Salt Spray Chambers - ACC46 SO2 gas dosing - how it works
ACC46 SO2 gas dosing
Designed to meet the requirements of ASTM G85 annex A4, this optional accessory provides a chamber mounted dispersion tube, through which SO2 gas can be introduced, at a rate determined by the user adjustable control valve, and monitored by an SO2 gas flowmeter. The gas can be switched on and off automatically, at user programmable times.
For completeness, the operation of the salt spray components and systems of the chamber, are also described below. This is because SO2 gas dosing is usually undertaken during salt spray testing.
SO2 gas cylinder
SO2 gas for gas dosing of the salt spray atmosphere is provided from a separate SO2 gas cylinder. The provision of the SO2 gas cylinder and cylinder connector (which should be manufactured from high grade corrosion resistant stainless steel) are the user's responsibility and are not included as part of this option.
SO2 flow meter and regulator
A precision graduated SO2 flow meter, with multi-turn needle valve, is factory fitted for monitoring and fine adjustment of the SO2 gas flowing into the chamber, during SO2 testing. This is located in-line between the SO2 gas cylinder and the SO2 dispersion tube inside the chamber, and mounted at a convenient height on the control panel. Operating in conjunction with this is a high grade stainless steel solenoid valve, which automatically turns the flow of SO2 gas to the chamber on and off and user programmable times. This enables complex gas dosing cycles, such as those specified in test standard ASTM G85 annex A4, to be programmed for fully automatic operation.
SO2 dispersion tube
SO2 from the gas cylinder is directed to the SO2 dispersion bar, via the SO2 flow regulator and gauge, where it is evenly introduced into the atmosphere of the chamber interior
Compressed air inlet
Compressed air from a separate compressed air supply is required for the chamber. The primary use of this is to atomise salt water into spray at the salt spray atomiser, located inside the chamber, during salt spray testing. The compressed air supply should be clean, dry and oil free, pressurised to within the limits 1.4 to 6.0 bar (20 to 87 P.S.I.). Between these pressures the air supply should be capable of delivering a flow rate of at least 75 Litres (2.6 cubic feet) per minute, which equates to a free flow at atmospheric pressure of approximately 102 standard litres (3.6 standard cubic feet) per minute.
Where a local supply is either unavailable or unsuitable Ascott offer an independent oil free air compressor as an optional accessory (ref.: ACC04). This requires connection to its own single phase electrical supply.
During salt spray testing, the compressed air utilised to generate the salt spray is bubbled through the air saturator (also referred to as a bubble tower or humidifier) in order to raise its humidity to c.100%RH at the point that it leaves the salt spray atomiser. This ensures a 'wet' and 'dense' salt spray is created. The temperature of the air saturator is user adjustable up to +63°C/145°F and is usually set several degrees above the chamber test temperature, in order to compensate for the cooling effect as the compressed air expands to atmospheric pressure on leaving the atomiser. Ascott air saturators are configured to top themselves up with water (distilled or deionised) automatically, but where a suitably pressurised supply of such water is unavailable, an optional accessory (ACC66) is offered to enable topping up of the air saturator by hand.
Salt solution reservoir
A separate salt solution reservoir is provided for ease of filling and cleaning. For floor standing chambers, it is mounted on castors for mobility. All reservoirs incorporate a transparent graduated sight glass for ease of monitoring the solution level inside. During salt spray testing, the salt solution (brine) is drawn from this reservoir by the chamber peristaltic pump, via a primary filter unit which removes any large undissolved salt crystals or other debris.
A single reservoir is supplied as standard with each new Ascott chamber (see chamber data sheets for capacity) - additional/larger reservoirs are available as optional accessories (ref; ACC02).
In addition, Ascott also offer an air agitating device which is fitted to the salt solution reservoir as an optional accessory (ref; ACC96) to keep the salt solution stirred and homogeneous, by continuously bubbling compressed air through it.
Salt solution pump and flow meter
During salt spray testing, the salt solution pump positively draws salt water into the chamber from the separate salt solution reservoir, by peristaltic action, so avoiding the need for a gravity fed system and the consequent difficulties in maintaining a constant 'head' of salt solution to be sprayed. This salt water is delivered, via a graduated flowmeter, to the salt spray atomiser inside the chamber. The speed of the pump is fully adjustable by the user, to allow the quantity of salt solution atomised and hence the fall-out rate, to be precisely controlled. Two pumps and flowmeters are fitted to 2000L size chambers.
Salt spray atomiser
This is a specially designed salt spray atomiser, made from thermally stable, high performance polymer, to guarantee a uniform salt spray and featuring an integral filter to minimise blockages. During salt spray testing, it is here that the compressed air, delivered via the air saturator, meets the salt water, delivered via the salt solution pump and flowmeter, to create a finely divided salt spray (also referred to as 'salt mist' or 'salt fog'). This oxygen rich and highly corrosive salt spray rises from the atomiser, into the chamber atmosphere, where it then 'falls-out' on to the test samples beneath. The fall-out rate can be adjusted and precisely controlled, according to international standards, by adjusting the speed of the peristaltic pump and the compressed air pressure. Two atomisers are fitted to 2000L size chambers.
Chamber heating mats
Silicone rubber encapsulated electrical heating mats are strategically bonded to the outside surfaces of the bottom and sides of the inner working chamber. Here they radiate heat uniformly into the chamber, under the control of the chamber temperature controller. In the salt spray/SO2 modes of operation, these heaters give the chamber a user adjustable temperature range from ambient to +50°C/122°F.
Forming the centre-piece of the ergonomically designed control panel is a state of the art Human Machine Interface (HMI). This incorporates alpha-numeric text messaging and digital displays of chamber variables such as temperature and time (see chamber data sheets for the type of HMI fitted). It is here that the user controls and monitors the various chamber functions. In addition, all chamber control panels incorporate an emergency stop and other safety facilities.
The chamber canopy is effortlessly opened/closed using pneumatic cylinders, which are activated at the touch of a button on the control panel. An automatic purge facility is incorporated to minimise the risk of corrosive salt spray/SO2 escaping into the laboratory when the chamber is opened.
Strategically located sensors, mounted inside the chamber and air saturator, monitor the climate continuously and convey this information to the Human Machine Interface (HMI), where it is displayed digitally at the control panel. Temperatures are monitored by PT100 precision temperature sensors and displayed to a resolution of 0.1°C. Most chambers (model specific - see chamber data sheets for details) are also equipped with a special design of corrosion resistant humidity sensor. This measures the relative humidity electronically inside the chamber and displays this at the HMI as %RH to a resolution of 1%RH. This humidity sensor has to be manually sheathed during SO2 testing, due to the risk of highly acidic H2SO4 forming on the sensor and damaging it.
Test samples are accommodated within the chamber generally by placing on, or by suspending beneath, sample racks. A full set of sample racks are provided as standard with each new chamber (see chamber data sheets for quantity) in a choice of styles. Additional sample racks are available as optional accessories, if required, as is a reinforced false floor (ref; ACC19) for supporting large/heavy test samples directly on the chambers internal base.
This vents spent salt spray/SO2 from inside the chamber to atmosphere. The chamber should therefore ideally be located adjacent to an outside wall, and a hole made through the wall to accommodate the exhaust pipe (diameter varies depending on chamber model). The end of this pipe should be directed away from persons and property, and be shielded from wind to prevent any disturbance to the conditions inside the chamber.
A floor level drain is required to remove excess salt fog condensate etc. which accumulates over the internal base of the chamber interior.
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