A single-lift gasometer stores gas beneath an inverted bell whose open rim dips into water. Gas entering the holder raises the bell; withdrawal lets it descend. The effective weight of the freely moving bell sets its approximate delivery pressure. Explore that force balance and the volume swept through a selected lift with the calculator below.
Gasometer (Form 1) Interactive Calculator
Compare ideal pressure, swept volume and a chosen water-head allowance by varying effective bell weight, diameter, lift, fill and factor.
Equation Used
Effective bell weight divided by circular plan area gives ideal gauge pressure. Seal Factor creates a comparison depth, not a verified requirement. Gas Volume is the volume swept through the selected fraction of lift, excluding fixed space beneath the crown.
- Bell weight is the effective net downward weight creating gauge pressure.
- Bell plan area is circular.
- Water gauge conversion uses 1 mm w.g. = 9.80665 Pa.
- The chosen factor compares an assumed depth with pressure head; actual immersion is not checked.
How a water-sealed bell holds gas
The bell is open underneath. Its rim remains immersed while a gas space forms above the water inside it. A pipe connects that space to the gas system, and guides constrain the bell as it rises or falls.
Gauge pressure acting over the bell’s horizontal area supports its effective downward load. In this simplified balance, the load and area remain constant, so calculated pressure does not change with fill. Buoyancy changes, guide friction, acceleration and loads from wind or accumulated material can change the behaviour of a real holder.
The water inside the bell stands below the outside surface by the pressure head. The bell rim must remain below that depressed surface to retain the seal. The least immersion throughout travel matters; a seal cannot be checked from the lowest bell position alone.
Parts to identify
- The crown and side walls form the moving bell.
- The open lower rim forms a seal with the tank water.
- The tank contains the sealing water.
- Guides and rollers constrain lateral motion.
- Gas piping connects to the space beneath the crown.
Gas storage and industrial heritage
Water-sealed holders stored manufactured gas and balanced supply with demand. Historic England describes the early single-bell arrangement and later telescopic forms in its introduction to gasworks and gasholders.
A documented single-bell example is Gasholder No. 2 at Fulham Gasworks, built in 1829–1830. Its listing describes twelve cast-iron guide standards, a wrought-iron bell and a brick tank. This example illustrates the components; its dimensions and service history are not inputs to this calculator.
Pressure, volume and a chosen seal allowance
For bell diameter D in metres, plan area A = πD²/4. Enter effective downward force W in kilonewtons: gauge pressure P = 1000W/A pascals. The displayed pressure head h is P/9.80665 millimetres of water.
Bell Lift L is the selected available travel in metres. Fill F is the percentage of that travel. The calculated swept volume is V = ALF/100 cubic metres. It excludes any fixed gas space beneath the crown and is not a volume corrected to standard temperature and pressure.
Seal Factor f multiplies pressure head: the displayed comparison depth is fh and the displayed allowance above the head is (f−1)h. The factor is your assumption. These outputs do not measure actual immersion or establish a safe seal, tank depth or structural design.
Example using the default entries
With 1039.59 kN effective weight and a 30 m diameter, plan area is 706.86 m². Pressure is approximately 149.97 mm water gauge. At a chosen factor of 1.5, the comparison depth is 224.96 mm and the allowance above the pressure head is 74.99 mm.
A 10 m lift at 100% fill gives 7068.58 m³ swept volume. At 50% fill, it gives 3534.29 m³ while the simplified pressure remains unchanged. At 0% fill, swept volume is zero; that does not mean every gas pocket inside a real holder has disappeared.
Doubling effective weight doubles pressure and both seal comparison outputs. Doubling diameter gives four times the swept volume and one quarter of the pressure for the same weight and lift. A factor of 1 gives zero allowance above the calculated pressure head.
What the calculation leaves out
A single bell has one available lift. Telescopic holders use multiple moving sections and additional seals; their engagement changes the loads and geometry. Their behaviour cannot be obtained by treating this single-bell drawing as a multi-lift design.
No structural thickness, guide load, corrosion allowance, gas flow capacity or dynamic stability is calculated. The drawings are explanatory sketches, not scaled installation drawings. The bell moves between zero lift and the selected fill at a fixed display pace; numerical outputs describe the selected fill setting. Diameter and lift adjust the drawn proportions, while the water-level difference is exaggerated and does not scale with pressure. The rim remains visibly immersed throughout this illustrative motion. Use the numerical outputs to study the ideal relationships rather than infer actual clearances or water levels from the artwork.
Frequently asked questions
Why does Fill change volume but not pressure?
The model keeps the effective weight and bell area constant. Pressure is weight divided by area; Fill changes only the fraction of selected lift used for swept volume.
Is Bell Weight a mass in kilograms?
No. The input is force in kilonewtons. It represents effective downward load after buoyancy or counterbalancing, not automatically the dry mass of the steelwork.
Does Seal Factor specify a required safety margin?
No. It is an editable multiplier for comparing water head with a hypothetical immersion. The tool does not check actual seal geometry or operation.
Does zero calculated volume mean the holder is gas-free?
No. The result is swept volume relative to the selected zero-lift position and excludes fixed spaces.
Can I use this result to change a working holder’s pressure?
The arithmetic alone cannot establish whether a pressure change is permissible. It does not check structural capacity, guide loads or available immersion.
References
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