Primus against the recorder PC¶

The Primus stamps drift a median +104 ppm against the recorder PC (measured by breath matching through the SNUADC stamp model). Its small corrections every 7,230 s, each about 21 s after a pause in the AWP waveform, do not keep up with that drift.


Clock	Primus clock
Measured against	Recorder PC clock
Modes	all
Status	measured
Drift	104 ppm (p10–p90 28 to 184; 178 recordings; whole-recording lines give +87 ppm median in 1,060 (p10–p90 −49 to +229))
Corrected every	about 7,230 s (2.0 h; p90 twice that)
Correction size	whole 16 ms samples either way; median −16 ms (one sample back), p10–p90 −144 to +144 ms

Correction¶

Small steps, whole 16 ms samples in either direction (median −16 ms; p10–p90 −144 to +144 ms), every 7,230 s or twice that, often two at once; the first at any time after the Primus starts. The AWP waveform stops for about 21 s just before each correction and that AWP is lost (The Primus AWP stops for about 21 s every 7,230 s); CO₂ carries on. Measured in 4,529 recordings (When each device's clock is corrected, in every recording). These steps imply only 1–11 ppm (Primus drift implied by its corrections), far less than the drift: the stamps also step at the AWP pauses, by amounts unrelated to the drift built up: a median −1 ms (p10–p90 −164 to +187 ms) where CO₂ carries on, up to seconds where CO₂ stops too (The Primus stamp step at each airway-pressure pause), so over a recording the Primus wanders from the PC by a median 1.5 s.

Parameters¶

Parameter	Value	Status	From
Primus drift used in the delay fits	115 ppm	assumed	—
Primus drift against the recorder PC, from breath matching	104 ppm (p10–p90 28 to 184; 178 recordings; whole-recording lines give +87 ppm median in 1,060 (p10–p90 −49 to +229))	measured	The Primus ventilator against SNUADC, by breaths
Primus drift implied by its corrections	about 1–11 ppm	derived	How often each device is corrected
Primus correction interval	about 7,230 s (2.0 h; p90 twice that)	measured	When each device's clock is corrected, in every recording
Primus correction size	whole 16 ms samples either way; median −16 ms (one sample back), p10–p90 −144 to +144 ms	measured	When each device's clock is corrected, in every recording

Evidence¶

Study	Date	Status	Summary
When each device's clock is corrected, in every recording	2026-10-19	draft	From the steps between records in all 6,388 recordings: the BIS is corrected every 8,192 s and the Primus every 7,230 s, on their own timers (the first correction comes at any time after the device starts); the SNUADC is reset when 0.49 s of drift has built up. With the full rerun of the correction fit, it gives each device's drift against the recorder PC in 4,728 BIS and 2,728 SNUADC recordings, and shows that the Primus's 20–23 s AWP pauses belong to its correction cycle. Every rate here is relative to the recorder PC, whose own clock drifts and is corrected.
How often each device is corrected	2026-10-17	current	The devices re-synchronise in different ways: SNUADC when its drift reaches about half a second, BIS on a fixed 8,192 s timer, and the Primus by small steps about every two hours that do not keep up with its drift (+104 ppm, measured later by breath matching).
The Primus ventilator against SNUADC, by breaths	2026-10-12	open	Each breath shows in the Primus airway pressure and, through the chest, in SNUADC's central venous pressure. Tracking that lag puts the Primus on the SNUADC timeline to about 55 ms over the whole release, with a line per stretch between its AWP pauses (49 ms in the pilot). At scale it measures the Primus drifting a median +104 ppm against the recorder PC between its AWP pauses, with steps at the pauses that do not reset it.