dettube — detonation-tube quick-look tools

Reads a detonation- or shock-tube shot recorded as LabVIEW TDMS, plots every channel, picks the pressure and flame fronts, and exports both to CSV.

Nothing about any particular tube is built in. A rig file says where the gauges are and what thresholds suit them, so the same package serves any tube whose DAQ writes TDMS.

Install

pip install -e .            # from this folder, for development
pip install .               # or a plain install

Needs Python 3.9+. Dependencies (npTDMS, numpy, matplotlib) come with it. The folder picker uses tkinter, which ships with most Python builds; on some Linux distributions it is a separate OS package (apt install python3-tk).

First run

dettube rig --template myrig.toml     # write a documented example
$EDITOR myrig.toml                    # station positions, channel numbers, thresholds
export DETTUBE_RIG=$PWD/myrig.toml    # set once, or pass --rig every time
dettube rig                           # check what got loaded

The rig file is searched for in this order: --rig PATH, $DETTUBE_RIG, ./rig.toml, ~/.config/dettube/rig.toml. Without one, every command stops and says so rather than guessing — a wrong station position quietly corrupts every velocity downstream of it, so guessing is the one thing this must not do.

Commands

dettube rig        [--template FILE]
dettube plot       <shot> [--sensor pt|pdt] [--per-station] [--show]
dettube velocity   <shot> [--sensor pt|pdt] [--show]
dettube conditions <shot> [--csv FILE]
dettube export     <shot> [--raw] [--full] [--every N]

All of them take --rig FILE.

Omit <shot> to get a folder picker. Each subcommand is also installed on its own as dettube-plot, dettube-velocity, dettube-conditions and dettube-export.

Command	Writes
dettube plot <shot>	PT_stack_<shot>.png
dettube plot <shot> --sensor pdt	PDT_stack_<shot>.png
dettube plot <shot> --per-station	six station_<name>_<x>m_<shot>.png
dettube velocity <shot>	velocity_PT_<shot>.png
dettube velocity <shot> --sensor pdt	velocity_PDT_<shot>.png
dettube conditions <shot>	a table on the terminal; --csv writes <shot>_conditions.csv and <shot>_events.csv
dettube export <shot> --raw	six CSVs in <shot>/csv/

Layout

dettube/
  rig.py       rig-file format, search path, template
  core.py      TDMS reading, ignitor-spike detection, the loaded rig
  analysis.py  front picking, segment velocities, causality and sanity checks
  conditions.py  pre-ignition readings from the slow process instruments
  plots.py     figures
  export.py    CSV
  cli.py       command line

Everything that describes a tube lives in the rig file and nowhere else. That separation is not tidiness: the station positions in the project this grew out of were wrong for months because the same numbers were copied into four source files and drifted apart. One file, loaded at run time, cannot drift.

What the plots show

What it draws

Every PT channel at true amplitude, offset vertically by its axial position, with both channels of each station overlaid. Nothing is normalised, so a station reading four times its neighbour looks four times bigger.

Colour is by side of the tube, not by station:

	Channels	Orientation
green	PT-01, 03, 05, 07, 13, 15 (odd)	45°
blue	PT-02, 04, 06, 08, 14, 16 (even)	225°

The two gauges at a station sit diametrically opposite each other, so in a healthy shot green and blue lie on top of one another at every station. Colouring by side rather than by station means a fault affecting one whole side of the tube shows up as a pattern running down the plot, instead of looking like one odd station. Green and blue were checked for colour-vision safety (ΔE 18.0 deutan / protan, 18.7 normal vision).

Station names, positions and channel numbers all come from the rig file — run dettube rig to see the ones in force.

Two things worth getting right there, because both have caught people out:

• What x = 0 means. If the ignitor is not at the closed end, distances from the datum are not distances from the ignitor. Velocities are unaffected, since they come from differences between stations, but any single distance is.
• Channel numbers need not be contiguous. If a section of tube is removed, its channels simply do not appear in the rig file, and the numbering jumps.

<< PAIR DISAGREES

Printed in red beside any station whose two gauges differ by more than 2×. Two transducers 150 mm apart on one flange ring should read within a few tens of percent of each other, so a flag means one of them is not measuring gas pressure. It is worth taking seriously. On the rig this was built against, one gauge tripped it on every shot — reading 4× to 11× its partner, and timing the front 1–2 ms early, which no amount of filtering removed. A median filter heavy enough to bring its amplitude down also destroyed the rise it was supposed to be measuring. The flag is what found it.

dettube_plot.py --per-station — one file per station, pressure and light together

dettube plot "D:\shots\03.100926" --per-station

Writes six PNGs, one per station — station_B2_3.50m_<shot>.png and so on. Each has the station's pressure pair on the upper panel and its photodiode pair on the lower one, sharing a time axis, so you can see what the pressure and the light did at the SAME place.

