Guide
How to check how much acetylene is in a cylinder: tare gross weight and the 0.91 m³/kg conversion
Weigh the acetylene cylinder, subtract the stamped tare (which includes porous mass, solvent and saturation gas at 15 °C), and convert net kilograms at about 0.91 m³ per kg (0.90 at 15 °C, 0.92 at 20 °C). A weight below tare means acetone has been lost and must be replenished by the filler.
- Gas density
- 1.0896 g/l at 20 °C and 1 atm; 1.109 g/l at 15 °C
- What the tare includes
- Cylinder
- Residual pressure for a solvent check
- Below 7 bar for cylinders with a working pressure of 17 bar or higher; below 4 bar for lower working pressures; emptied at about 1/8 of content per hour
- Scale tolerance
- ±20 g up to 6 l water capacity; ±50 g from 6 to 20 l; ±100 g above 20 l; scales checked daily and calibrated annually
- Condemnation on tare loss
- A cylinder that loses more than 5 % of its tare weight shall not be filled and shall be destroyed
The amount of acetylene in a dissolved acetylene cylinder is found by weighing, not by reading the gauge. The cylinder carries a stamped tare that includes everything except the deliverable gas; the difference between the gross weight and that tare is the acetylene, and one kilogram of it is about 0.91 cubic metres (0.90 at 15 °C, 0.92 at 20 °C). The same weighing tells the filler whether acetone has been lost, which is why Indian filling law is written entirely in kilograms.
What the stamped tare means
A DA cylinder's tare is not simply the empty steel weight. IS 16271:2014 cl 2.14 defines it as the weight of the cylinder with valve, porous mass, solvent, and the acetylene that saturates the solvent at atmospheric pressure and 15 °C. Rule 37(ix)(i) of the Gas Cylinders Rules 2016 requires it stamped inclusive of the valve, and IS 7312 lists tare weight in kg and, since Amendment 3 of 2007, the solvent weight in kg among the permanent markings.
The saturation gas is included because a cylinder is never empty of acetylene. At atmospheric pressure and 15 °C acetone still holds 34 g of acetylene per kg (Hölemann and Hasselmann, as compiled by S. A. Miller, 1965), and that gas is not deliverable in practice. Defining the tare this way means that:
- gross weight minus tare = deliverable acetylene, and
- a gross weight below the tare = a loss of solvent.
Both facts are used at every filling. Rule 41(1) requires the filler to record, for each charge, the empty cylinder weight without gas, the weight of solvent charged before gas charging and the full weight, and Rule 42 requires the label to show the weight of gas filled and the full cylinder weight.
The conversion from kilograms to cubic metres
Acetylene has a molecular weight of 26.038 and a gas density, as compiled by Miller from the measurements of Bottomley, Reeves and Seitlow, of 1.17167 g/l at 0 °C, 1.10898 g/l at 15 °C and 1.08959 g/l at 20 °C, all at 1 atm. Hence:
| Reference condition | Density | Volume of 1 kg |
|---|---|---|
| 0 °C, 1 atm | 1.172 g/l | 0.853 m³ |
| 15 °C, 1 atm | 1.109 g/l | 0.902 m³ |
| 20 °C, 1 atm | 1.090 g/l | 0.918 m³ |
The trade figure of 0.91 m³ per kg sits between the 15 °C and 20 °C values. Whichever reference KIGL prints on its label should be used consistently; the difference between 15 °C and 20 °C is under 2 %.
The procedure for a user
- Close the valve and let the cylinder stand at room temperature. Weight does not depend on temperature, but a cylinder that is frosted or wet carries water and ice that will read as gas.
- Weigh the cylinder on a platform scale reading to 100 g or better, with the cap on if the tare was stamped with the cap, and without the regulator.
- Read the tare from the shoulder stamping or data ring. Do not use the tare from a different cylinder of the same size; tares differ with the mass and solvent charge.
- Subtract: net acetylene (kg) = gross − tare.
- Convert: net × 0.91 ≈ m³ (use 0.90 for 15 °C or 0.92 for 20 °C if a reference temperature matters). Compare with the "weight of gas filled" on the filling label to see what fraction remains.
