrument. Accordingly, it's important to select an instrument with capacity as close as possible to the expected maximum force.

For example, our Series BG digital force gauges specify accuracy of ±0.2% of full scale ±1 digit (increment). In the case of the
BG100 force gauge (with capacity of 100 lb), the specification translates into an error of 0.25 lb. This is calculated as follows: 100 lb x 0.2% = 0.2 lb + 1 digit [0.05 lb] = 0.25 lb.

If the total observed force is 80 lb, a maximum error of 0.25 lb represents a percentage reading error of approximately 0.3%. If, however, the total observed force is only 2 lb, the percentage reading error is 12.5%. Such an error could easily mean the difference between an acceptable product and a reject.

A quick evaluation of the maximum expected load during your test and an evaluation of the available capacities of force and torque measuring instruments will go a long way in ensuring that your samples are tested as accurately as possible.

Questions? Comments? Feel free to post!