Two stacked panels, not two y-axes on one panel: barg and volts share no scale, and overlaying them invites reading a crossing as meaningful when it is only an artefact of how the two were scaled.

This is the view that shows the physics directly. At a healthy mid-tube station you see the gentle precursor compression arrive first, then several milliseconds later the light jump as the flame itself reaches the viewport, with a local pressure spike alongside it. The separation between those two is the thing the whole campaign is about.

Velocity

dettube velocity "D:\shots\03.100926" --sensor pdt

Writes velocity_<folder>.png and prints an arrival table and a velocity table.

How the front is picked

t = 0 is the ignitor's electrical spike. The first 1.5 ms after it are blanked, because the spike lands on every channel at once and would otherwise be picked as the front. The arrival is then the first crossing of max(0.30 barg, 10σ) that stays above for 0.1 ms, so one noisy sample cannot trigger it.

Where a station's two gauges differ in peak amplitude by more than 2×, the louder one is dropped and the quieter kept, and the choice is printed:

! A4 (6.00 m): PT-14 reads 28.6 barg against PT-13's 4.0 — dropped, using PT-13

Photodiodes — --sensor pdt

Both tools take --sensor pdt. The photodiode path differs in ways that matter:

• The clocks are different. Pressure and photodiodes are separate DAQ tasks with separate start times; the skew has been as large as 84 ms. The ignitor spike is always found in the PRESSURE record, then converted to the photodiode clock using each file's own wf_start_time. Both files must be present.
• No pair-disagreement test. The rig carries a MIXED population of photodiodes, so the two channels of a station are often different sensor types and are expected to differ. Instead each channel is judged on its own: it must produce at least 0.05 V and hold it for at least 2 ms at half height. That separates a real flame glow, which is tens of milliseconds wide, from the ignitor's sub-millisecond EMI spike. Channels are marked usable or --.
• The arrival is the PEAK of the glow, not its leading edge. Some units carry a baseline 10–40 % of their own signal, so an edge threshold lands inside the noise. The peak is self-consistent between stations, which is what a velocity needs, but it times the brightest part of the burning zone rather than its front — so read flame speeds as good to about ±30 %, not to the two figures the fit prints.
• Causality filtering. Stations whose arrival breaks the order of the others are dropped and named, keeping the largest self-consistent set. Where several sets are the same size, the tie goes to the one with the most light — the station that saw the brightest glow is the one most likely to be timing the flame. Signal-to-noise is deliberately NOT used for this: a quiet photodiode that barely sees the flame scores well on it.

UNRELIABLE warning

The flame path refuses to look confident when it should not be. It says UNRELIABLE, on the plot and in the terminal, if either:

• two or more of the six stations had to be dropped to get a causally ordered set, or
• any segment exceeds 1000 m/s — the sound speed of the combustion products, and the last step before DDT. In a tube that has produced 100–400 m/s flames, a segment above that is far likelier to be a mis-picked glow than a real event.

A high R² alongside these warnings means the wrong points fit a line well. It is not reassurance.

CAUSALITY WARNING

If the fitted arrivals put the front at a downstream station before an upstream one, the script says so on the plot and in the terminal. That is physically impossible, so it means a pick failed — go back to dettube plot and look at the raw traces before believing any velocity.

Conditions

dettube conditions "D:\shots\03.100926"

What the process instruments read before the shot, which is not always what the folder is named. In the set this was built against, a folder called 50%H2 holds a shot whose analyser read 48.4%. Folder names record the target; the instrument records the gas.

Each channel gets its pre-ignition median, a median absolute deviation, and the full range of the same window. The median and MAD are used deliberately: these channels carry impulsive noise — a flow transmitter on a closed line throwing single-sample spikes to 9 L/min about 0.4% of the time — and a mean and standard deviation over that describe the spikes rather than the reading. The range is printed beside the spread so the spikes stay visible instead of being averaged out of sight.

Three things are flagged.

Drift compares the last second before ignition with the second before that. Near zero means filling had finished and the mixture had settled, so the single number means something. Large means it had not, and the number is a snapshot of something still moving.

A sustained change after ignition — a run entirely outside the pre-ignition envelope, not an isolated spike, since these channels spike just as often before ignition as after. A sustained response arriving in under a millisecond is flagged TOO FAST: no analyser or sheathed thermocouple responds that quickly, so it is the ignitor's electrical pickup or a wiring fault, and only the channel's pre-ignition value can be used.

