- Author
- Byron Raal, CAS Founder-Editor About the author
- Checked against
- AS/NZS 1200AS/NZS 3788AS 4343
- Date last checked
- 10 August 2026
How do I work out the AS 4343 hazard level of my air receiver?
Use H = P x V x Fc x Ff x Fs, where P is design pressure in MPa, V is volume in litres, Fc is 10 for a gas such as clean compressed air, Ff is 1.0 for a non-harmful gas, and Fs is 1 for standard service. For a typical compressed air receiver this simplifies to H = 10 x P (MPa) x V (litres). A 1,000 L receiver at 800 kPa (0.8 MPa) gives H = 10 x 0.8 x 1,000 = 8,000, which sits in Hazard Level C. Registration is state and territory specific: design registration generally applies at Hazard Levels A to D, and item (plant) registration generally applies to boilers and pressure vessels at Hazard Levels A to C, subject to jurisdictional exceptions. Victoria has not required item registration since 1 July 2014. A widespread industry error compares the bare pressure times volume product with these thresholds. It is a different number, and it produces the wrong level.
Key takeaways
- The AS 4343:2014 hazard level formula is H = P (MPa) x V (litres) x Fc x Ff x Fs, not the raw pressure times volume (pV) product.
- For clean compressed air at standard service, Fc = 10, Ff = 1.0 and Fs = 1, so the formula simplifies to H = 10 x P (MPa) x V (litres).
- The band edges are powers of ten and half powers of ten: 10^2.5 (316.2), 10^3 (1,000), 10^4 (10,000) and 10^8.5 (316,227,766). The 10^8.5 figure at the top of Level B is genuine, not a transcription error.
- AS 4343:2014 contains a drafting tension at those edges. Table 2.1 as printed and Clause 2.2.8 do not agree about a value landing exactly on a boundary. CAS resolves it in favour of Clause 2.2.8, which sends the boundary value down to the lower level. See the section below.
- A common industry miscitation (“Class B above 100 L at 800 kPa”) is wrong by two hazard levels. For clean compressed air at standard service the Level B threshold is a little above 1,250 L at 800 kPa.
- Hazard level decides registration. It does not decide in-service inspection cadence for a compressed air receiver, which comes from the pV product in AS/NZS 3788:2024 Table 4.1 Item 6. The two numbers are not interchangeable.
Work out your own number
The arithmetic below is the whole method, and you can do it on paper. If you’d rather not, the AS 4343 hazard level calculator runs the same formula, shows each factor as it’s applied, and tells you what the result usually means for registration in your state. This page explains the standard behind the tool. It doesn’t repeat the tool.
Want this as a worksheet for your own receivers?
Free two-page PDF. It runs the AS 4343 hazard level calculation in five lines and pairs the answer with the registration checklist for your state or territory. Print it, work through one receiver, and you will know where every vessel on site stands. Email only, and the download is on the next screen.
We email you the PDF and nothing else unless you ask. Form not loading? Email byron@compressedairsolutions.com.au with your enquiry.
How the hazard level is computed
AS 4343:2014 Clause 2.2.1 specifies the hazard level computation method as H = P x V x Fc x Ff x Fs, where:
- P is design pressure in MPa. Use the design pressure on the nameplate, not the pressure the compressor happens to be running at.
- V is volume in litres.
- Fc is the compressibility and mass factor (Table 3.1): 0.1 for vacuum, 1 for liquid, 10 for gas.
- Ff is the fluid contents factor (Table 3.1 and Clause 2.2.2): 1/3 for non-harmful liquid, 1.0 for non-harmful gas, 3 for harmful, 10 for very harmful, 1000 for lethal.
- Fs is the location or service factor (Clause 2.2.5): 1 for standard service, and 3 where one of the special conditions in Clause 2.2.5(a) applies, including a human-occupancy vessel. Higher factors apply to the elevated special conditions the clause lists. If you’re not sure whether a special condition applies to your vessel, that’s a question for a competent person, not for a web page.
Clean compressed air is treated as non-harmful gas up to 120 degrees C. That is an air-specific exception: Clause 3.2.6 and Table 3.1 set the generic non-harmful gas ceiling at 90 degrees C, and lift it to 120 degrees C for air. Above 120 degrees C the air is treated as harmful gas. Contamination, including oil carryover, may also change the contents classification, depending on what’s in the air and how much, so establish the right Ff for the actual service with a competent person.
For clean compressed air at standard service, three of the five factors are fixed, and the formula collapses to:
H = 10 x P (MPa) x V (litres)
Worked the other way, at 800 kPa the band edges land at roughly 40 L, 125 L and 1,250 L. A 100 L receiver at 800 kPa gives H = 800 and sits in Level D. A 1,000 L receiver at the same pressure gives H = 8,000 and sits in Level C.
