Glider ballast calculator

A glider changes depth by changing its buoyancy. Ballasting sets the vehicle's fixed mass so the buoyancy engine's travel spans neutral across the water it will fly in. Densities here come from the same full-precision TEOS-10 engine as the seawater calculator.

Under testing. This calculator is new and still being checked against use. It is provided as-is, with no warranty of any kind, and is used at your own risk. The vehicle values it ships are illustrative, not manufacturer data, and the result is an input to a tank test rather than a replacement for one.

The glider

Vehicle

Illustrative values, not vehicle data. Each volume is the mass divided by 1025 kg/m³, and the two hull terms are order-of-magnitude only. Replace all four with values from your ballast sheet or a tank test.

Total mass in air

Displaced volume

Hull compressibility

/dbar

Hull thermal expansion

/°C

Volume measured at

°C

Buoyancy pump travel

± cc

The ballast tank

Salinity

Value

Temperature

°C

Measured buoyancy

The reading from floating the glider in the tank, positive if it floats. Entering it determines the mass above. Leave it blank to use the mass as entered.

The water it will fly in

Ballast for	Point	Salinity	Temp °C	Depth m
	Surface
	Mixed layer
	Bottom inflection

Salinity given as

Position

A position applies the Absolute Salinity anomaly and sets the gravity used to convert depth to pressure. On a 50 L glider the anomaly is worth about a gram.

The answer

Ballast change: +48g
Then reads in the tank: −1433g
Neutral at: 1025.660kg/m³
Ballasted mass: 54.048kg

Add or remove that mass of ballast, then confirm the glider reads the tank figure in the tank water above.

Point	Density	Displaces	Buoyancy	Pump to neutral
Surface	1024.023	52.716 L	−65 g	+64 cc
Mixed layer	1025.660	52.696 L	0 g	0 cc
Bottom inflection	1032.229	52.597 L	+244 g	−236 cc

Can it surface?: +191g with the pump fully out
Can it dive?: −14g with the pump fully in

The tank is 27.3 kg/m³ lighter than the target water, so the expected tank reading is -1433 g.

The hull compresses less than the water: +18 g per 100 dbar of depth, at this temperature and salinity.

Cooling by 5 °C changes buoyancy by +43 g.

How to use it

Select the vehicle, then replace the values in the glider panel with those from your ballast sheet or tank test. The values that ship are illustrative.
Enter the tank water. A measured tank buoyancy, if you have one, determines the vehicle's mass, and the mass field follows it.
Enter the three water points from a cast, a climatology or a forecast: the surface, the mixed layer, and the deepest inflection of the mission.
Select which point to be neutral at. The ballast change is the result.

What it does

One equation

Buoyancy is the weight of water displaced less the weight of the vehicle: B = ρ(S_A, t, p) · V(t, p) − m. The density is in situ, at the point's own pressure and temperature. The displaced volume is the hull's, adjusted by its compressibility and thermal expansion.

Ballasting for a point sets m so that B is zero there. The rest of the page follows from that.

Buoyancy against depth

Whether a glider gains or loses buoyancy with depth is a comparison between two compressibilities rather than a property of gliders. A hull that compresses less per dbar than seawater gains buoyancy on descent; one that compresses more loses it. Seawater is about 4.3×10^-6 per dbar. The page reports the result for the entered vehicle in grams per 100 dbar, and does the same for a five-degree change in temperature.

Surfacing and diving

Neutral buoyancy at one point is not sufficient. The engine also needs enough travel to be positive at the shallowest point and negative at the deepest, with the pump at its stops. Both margins are computed at the shallowest and deepest of the three entered points, by pressure rather than by the order they were entered. A vehicle that cannot do both is reported as a warning.

Where it stops

The hull model is first order in pressure and in temperature, which is what a ballast sheet's two coefficients support. It does not account for oil volume changing with temperature, air trapped in the fairing, water absorption by foam, or vehicle attitude. It takes the three water points as given. The result is an input to a tank test, not a replacement for one.

Credits

Densities from TEOS-10 — see the seawater calculator for what that covers and how it is checked. The ballast arithmetic is this site's own. There is no published standard for it as there is for the equation of state, so it is checked against identities, against TEOS-10's compressibility, and against hand calculations.