A channel pinned at one end of its span is marked * — or ! if it is not reading at all. The two are told apart by counting distinct values: an input still being digitised dithers over hundreds of codes even when the reading it carries is a true zero, whereas a channel nobody is sampling repeats one value exactly. What this does not establish is whether the instrument behind a live input is powered and spanned; a transmitter parked at the bottom of its output range looks identical to one correctly reporting zero. Only a span check, or a shot that actually contains the species, settles that.

Add on and note to a [[conditions.channel]] entry in the rig file to carry what you know about an instrument into every report. Where it is mounted is worth writing down: an analyser on a supply header measures the supply, and that equals the tube only once the tube has been purged to equilibrium.

How the shot was set up

If the control system writes a log beside the data, point the rig file at it with [event_log] and dettube conditions reads it too. It is worth doing: which valves were open, and how long a spray had been running before ignition, are recorded there and nowhere in the TDMS. Without it the only surviving record of a shot's configuration is its folder name, which states what was intended rather than what happened.

How the shot was set up — Ignition Report20260910_110457.csv
  ignition logged at   11:04:58.876
  shut    water curtain SV01 (centre of A1, 1.00 m)
  OPEN    water curtain SV04 (centre of B3, 4.50 m)
  mist valve opened at 11:04:53.358   5.52 s before ignition

The log's own ignition timestamp is checked against the ignitor spike measured in the pressure record, because the two are not the same moment and the gap is not small. On the rig this was written for they sit exactly 8 hours apart — the control system logs local time, the DAQ logs UTC, and nothing in either file says so. Anything correlating the two without allowing for it is out by 8 hours silently. Past that whole-hour part the logged time still runs 0.9 to 5.3 s early from shot to shot, which is why the spike and not the timestamp is t = 0.

glob accepts a list, so a rig can keep matching an older filename alongside the current one and old shots stay readable.

CSV export

dettube export "D:\shots\03.100926" --raw

Writes into a csv/ folder beside the data. Results come from the same picking code the plots use — there is no second copy.

File	Contents
<shot>_arrivals.csv	one row per station per sensor: channel used, arrival time, peak, used/dropped, plus every rejection note as its own row
<shot>_velocities.csv	one row per segment per sensor: length, transit time, velocity, fit, R², and any warning
<shot>_raw_PT.csv etc.	raw samples, one file per DAQ group

Every raw file carries a t_ms column measured from the ignitor spike, so the four groups line up with one another. They do not line up by themselves: PT, PDT, Mixed and Pitot are separate DAQ tasks and the start-time offsets between them run from −23 ms to +68 ms across the shots checked.

On size. The photodiode record is 16 channels at 100 kHz — written whole that is ~700 000 rows and well over 100 MB of text. The default is the window −20 to +120 ms around the spike, about 1.4 MB. --full writes everything and --every N decimates. Sizes are printed as files are written.

What it will tell you it could not do

Rather than write a silently wrong file, the export skips a group and says why:

Pitot: the ignitor spike falls at 2531 ms but this record is only 1800 ms long — skipped
Pitot: 'Pitot Tube Outputs' in Pitot Output20260828_104336.tdms has no sampled data — skipped

Both of those are real, from the September shots: the Pitot data is unusable in two of the four — empty in one, and stopped recording before the shot in another. Worth knowing before anyone plans an analysis around it.

It also reports when channels in one group have different lengths and it has had to trim to the shortest, naming them. That is normal on this rig — the last pair usually runs a few thousand samples short — but a group where ten of sixteen channels are short is worth a look.

Two things the tools handle for you

The TDMS files will not open as they are. The VI pre-allocates each file and does not trim it on close, so a few hundred bytes of zero padding sit past the last segment; npTDMS stops with Segment does not start with b'TDSm', and the rig's own .tdms.log files record the same thing as TdsErrNotTdsFile(-2503). The reader walks the segment chain and reads up to the last complete segment. Nothing is lost. Worth fixing in the VI so nobody has to work around it.

Ignition Time in the CSV is not the moment of ignition. On the shots checked it was 0.5–1.4 s early. The script instead uses the ignitor's own electrical spike, which lands on every PT channel simultaneously and is the best time reference in the recording. It is reported in the plot title so you can see where in the record the shot actually happened.

Known data-integrity issues to watch

• The TDMS writer looks size-capped. Every TDMS file is byte-identical in size between shots, and for one August shot the Excel export carried 710 000 samples against 700 000 in the TDMS. A long shot will lose its tail with no error. Worth checking before a campaign that matters.
• Record lengths are inconsistent between streams within one shot, and one September shot recorded an empty Pitot file.
• Water-curtain state lives only in Ignition Report*.csv, not in the TDMS. Water flow rate and supply pressure are not recorded anywhere; the nozzle is hand-noted in the analysis workbook as "Flat Fan 40deg 005, 2 bar". Adding flow and pressure as Mixed-Outputs channels would put them in the TDMS automatically.