How often each device is corrected¶
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).
| Date | 2026-10-17 |
| Status | current |
| Methods | Reading the stamps |
| Techniques | Sawtooth fit |
| Clocks | SNUADC sample clock, BIS clock, Primus clock, Recorder PC clock |
| Devices | SNUADC, BIS monitor, Primus anaesthesia machine |
| Variables | ECG lead II (SNUADC), BIS EEG, Airway pressure (Primus) |
Data¶
Record stamps of SNUADC ECG II, BIS EEG1 and Primus AWP in the 250 cases with clock fits.
Method¶
Corrections were taken from the stamp fits. Intervals count only consecutive corrections inside one stretch between gaps.
Results¶
- SNUADC (step mode) corrects on a drift threshold. Its interval follows its rate: about 0.5 s ÷ rate predicts a median of 2.08 h, and 2.06 h is observed (p10–p90 1.70–2.31 h, 182 intervals; the full release gives 2.08 h in 642 recordings, When each device's clock is corrected, in every recording).
- BIS corrects on a fixed timer of 8,192 s (2¹³ s) in 95% of 266 intervals. The step takes whatever drift has built up (+83 to +289 ms, median +155 ms); 0.155 s per 8,192 s implies 19 ppm, against 23 ppm from the tooth fits (25 ppm and a median step of +0.17 s in the full release).
- Primus AWP shows small corrections of −64 to +80 ms (median +8 ms) about every 7,230 s (2.0 h; p90 twice that), besides gaps of +20 to +22 s (median 5.5 per case).
| Parameter | Value | Status | From |
|---|---|---|---|
| Primus drift implied by its corrections | about 1–11 ppm | derived | How often each device is corrected |
Discussion¶
The Primus corrections imply a drift of only about 1–11 ppm against the recorder. Breath matching later measured the drift itself at about +104 ppm (Primus drift against the recorder PC, from breath matching, The Primus ventilator against SNUADC, by breaths): these small corrections do not track the Primus drift, so their size is not its rate.
Open questions¶
- What the Primus's small corrections correct (unexplained phenomenon). The corrections imply 1–11 ppm, but breath matching measures the Primus drifting about +104 ppm against the recorder PC (Primus drift against the recorder PC, from breath matching), and the stamps also step at the AWP pauses by amounts unrelated to the drift built up. What the small steps correct, if not the drift, is open. (This replaces the earlier conflict between 1–11 ppm and the assumed 115 ppm, which breath matching settled.)
Worked examples¶
| Example | Side | Status | Summary |
|---|---|---|---|
| Pulse pressure variation gated by ventilator breaths | time | draft | Pulse pressure variation per ventilator breath, in 3,448 recordings: the ventilator's clock sits a median 1.8 s off the arterial line's, which moves half of all breaths' values by a point or more and scatters where in the breath the pulse pressure peaks; a recording's median PPV hardly changes. |