Slocum Ballasting Procedure¶

Ballasting adjusts the glider's mass and mass distribution so it is neutrally buoyant and correctly trimmed in the water it will actually be deployed into — not the tank it's ballasted in. Get it wrong and the glider flies asymmetric dive/climb profiles, wastes buoyancy-pump energy fighting an unwanted heaviness or lightness, or in a bad case just sits on the bottom or can't submerge.

Before you start: vacuum¶

The glider must be closed with a proper vacuum before it is powered up — see the one-pager. Evacuate to at least 6 in Hg (shallow glider) or 7 in Hg (deep glider); pulling a higher vacuum than the minimum is better practice, since some air bleeds in once the glider is powered. Seal with the 7/16-20 PEEK MS plug torqued to 15 in-lb — PEEK is delicate, so use the correct tools and torque.

The goal¶

Ballasting is done with the glider in a defined reference state:

The ballast command (from lab_mode) drives all of these to that reference state in one step.

Setting the glider to ballast mode¶

1. Power the glider — external power cable (15 VDC) or the green go-plug.
2. If in PicoDOS, type app to start GliderDOS.
3. Ctrl-C (when prompted) to get control of the glider.
4. callback 30
5. lab_mode on
6. ballast — deflates the air bladder and zeroes the buoyancy pump and pitch vernier.
7. On external power: exit, wait for the glider to confirm it's OK to remove power, then pull the plug and fit the red dummy plug. On internal (battery) power the glider can stay on.

Reading tank density in real time¶

To compare the tank to the target deployment water you need live science data. Either drive the sensors directly:

put c_science_all_on 0     (all sensors, fastest sample rate; -1 = off)
put c_science_on 3         (display to screen; 1 = off)
put c_science_send_all 1   (send to the flight Persistor; 0 = off)

or use the shortcut mission files: loadmission sci_on.mi / loadmission sci_off.mi. Use the CTD's temperature and conductivity to compute the tank's salinity and density, and enter them — along with the target water's temperature and density — into TWR's Ballast Adjustment spreadsheet.

Immersing the glider¶

1. Lower the glider into the test tank. Purge every trapped air bubble from the nose cone and aft cover — bump the nose gently against the tank wall and tap the aft cover until nothing comes loose. HD (deep) pumps have a much larger internal volume to clear, so take extra care.
2. Fit the wings into the wing rails. If the tank isn't large enough to fly the wings, lay them stacked on the aft hull section, aligned with the wing-rail holes, and hold them with a wrap of electrical tape.

Adjusting the glider's weight¶

If the glider is too light, add weight. Two families of options:

• External hull weight (fast, no need to open the glider) — at the fore/aft end-cap joints, and at the fore/aft ends of the payload bay. This weight displaces water, so before converting it into an internal equivalent, correct for displacement: external stainless steel weight × 0.875, or external lead weight × 0.912, gives the internal weight it corresponds to.
• Internal weight and balance — stainless weights or lead shot in the two forward ballast bottles (60 mL each); factory steel weights in the unused battery brackets on 200 m gliders running lithium packs; pie-shaped stainless roll weights at the fore/aft ends of the payload bay; shifting payload or batteries; weight bars with tapped holes in the payload bay for bolt-on plates or ballast bottles.

If the glider is too heavy, hang spring scales from the fore and aft end-cap attachment points to read off the imbalance and total excess weight before removing anything.

Correcting tank weight to target water¶

The glider is ballasted neutral in the tank, but it needs to be neutral in the target deployment water, which almost never has the same density or temperature. The correction depends on the glider's displaced volume, a thermal expansion coefficient (β ≈ 535 × 10⁻⁶), and the density/temperature difference between tank and target water — use the Ballast Adjustment Spreadsheet rather than computing it by hand.

Confirm the exact figure for your specific hull/payload configuration with Teledyne — these are starting points, not a substitute for the spreadsheet.

H-moment¶

H-moment is the distance — 5–6 mm is ideal, 4–7 mm acceptable (bias toward 7 mm for heavy thruster use) — between the glider's center of buoyancy and center of gravity. It's the glider's equivalent of a boat's righting moment.

• Too large (stiff): the pitch battery has to travel further on every inflection, burning more energy; in a bad case the glider can't dive and climb at all and just "pancakes" through the water.
• Too small (twitchy): the glider makes constant small pitch corrections to stay stable, burning energy; in a bad case it can flip over and stay flipped.

Measure it with the glider powered and neutral in the tank: report ++ m_pitch m_battpos streams pitch (radians) and battery position (inches) every cycle — feed the readings into the Ballast/H-moment spreadsheet. Adjust with the pie-shaped roll weights at the payload bay ends: moving mass low increases H-moment (stiffer), moving mass high decreases it (twitchier). Measure H-moment before the final mass adjustment, and re-measure it any time you change the glider's configuration (new sensor, different battery, added payload) — you don't need to re-measure it on every routine ballast.

Equipment¶