Uncertainty of Measurement and Surface Texture Flashcards
What are the SOURCES OF UNCERTAINTY?
Incomplete definition of the test.
- Imperfect realisation of the definition of the test procedure.
- Inadequate knowledge of the environmental conditions (assessment of EC and effects on measurements).
- Sampling strategy not being truly representative.
- Personal bias in reading analogue instruments.
- Values assigned to measurement standards.
- Values assigned to reference materials.
- Changes to instrumentation since last calibration.
- errors in constants, correction factors, other parameters.
- Approximations/assumptions included in data evaluation.
- Random effects (environment, human being, electricity/gas/water services).
WHY ESTIMATE UNCERTAINTY ?
- Allows meaningful comparison of results from different laboratories, or within the same laboratory.
- Comparison of results with standards/reference values.
- Identification that tolerances included in a formal specification have been met.
- Identification of aspects of the test methods/equipment which require particular care.
- All UKAS accredited laboratories are required to make statements of estimated uncertainties of measurement.
What are the two TYPES OF EVALUATION?
The Guide adopts the approach of grouping uncertainty components into two categories, based on their method of evaluation; Type A and Type B.
* Type A evaluation of uncertainty is by a statistical calculation from a series of repeated observations.
* Type A evaluations will involve standard deviation calculations, and are known as Type A Standard Uncertainty.
* Type B evaluation of uncertainty is by means other than that used for Type A. For example, data may come from calibration certificates, manufacturers specifications, previous experience, etc.
* Type B evaluations are also characterised by estimated standard deviations, and are known as Type B Standard Uncertainty
How to work out combined UNCERTAINTY and expanded uncertainty?
What is used for VERTICAL DISPLACEMENT UNCERTAINTY of a stylus?
A Type A1 or Type A2 artefact should be used to calibrate the accuracy of the stylus pick-up and transducer arrangement, or in other words the vertical magnification factor of the instrument.
What are the Type A standards?
Type A standards are for calibrating the vertical profile component of stylus instruments
Type A1 standards which are designed to be insensitive to the shape or condition of the stylus tip.
Type A2 standards which are designed to be insensitive to the shape or condition of the stylus tip.
How many contributions must be made to the Zm UNCERTAINTY?
Under ISO 5436: 2000, a minimum of five measurements should be taken to determine dm.
* However, Zm will depend on a number of additive factors, the most influential of which are represented in the following relationship
What is Zp,
What is Zref
What is Zn
What is Zpl
What is Ztip
- Zp is the “Traced Profile” term, and is the actual height value at any given point in the measurement
- Zref is the “Slideway Profile” term, and denotes any imperfections in the slideway or skid
- Zn is the “Instrument Noise” term which is the random measurement error of the instrument
- Zpl is the “Plastic Deformation” term, which is dependant on the stylus, work piece material, stylus force, shape, local curvature and slopes on the surface.
- Ztip is the “Effect of Tip Geometry” term, and is only required when the measured surface has wavelengths which are less than the radius of the stylus tip
Look at Surface texture uncertainty equations for vertical displacement
What is used to measure TRAVERSE DIRECTION UNCERTAINTY?
Calibration in the horizontal direction
A Type C artefact should be used with an Rsm value as close as possible to that of the surface being measured, with at least twelve measurements taken over different sections of the X-axis.
Look at Surface texture uncertainty equations for traverse direction uncertainty
