Probe Calibration

Probe Calibration is the reserved term, fixed by Seventh Concordance, Annex 2, for the alignment of an Ambient Reality Probe to a single anticipated condition before deployment. It is distinguished throughout the archive from mere Calibration, which may be performed on any instrument for any purpose, and from Conceptual Calibration, which requires no instrument at all. A probe calibrated to no specific condition is, in the wording of the Annex, "a lamp without a wick" — serviceable in appearance, useless in report.

The distinction matters chiefly because an uncalibrated or wrongly calibrated probe returns data indistinguishable from the data of a verified Ambient Absence, and the Measurement Directorate has twice had to strike whole quarto volumes from the Index of Ambient Conditions on this account.

Procedure and the Vance Lumen

Probe Calibration is conducted at the Central Calibration Spire, where the Vance Lumen is kept. The Lumen is brought to the anticipated condition of the target absence, and the probe is adjusted until it agrees with the Lumen on every reading the instrument is capable of taking. Agreement is certified by two attending Librarians, neither of whom may have calibrated the probe in question during the preceding quarter — a rule introduced after the Incident of the Fogged Calibration, in which a single clerk certified his own work four times and produced a consistent, sincere, and entirely false record.

Since Stairwell Calibration Ordinance 44-E took effect, calibration certificates must also record the pressure of the stairwell at the moment of agreement. This is widely regarded as unnecessary. It is also mandatory, and the Spire keeps a boy on the third landing whose sole occupation is to read the gauge aloud.

The SMP-Ξ question

A lasting scholarly dispute concerns whether Probe Calibration is a stage of Static Measurement in its own right or merely a preliminary to the second stage, in which the Mk.IV Ambient Reality Probe is typically employed. The SMP-Ξ (Xi) framework, as circulated in the Directorate's working recension, places calibration outside the measurement proper. The Office of Stairwell Calibration holds the opposite, citing Chapter 3, where calibration is counted as the first cycle and the probe never leaves the stairwell between cycles.

"A probe that has been calibrated has already begun to measure; it is measuring the Lumen. That the Lumen is not the subject is a matter of intent, and intent is not a stage."

— Stairwell Calibration Ordinance 1-A, marginalia attributed to the Fourth Registrar

Fourth Concordance, Amendment 712, forbids reopening the question, though it does not say which question, and both parties have claimed the prohibition favours them.

Failure modes and the Zeta Drift

Four failures are recognised: over-calibration, in which the probe is brought into agreement with the Lumen so precisely that it can no longer disagree with anything; under-calibration, the ordinary case; lateral calibration, in which the probe is aligned to a condition adjacent to the intended one and reports a neighbouring absence with great confidence; and Zeta Drift, a slow divergence of the probe from its certificate first described on Floor 2041 in the winter of the long frost.

Zeta Drift is not detectable at the Spire. It appears only after the probe has descended at least nine floors, and the Atlas of Unfixed Points, Vol. VII must therefore be consulted on arrival rather than at issue. Probes exhibiting drift are retired to the Seventh Helical Gallery, where the Memory Caskets are used to test whether the probe's earlier certificates were ever internally consistent. Most were not.

Record of the 1899 recension

The calibration ledgers of the Spire were recopied in 1899 under the direction of Clerk Annis Vell, who objected in writing to the practice of entering certificates by floor number rather than by date. Her objection was upheld by Third Concordance and overturned eleven days later by the Seventh. Vell's marginal note survives in the recension's flyleaf:

"The floors do not agree among themselves, and I am asked to make the book agree with the floors."

— Clerk Annis Vell, flyleaf of the 1899 Calibration Recension, Central Calibration Spire

The recension was completed by floor number. It is the edition still in use, and three of its volumes cannot be located, having been filed under their correct dates.

Access and visiting rules

Probe Calibration is witnessed by appointment through the Directorate. Visitors are asked not to speak during the agreement of readings, not to carry a lit lantern above the second gallery, and to leave the stairwell gauge to the boy on the third landing, who has the reading by heart and dislikes being corrected. Instruments brought from below Floor 1900 are calibrated in a separate chamber and are not permitted to share the Lumen.