The bands, and the tension in the standard
Table 2.1 of AS 4343:2014, as printed, sets the bands like this:
| Hazard level | Table 2.1 as printed |
|---|---|
| A | H > 10^8.5 (above 316,227,766) |
| B | H >= 10^4 to < 10^8.5 |
| C | H >= 10^3 to < 10^4 |
| D | H >= 10^2.5 to < 10^3 |
| E | H < 10^2.5 (below 316.2) |
Clause 2.2.8 then says: “If the product P x V or P x D equals the value of boundary between two categories, then the lower category shall apply.”
Those two statements don’t agree. Read literally, Table 2.1 puts H = 10,000 at the bottom of Level B, because the B row opens with a greater-than-or-equal sign. Clause 2.2.8 sends the same value to Level C. The same conflict sits at the C/D edge (H = 1,000) and the D/E edge (H = 316.227766).
The A row is the tell. It is the only row in the table that uses a strict greater-than, which is exactly what Clause 2.2.8 requires: a vessel landing precisely on 10^8.5 stays in B rather than being promoted to A. The clause and the top row of the table agree with each other. The lower rows do not follow the same convention.
CAS resolves this in favour of Clause 2.2.8, consistently across all nine pressure vessel registration pages and in the calculator’s classification logic. A value landing exactly on a boundary takes the lower level:
| Hazard level | CAS reading (Table 2.1 read with Clause 2.2.8) |
|---|---|
| A | H above 316,227,766 (10^8.5) |
| B | above 10,000 up to and including 316,227,766 |
| C | above 1,000 up to and including 10,000 |
| D | above 316.2 up to and including 1,000 |
| E | 316.2 (10^2.5) and below |
Two things follow, and both are worth knowing before an audit.
First, the practical effect is small but real, and it always runs in the operator’s favour. Exactly 1,250 L at exactly 800 kPa gives H = 10,000 and is Level C on this reading, not Level B.
Second, if an inspector, a supplier or a regulator hands you a classification that reads a boundary value the other way, they are not making an error against the text of Table 2.1. They are reading the table without Clause 2.2.8. Say so plainly, cite the clause, and let the paperwork record which reading was applied. Where a boundary value is genuinely load bearing for a registration decision, get it confirmed by a competent person rather than settling it from a table on any website, including this one.
Note also that 10^8.5, or 316,227,766, is a genuine boundary in the published table and not a typo for 10^8 or 10^5. Nothing in compressed air service comes anywhere near it, which is why the A/B edge almost never matters on an industrial site.
The error this page is built to prevent
A widespread industry miscitation treats the pV product, pressure multiplied by volume in kPa.L or MPa.L, as directly comparable to the AS 4343 hazard level thresholds. It’s not the same number. H carries the Fc, Ff and Fs factors, and for gas contents Fc alone is 10, so a bare pV product understates H by a full order of magnitude.
The specific instance is a piece of guidance that circulated for years: “Class B for receivers above 100 L at 800 kPa”. Applying the full formula, a 100 L receiver at 800 kPa gives H = 800, which is Level D. Level B starts a little above 1,250 L at the same pressure. The miscitation is wrong by two hazard levels. A site that believed it may have paid for item registration on vessels that only ever needed design registration, or none at all. And the habit it grew from, comparing a bare pV product against the AS 4343 thresholds, cuts the other way too: raw pV understates H tenfold for clean air, so a genuine Level C receiver reads as Level D and sits unnotified.
Auditing one site’s hazard level paperwork against the formula above is a one day exercise. It’s usually the cheapest compliance work available.
Hazard level is not inspection cadence
This trips up more people than the boundary question. AS 4343 hazard level decides registration. For a compressed air receiver, in-service inspection cadence comes from a different standard and a different number.
AS/NZS 3788:2024 Table 4.1 gives compressed air containing vessels their own row, Item 6, and classifies them by the pV product rather than by hazard level. Three bands:
- pV below 100 MPa.L: no commissioning inspection, no first in-service inspection. External, internal and extended intervals fall to Note 6.
- pV from 100 to 150 MPa.L: commissioning inspection required, no first in-service inspection. Intervals again fall to Note 6.
- pV above 150 MPa.L: commissioning inspection required, no first in-service inspection, external inspection at 2 yearly intervals, internal inspection at 4 yearly intervals, and an extended interval of 12 years available under the standard’s conditions.
