What the receiver size and bar-litre checker works out
A receiver is the steel tank that sits between the compressor and the pipework, storing air at pressure so the compressor is not switching on and off every time a tool draws a breath of air. The checker takes the figures that describe your system, your air demand, what the compressor delivers, and the pressure at which it switches on and off (cut-in and cut-out), and returns three things: the receiver size in litres, the bar-litre product at maximum pressure, and whether that product puts the vessel over the threshold that triggers a written scheme of examination (a documented inspection programme) under the Pressure Systems Safety Regulations 2000.
The arithmetic is straightforward once the inputs are right, but the inputs have to come from the vessel actually installed, its data plate and its measured working pressure. Get the maximum allowable pressure wrong by a bar and the bar-litre product can move enough to change the answer entirely.
Bar-litres, defined
Bar-litres is the product of a receiver's capacity in litres and its maximum allowable pressure in bar. It is not a measure of how much air is in the tank at any given moment; it is a fixed property of the vessel itself, set by its size and its pressure rating, and it is the figure the Pressure Systems Safety Regulations 2000 uses to decide whether a written scheme of examination applies.
Suppose a receiver holds 24 litres and is rated to a maximum of 10 bar. The bar-litre product is 24 × 10 = 240. Suppose instead the receiver holds 270 litres at the same 10 bar rating: the product is 2,700, more than ten times over. The size of the tank moves the figure a long way faster than the pressure does, which is why a small compressor with an oversized receiver bolted on can cross the threshold that a similarly powered unit with a modest tank never reaches.
The 250 bar-litre line and what crossing it means
Regulation 2(1) of the Pressure Systems Safety Regulations 2000 (SI 2000/128, Great Britain) sets 250 bar-litres as the point above which a written scheme of examination is required for the vessel. Below that line, the regulations still apply to the system as a whole, but the specific written-scheme duty in regulation 8, requiring the user to ensure the system is examined in accordance with the scheme at the intervals it specifies, does not bite on the receiver alone.
Once a vessel is over the line, examination has to be carried out by a competent person (someone with sufficient knowledge and experience to examine a pressure system safely, as the regulations define the term), at intervals set out in the scheme. HSE's approved code of practice L122 gives guidance on how those intervals are normally set. None of this is a decision the checker makes for you: it tells you where the bar-litre arithmetic lands, and whether a written scheme follows is then a question for the competent person assessing your actual installed system.
A receiver just under the line, and one just over
Say a workshop runs a 24-litre receiver rated to 10 bar. The bar-litre product is 240, under the threshold, so the written-scheme duty does not apply to that vessel on its own. Add a modest amount of extra capacity, say a 26-litre tank at the same pressure, and the product becomes 260: over the line. The step from unregulated to regulated can turn on a couple of litres of tank capacity or a single bar of rated pressure, which is exactly why guessing the figures is the mistake to avoid.
Cut-in and cut-out pressure, and why they matter to sizing
Cut-in and cut-out are the pressures at which the compressor's control switch starts and stops the motor: cut-in when stored pressure has fallen enough to need topping up, cut-out when it has built back up to the set maximum. The gap between the two, together with demand and delivery, decides how large a receiver needs to be to avoid short-cycling the compressor, starting and stopping it too often for the motor and contactor to cope with comfortably.
A bigger receiver smooths out the cycling for a given demand, but it also raises the bar-litre product if it is rated to run at the same maximum pressure, since capacity and pressure both feed the same multiplication. Sizing for comfortable running and sizing against the 250 bar-litre threshold are two different questions that happen to share the same inputs, and a receiver chosen purely to reduce cycling can land on the wrong side of the threshold without anyone intending it to.
Mistakes worth checking for
- Using the compressor's rated pressure. The two are often the same number but not always, and the bar-litre figure depends on the vessel's rating, stamped on its own data plate.
- Treating a small tank as automatically exempt. A small receiver run at a high pressure can cross the threshold just as easily as a large one at low pressure; the product is what counts, not either figure alone.
- Forgetting that more than one receiver in a system adds up. Where a system has more than one vessel, each one's bar-litre product and its combined effect on the written-scheme question is something a competent person needs to look at.
- Taking a screening answer as a certificate. A figure that comes out under 250 bar-litres tells you the specific written-scheme duty in regulation 8 does not apply to that vessel; it says nothing about the condition of the vessel, its fitness for continued use, or the wider duties the Pressure Systems Safety Regulations place on the system as a whole.
What the checker can tell you, and what it can't
The checker applies the bar-litre arithmetic to the numbers entered and states where the result sits against the 250 bar-litre line. It can return the wrong answer for a system where the receiver's actual data plate figures were mistyped, where the vessel has been modified or re-rated since manufacture, or where cut-in and cut-out settings have drifted from what is actually programmed into the control switch. It also cannot tell you whether a vessel already over the threshold has a written scheme in place, whether that scheme is being followed, or whether the vessel is fit to keep running: those are judgements a competent person makes against the physical vessel.
Whether your own system needs a written scheme of examination, and whether it complies with the Pressure Systems Safety Regulations more broadly, is decided by a competent person's assessment of the system you actually have installed.
Where to go next
The full arithmetic behind bar-litres, the regulation and paragraph numbers, and the decision tree for a written scheme are set out in Sizing the Air Receiver, and the 250 Bar-Litre Line. If a receiver is turning out to be a legal question as well as a sizing one, cases HSE has actually taken on pressure systems are worked through in What HSE's Pressure Systems Cases Teach a Workshop Owner. Once the receiver and demand figures are settled, Get Compressor Quotes: The Figures to Send, the Questions to Ask covers what to hand a distributor next.