Dacorum Sub Aqua Club

Compressor

This page provides information the air compressor and gas blending equipment.

We bought and installed the compressor in 2005 with financial help from Awards for All and technical help from Dacorum Borough Council.

Current configuration

We have an electric-powered diving air compressor, an air bank of three cylinders, a humidity indicator and a mixing panel with three 232 bar whips and a 300 bar whip.

The compressor is an LW 280 ES Silent that is able to compress 280 litres per minute (9.9 cfm) to 350 bar using 7.5 kW of power.

The compressor has an automatic filter drain which operates every 15 minutes with a loud woosh. The filters collect moisture, particles and most contaminants from the compressed air preventing them being passed to the diving cylinder.

The air bank consists of three 50 litre, 300 bar "J" cylinders that are automatically filled by the compressor when the pressure in a bank cylinder falls below 200 bar. The reason for having a bank is that the compressor is relatively slow at compressing air. The bank is filled automatically during idle periods storing a large volume of pressurized air so that a batch of cylinders can be filled quickly one after the other without being delayed by the slow running compressor.

There are two pressure gauges in the centre of the panel. Both measure the filling pressure. The digital gauge is useful for controlling the rate of decanting. The analogue gauge is a backup for the digital gauge.

The humidity indicator monitors the functioning of the filter that removes water from the compressed gas. Three light indicate the condition of the filters. If the red or amber light is not replaced by the green light 2 or 3 minutes after the compressor starts, the filter has failed and air from the compressor should not be used as it may lead to increased internal corrosion in your cylinder and the bank cylinders.

Using the compressor

The compressor is administered by the Compressor Officer (CO). Please, contact the CO in the event of any problem or to find out to get access to the compressor and to be trained in its use.

At present the shed is padlocked but there will soon be a combination lock will allow members access to the blending panel only. Normally club members will not have access to any equipment except the panel; the compressor and banks will be locked away and inaccessible.

Fills are not available to non-members.  

You MUST be trained and approved to operate the compressor unsupervised.

See the Using the compressor.

In addition to filling the cylinders with air we now have the ability to fill all the common diving gas mixes; nitrox, trimix and heliox. There is a lock to control access these additional cylinders to limit use of those expensive and potentially dangerous gases to paying and suitably qualified members.

The panel has inlets for three oxygen and three helium bank cylinders.

We also have a Haskel booster pump, which allows us to "top-up" cylinders to 232 bar with helium or oxygen from lower pressure bank cylinders and will help reduce costs by allowing oxygen and helium bank cylinders to be fully drained.

The 232 bar whips incorporate a one-way valves that prevent gas from diving cylinders contaminating the panel and banks. The third 232 bar whip has been left without a one-way valve so that it can be used to measure the initial cylinder pressure for the calculation needed in partial pressure nitrox filling.

Cost of gas

Air fills are free to DSAC members. Nitrox, Trimix and Heliox are all payable after you have filled your cylinders at a cost & maintenance rate. Commercially, it's not uncommon for a nitrox fill to cost £6 and a trimix fill £30, our costs are significantly lower than these prices.

We will need to buy oxygen and helium from suppliers such as Air Products. There are three components to the cost of this gas: the gas we actually decant and breathe, the rental of the cylinders and the gas we're unable to decant from the cylinders because the bank cylinder pressure is too low. There are two ways to reduce this last cost: either decant from the bank using the cascade system, where there are several bank cylinders and we use the lowest pressure cylinder first or buy a booster pump to drain the cylinder. Both techniques have costs. The cascade system is less physically efficient leaving more expensive gas in the cylinder and may involve renting more cylinders than are needed. Booster pumps are cost several thousand pound to buy and need professional maintenance.