Worked example, illustrative only: a cylinder stamped tare 72.0 kg is delivered with a label showing 7.6 kg of gas filled (gross 79.6 kg). After a week on a cutting job it weighs 76.5 kg. Net gas is 4.5 kg, about 4.1 m³, or 59 % of the fill. The gauge on a warm afternoon might show 14 kg/cm²; the weight says the cylinder is a little over half full.
Rules that matter
A user only needs the net weight. The filler needs one more number: whether the solvent is still at its specified charge. IS 16271:2014 cl 3.3.1, which draws on ISO 11372:2011, sets out the method:
The acetone deficiency check the filler makes
- Special care is needed if the cylinder has been exposed to a significant temperature change in the preceding 3 hours; the cylinder must reach room temperature before measurement.
- The cylinder is emptied to a low residual pressure, typically below 7 bar for cylinders with a working pressure of 17 bar or higher and below 4 bar for lower working pressures, at a slow rate of about 1/8 of the maximum acetylene content per hour so that cooling and carry-over do not distort the result.
- Pressure, temperature and weight are measured. Annex B gives the acetylene-to-solvent ratio F as a function of the residual pressure and temperature, so the residual acetylene mass can be deducted.
- If the weight after deducting residual gas is below the stamped tare, the solvent loss is replenished with the same solvent type; the solvent type is never changed for a given cylinder. If it exceeds the stamped tare, the cylinder is examined and the reason determined before further handling.
- During replenishment the pressure is limited to a few bar above the emptying pressure so that the porous mass is not damaged.
AIGA 037/16, which adopts EIGA's method, gives the scale tolerances for this measurement: ±20 g for cylinders up to 6 litres water capacity, ±50 g from 6 to 20 litres and ±100 g above 20 litres, with scales checked daily and calibrated annually. The Gas Cylinders Rules 2016 Schedule IV A-2.2 requires weighing equipment at a testing station to have an error not greater than ±0.1 %, checked by test weight daily when in service, with test weights themselves calibrated every two years.
The whole system rests on the stamped tare being genuine. CGA C-13's prefill check includes looking for evidence of an altered tare, because a re-stamped tare silently breaks every calculation above. The filling inspection page covers the rest of the pre-fill checks.
Why weight matters for safety as well as stock
Two documented cylinder explosions with published investigations, in Great Britain in 2010 and Taiwan in 2005, both involved a settled or voided porous mass together with too little acetone relative to the acetylene, and in the Taiwan case poor residual-gas measurement was identified as what hid the shortage. The British cylinder was 75 years old with a charcoal mass, so read-across to a modern monolithic fleet is limited, but the solvent lesson stands. Solvent deficiency shifts the acetylene-to-solvent ratio past the backfire line set at type approval, and the Explanation to Rule 37 of the Gas Cylinders Rules 2016 states the consequence directly: a shortage of solvent makes the cylinder less resistant to decomposition from flashback, and replenishment before refilling is essential. The weighing described above is therefore a safety-critical measurement, not a stock-control convenience.
Rule 36(1) adds a hard limit: a cylinder that loses more than 5 % of its tare weight shall not be filled and shall be destroyed. For a DA cylinder, whose tare includes the mass and the solvent, a loss of that size means serious shell corrosion or loss of mass, and either voids the filling arithmetic entirely.
Acetone leaves in two ways, and weight catches both
Acetone is lost as vapour, governed by pressure and temperature, and as liquid, governed by withdrawal rate and orientation. The vapour fraction of the withdrawn gas is about 1.2 % at 15 atm and 20 °C but 23 % at 1 atm, and 55 % at 1 atm and 40 °C (from Hölemann and Hasselmann's data as compiled by Miller), so most vapour loss happens in the last atmospheres of a run-down cylinder, as the carry-over page sets out. Liquid loss comes from over-drawing, which froths the swollen solvent out through the valve, or from a cylinder lying down with liquid at the valve. A weight below tare at return does not say which happened, but it records that one of them did, and the filler makes it up before the next charge.
Rules that matter
Rules to apply
- Use net weight for every decision about contents: ordering, job planning and deciding when to change over.