One caution at the first edge. As printed, the operators in the first two rows of Item 6 overlap at exactly pV = 100 MPa.L, so the table claims that value twice, once with commissioning “No” and once with commissioning “Yes”, and Amendment 1:2025 does not touch Table 4.1. Nothing on this page turns on landing exactly on 100, and nothing on your site should either. If your receiver computes to exactly 100 MPa.L, the conservative course is to treat the commissioning inspection as required, and to have a competent person record the call.
Note 6 itself is worded in “should”, not “shall”, and it does not state intervals. It neither exempts a vessel from periodic inspection nor mandates one. So for a receiver at or below 150 MPa.L the honest answer is that the table stops short of setting an interval, and the interval has to be established with the competent person rather than read off the standard. Anyone quoting you a fixed cadence for a small receiver as though the standard prescribed it has gone beyond the text.
For scale, a 1,000 L receiver at 1,000 kPa gives pV = 1,000 MPa.L, well inside the top band. A 200 L receiver at 800 kPa gives 160 MPa.L, also inside it. Most plant room receivers are in the 2 yearly external and 4 yearly internal regime, and the Note 6 bands only come into play for genuinely small vessels.
Confirm your registration duty with a specialist
If your receivers were registered against the old “Class B above 100 L” guidance, you might be paying for plant registration you never needed, or running a Level C vessel that was never notified. Both are worth knowing before your next inspection or audit.
Send three things and I’ll have an independent pressure-equipment specialist sanity-check your hazard levels against the AS 4343 formula above:
- your site postcode
- your state or territory
- what you need proved (over-registered? due for inspection? buying, selling, or relocating a vessel?)
That is enough for an initial enquiry. Include your location and what you need to resolve; equipment records and operating details may be needed for a specialist assessment. CAS does not sell equipment or carry out inspections. We can review your brief and discuss a suitable Australian provider if one is available. We ask for your written permission before an introduction and explain any referral payment arrangement. There is no cost to enquire.
Form not loading? Email us directly at byron@compressedairsolutions.com.au with your enquiry.
Frequently asked questions
What is the AS 4343 hazard level formula?
H = P x V x Fc x Ff x Fs, where P is design pressure in MPa, V is volume in litres, Fc is the compressibility and mass factor (10 for gas), Ff is the fluid contents factor (1.0 for non-harmful gas such as clean compressed air), and Fs is the location or service factor (1 for standard service). For a typical clean compressed air receiver at standard service this simplifies to H = 10 x P (MPa) x V (litres).
Why can’t I just multiply pressure by volume to get my hazard level?
Because the pV product on its own omits the Fc, Ff and Fs factors that AS 4343 requires. For gas contents, Fc alone is 10, so a bare pV product understates H by an order of magnitude. The pV product is not useless: it is the number AS/NZS 3788 Table 4.1 Item 6 uses to set inspection cadence for compressed air receivers. It is simply not the number AS 4343 uses to set hazard level.
What happens if my result lands exactly on a hazard level boundary?
AS 4343:2014 is not fully consistent on this point. Table 2.1 as printed opens the B, C and D bands with a greater-than-or-equal sign, which would put a boundary value in the higher band. Clause 2.2.8 says that where the product equals the value of a boundary between two categories, the lower category applies, and the A row of the table follows that convention. CAS applies Clause 2.2.8: an H value of exactly 10,000 is Level C, not Level B. Where a boundary value is load bearing for a registration decision, have a competent person confirm the classification and record which reading was applied.
Is 316,227,766 really a hazard level boundary?
Yes. The AS 4343 band edges are powers and half powers of ten, and the top of Level B is 10^8.5, which is 316,227,766. It looks like a typographical accident and is not one. No compressed air receiver approaches it.
At what temperature does compressed air stop being a non-harmful gas?
120 degrees C. AS 4343 sets the generic non-harmful gas ceiling at 90 degrees C in Clause 3.2.6 and Table 3.1, and air is the explicit exception at 120 degrees C. Above that, or where the air is contaminated or oil laden, the contents classification changes and Ff rises with it.
Does my hazard level decide whether I have to register my receiver?
It decides the trigger, but the duty is state and territory specific. In model WHS jurisdictions, pressure equipment other than pressure piping at Hazard Levels A to D generally requires design registration, and item (plant) registration generally applies to boilers and pressure vessels at Hazard Levels A to C, subject to jurisdictional exceptions such as gas cylinders, automotive LP Gas fuel vessels, serially produced pressure vessels, AS/NZS 1200 exclusions, and in NSW an additional exception for pressure vessels that do not require periodic internal inspection. Victoria has not required item registration since 1 July 2014, though design registration and inspection and maintenance duties still apply. Confirm your duty with your state or territory WHS regulator, and see the pressure vessel registration guide for the lodgement pathway in your jurisdiction.