Choosing the instrument before choosing the strip

The extension we need to distinguish

The Space Elevator Initiative needs to understand how a loaded ribbon stretches and where it fails. Before approaching a scarce carbon specimen, Mara Voss and Jonas Reed want an ordinary 100 mm strip to tell them whether their gauge sees the strip itself or the movement of the frame. The earlier readiness study left them with five unsecured channels and an unassigned reference custodian.

Their first selection is a specification envelope. SEI-P1 and SEI-P2 are internal requirement sets for seeking equipment, not supplier products, quotations or instruments already in a room. P1 requires 2 micrometre displacement resolution; P2 allows 10. Both require separately recorded force, crosshead travel and grip movement. The fixture calculates extension and strain only: no force reading, section area or tensile-strength result has been supplied.

An interval rather than a reassuring average

For P1, the planning allowances in micrometres are 1 for half a resolution step, 6 for calibration, 4 for alignment, 3 for temperature, 5 for mounting and 3.5 for half the seven-micrometre repeat range inherited from the prior fixture. Adding these conservatively gives a 22.5 micrometre extension bound. P2 sums to 85. This addition permits correlated errors; it does not assume that errors conveniently cancel.

At a nominal 200 micrometre extension, the P1 interval is 177.5–222.5 micrometres. With a nominal gauge length of 100,000 micrometres and a 100 micrometre length bound, the corresponding strain interval is 0.00177323–0.00222723. That interval is conditional on every input bound being valid. Repeat half-range is a planning allowance; three repeat values do not establish a statistical coverage level.

The selection has a cost in readiness

Voss sets a 25 micrometre design requirement for this small extension. It is a chosen discrimination target, not a newly discovered material property. P1 fits within it; P2 does not. Dropping the temperature allowance yields an incomplete budget, rather than a smaller and apparently better uncertainty.

Reed keeps the reference step outside the controller record. Voss owns specimen handling and Reed owns metrology planning; the independent reference-custody role is still unassigned. A separately documented 1,000 micrometre step must be checked before and after a run, but one step cannot establish linearity over the whole measurement range. Frame, sensors, reference and fixture access remain unsecured. The actual readiness case is held even with the favorable P1 numbers.

A request that can now be answered

The next equipment inquiry must ask for usable range, resolution, calibration evidence, mounting geometry, temperature sensitivity and access to raw readings. P1 is retained as the preferred requirements envelope; P2 is deferred for the present 200 micrometre task. If supplier data or a borrowed rig cannot support the P1 bounds, the budget must expand and the target must be reconsidered. No purchase, equipment offer or completed specimen run is recorded.

The 25 um requirement and component bounds are authored design inputs, not manufacturer data or measured uncertainty. The sum is conditional on bounds covering actual errors; repeat half-range is a planning allowance, not a coverage claim. A 1000 um reference step does not establish full-range linearity. No stress, strength or tether qualification follows.

Results at a glance

Related records

• The bench that is still on paper