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# ME 3263 Introduction to Sensors and Data Analysis (Fall 2018) | |
## Lab #1 - Measurements of machining precision and accuracy | |
### How can you measure something? | |
All measurements have traceable standards. There are seven base units in SI - | |
meter (length), second (time), Mole (amount of substance), Ampere (electric | |
current), Kelvin (temperature), Candela (Luminous intensity), and kilogram | |
(mass) [1](https://www.nist.gov/pml/weights-and-measures/metric-si/si-units). | |
Any measurement you make should have some method to check against a reference. | |
In this lab, we will use calipers that measure dimensions i.e. meter\*10$^{-3}$ | |
(length). Calipers can always be verified to work with gage blocks. | |
### Sources of measurement variations | |
No measurement is exact. No surface is compeletely flat. Every measurement you | |
make has two types of uncertainties, *systematic* and *random*. *Systematic* | |
uncertainties come from faults in your assumptions or equipment. Here are some | |
examples for caliper measurements: | |
- Your calipers were dropped on the floor and bent, now all of your measurements | |
will be too long or too short. | |
- The bar stock was not machined with square edges so each measurement along the | |
sides increases or decreases | |
- You align the calipers with the bar stock, instead of measuring | |
point-to-point. Now your measurements are only the maximum length from | |
edge-to-edge | |
*Random* uncertainties are associated with unpredictable (or unforeseen at the | |
time) experimental conditions. These can also be due to simplifications of your | |
model. Here are some examples for caliper measurements: | |
- You assume the surface is flat, but it is in fact rough | |
- The temperature in the room changes the dimensions through thermal expansion | |
- Your calipers are not aligned parallel to the edges of the bar stock | |
In theory, all uncertainies could be accounted for by factoring in all physics | |
in a problem e.g. Temperature-dependence, Coriolis effect, electrical | |
disturbances in your readings. In practice, the diminishing return on investment | |
prevents this practice. For something like measuring bar stock, it is more than | |
sufficient to report the average and standard deviation with good calipers. | |