- Change over while some positive pressure remains; do not run the cylinder down to recover the last of the charge, which costs acetone and delivers acetone vapour to the torch.
- If a cylinder weighs less than its stamped tare, do not use it; return it and tell the filler.
- Never alter, grind or overstamp a tare. A cylinder with an illegible or incorrect stamping is rejected at periodic inspection under IS 8433:2017 Annex B.
- Keep the filling label on the cylinder; it carries the gross weight when full that your weighing is compared with.
KIGL's dissolved acetylene is filled by weight with the gas weight on the label; for what else to look for when a cylinder is delivered, see buying acetylene — what to check and the safety data sheet.
How — frequently asked questions
How do I calculate how much acetylene is left in my cylinder?
Weigh the cylinder on a platform scale, subtract the tare stamped on the shoulder, and multiply the net kilograms by about 0.91 to get cubic metres (0.90 at 15 °C, 0.92 at 20 °C). For example, a cylinder stamped tare 72.0 kg that weighs 76.5 kg holds 4.5 kg, about 4.1 m³. The label from the last filling gives the gross weight when full for comparison.
Where is the tare weight marked on an acetylene cylinder?
It is stamped on the shoulder or a data ring under Rule 37(ix)(i) of the Gas Cylinders Rules 2016, inclusive of the valve, and IS 7312 requires the tare in kg among the permanent marks. It may be prefixed with a letter indicating the tare convention; the Rules require the tare to include the porous mass and the solvent saturated with acetylene at 15 °C.
Why does the tare include acetone and some acetylene?
Because a cylinder is never truly empty. Even at atmospheric pressure the acetone holds acetylene in solution, 34 g per kg at 15 °C. IS 16271 defines the tare as cylinder, valve, porous mass, solvent and that saturation gas at 15 °C, so that the net weight above tare is the deliverable acetylene and a weight below tare is a solvent loss.
What does it mean if the cylinder weighs less than the stamped tare?
Acetone has been lost, usually from over-drawing, running the cylinder down to very low pressure or storing it on its side. IS 16271:2014 cl 3.3.1 requires the filler to replenish the solvent loss before filling, with the same solvent type. A user should return such a cylinder and report it; the deficiency affects its resistance to flashback.
How accurate does the scale need to be?
AIGA 037/16 gives ±20 g for cylinders up to 6 litres water capacity, ±50 g up to 20 litres and ±100 g above, checked daily against a test weight and calibrated annually. The Gas Cylinders Rules 2016 Schedule IV require testing-station weighing equipment with error not greater than ±0.1 % and daily test-weight checks. A workshop platform scale reading to 100 g is adequate for stock control.
Can I just use the pressure gauge instead?
No. The gauge reads the equilibrium pressure at the cylinder's temperature, which for the same contents moves by about a third between 15 °C and 30 °C, and collapses when the cylinder is cold or being drawn quickly. Weight is the measure Indian law uses; the gauge is only a cross-check on the solvent ratio once the weight is known.
How does the filler allow for gas still in the cylinder when checking acetone?
IS 16271:2014 Annex B gives an equation for the acetylene-to-solvent ratio from the residual pressure and temperature, so the residual acetylene mass can be deducted from the measured weight before comparing with the tare. The check is valid only after about 3 hours at room temperature and with the residual pressure below 7 bar for cylinders with a working pressure of 17 bar or higher, or below 4 bar for lower working pressures.
References
- Asia Industrial Gases Association. (2016). Permissible Charge / Filling Conditions for Acetylene Cylinders (AIGA 037/16).
- Bureau of Indian Standards. (2014). Dissolved Acetylene Gas Cylinders — Inspection at Time of Filling (IS 16271:2014).
- Government of India. (2016). Gas Cylinders Rules, 2016 (G.S.R. 1081(E)). Petroleum and Explosives Safety Organisation.
- Linstrom, P. J., & Mallard, W. G. (Eds.). (n.d.). NIST Chemistry WebBook (NIST Standard Reference Database 69). National Institute of Standards and Technology. [acetylene gas-phase data] https://webbook.nist.gov/cgi/cbook.cgi?ID=C74862
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