Adjusting downstream second stages
Posted by Pia Limpiyasrisakul on
NOTES:
The text below uses some simplified schematics to illustrate the effect of different adjustment options for downstream second stages or demand valves. As to isolate the effect of the different adjustment options, one adjustment option at the time is being shown whilst the other adjustment option remains fixed.
In reality both adjustment options go together hand in hand and changing one might require an adjustment of the other option as well.
The illustrations are for balanced second stages; however the principle remains the same for unbalanced second stages.
A WORD OF CAUTION:
Always follow the manufacturer’s recommendations with regards to setting-up and tuning second stages. Each brand and regulator type will have their own specific settings and procedures. These procedures and setting values can be found in the manufacturer’s literature and must be adhered to in order to ensure the correct and safe operation of the regulator. Many manufacturers will require either preset positions or gauges ensure the lever height is set correctly.
GENERAL RULES:
• When considering the options to adjust the cracking effort of second stages, the technician has the following options at his/her disposal:
o Exchanging parts as required with OEM parts suitable for the particular type of regulator (this could be part of the routine maintenance procedure or part of defective/worn out component replacement)
o Adjust the intermediate pressure of the first stage
o Adjust the lever height of the second stage
o Adjust the spring load of the second stage
o Adjust the intermediate pressure of the first stage
o Adjust the lever height of the second stage
o Adjust the spring load of the second stage
• It is futile attempting to adjust second stages if either the intermediate pressure of the first stage is unstable or outside the manufacturers specifications/range. In both cases, this issue has to be addressed prior to adjusting the second stage.
• The cracking effort will increase as the intermediate pressure decreases and vice versa. Therefore it makes sense to set up the cracking effort at the supply pressure that yields the highest intermediate pressure.
• Ensure proper cycling of both the first and second stage to ensure the soft- and hard seats are properly bedded in.
• When adjusting second stages, the final goal is to:
o Adjust the second stage for maximum lever height in order to allow for the maximum poppet-orifice separation that in turn will give the greatest flow through the second stage.
o Adjust the second stage for minimal spring tension in order to allow for the most favorable cracking effort.
o Adjust the second stage for minimal spring tension in order to allow for the most favorable cracking effort.
• In order to achieve the goal mentioned above, it’s good to remember that:
o The amount of inhalation or cracking effort depends on the amount of force that is required to depress the lever; iow. the amount of force required to pull the diaphragm down.
o The amount of force required to depress the lever and start the flow through the valve (iow. ensuring a positive poppet-orifice separation) depends on the amount of the spring tension.
o A minimum amount spring tension is required to overcome the downstream force introduced by the intermediate pressure – the intermediate pressure tries to push the valve towards an open position -. This minimum amount of spring tension ensures that poppet-orifice assembly remains airtight (no leakages or free-flow).
o The amount of force required to depress the lever and start the flow through the valve (iow. ensuring a positive poppet-orifice separation) depends on the amount of the spring tension.
o A minimum amount spring tension is required to overcome the downstream force introduced by the intermediate pressure – the intermediate pressure tries to push the valve towards an open position -. This minimum amount of spring tension ensures that poppet-orifice assembly remains airtight (no leakages or free-flow).
CHANGING THE ORIFICE POSITION – adjuster/nut fixed -:
• Turning the orifice in will:
o Decrease the lever height
o Increase the spring tension (spring compressed)
o Result in an increase of cracking effort
o Increase the spring tension (spring compressed)
o Result in an increase of cracking effort
• Turning the orifice out will:
o Increase the lever height
o Decrease the spring tension (spring relaxes)
o Result in an decrease of cracking effort
o Decrease the spring tension (spring relaxes)
o Result in an decrease of cracking effort
ADJUSTING THE SPRING TENSION – orifice position fixed -:
• Turning the adjuster/nut in will:
o Increase the lever height
o Increase the spring tension (spring compressed)
o Result in an increase of cracking effort
o Increase the spring tension (spring compressed)
o Result in an increase of cracking effort
• Turning the adjuster/nut out will:
o Decrease the lever height
o Decrease the spring tension (spring relaxes)
o Result in an decrease of cracking effort
o Decrease the spring tension (spring relaxes)
o Result in an decrease of cracking effort
DISCLAIMER:
Nothing contained in these notes or shall be construed to over-ride or replace the relevant standards or manufacturer’s recommendations, manuals, data or product specific training. The contents are believed to be correct to the best of our knowledge and are offered in good faith. No warranty is expressed or implied. The author, Scuba Clinic Co., Ltd. accept no liability for any loss, damage or injury however caused resulting from information contained in these notes. It is the responsibility of the reader to verify the correct information, practises and procedures prior to commencing work.