# Uncertainty in Length Measurement using a Caliper

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#### bearvn

Hello all,
I used a caliper to measure linear dimensions (for e.g. length of an object). Below is my uncertainty budget:
1. Repeatability (type A)
2. Resolution of caliper (half of resolution/1.732)
3. Uncertainty of the caliper (get from calibration certificate)
4. Uncertainty of coefficient of thermal expansion (CTE):
Because the CTE of caliper is not known exactly, I assumed that it is within +/-10%. Uncertainty of CTE = Length*CTE*10%*T, where T is the range of fluctuation of room's temperature. Then I treat this uncertainty as rectangular distribution. Is this estimation right?
5. Uncertainty of parallelism of caliper's jaws. How can I estimate this uncertainty?

Furthermore, did I miss any components?
In case that I use this caliper to measure an object at 25 Celcius degree of room's temperature which is different from its calibration temperature, it is neccessary to add correction of thermal expansion to the measured values (L*α*(20-25)), isn't it?

S

#### sunilgaidhani

You can neglect the Temperature effect as it will have minor contribution, as resolution of your vernier is not enough to read this difference.

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#### bearvn

You can neglect the Temperature effect as it will have minor contribution, as resolution of your vernier is not enough to read this difference.

#### howste

##### Thaumaturge
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I suspect this would be similar to drift. How far from nominal is the parallelism at each calibration? If you adjust the parallelism to nominal at each calibration, it seems like the uncertainty value would be the maximum observed value found at calibration. The distribution should be treated as rectangular.