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Measuring cracking effort

Posted by Pia Limpiyasrisakul on

Cracking or inhalation effort can be defined as the minimum amount of inhalation effort or vacuum required to start the airflow through the second stage regulator. 
In other words it is the amount of inhalation/effort/vacuum required to just open the valve of the second stage or, to just separate the soft seat (poppet) from the hard seat (orifice) of the second stage regulator and start air flowing through the second stage.
- After calibration of the differential pressure gauge, install the mouthpiece assembly onto the second stage by means of a band seal.  The hose of the mouthpiece assembly is connected to the inlet port of the differential pressure gauge. The regulator can now be pressurised.
- Gently inhaling or “sipping air” through the mouthpiece assembly will create a vacuum inside the second stage that will pull the onto the diaphragm. This flexing action of the diaphragm pushes onto the lever which in turn tries to pull the poppet away from the orifice. The total amount of inhalation effort required to slightly open the second stage and start the airflow will be displayed on the differential pressure or on a Magnehelic® gauge.
- Two methods are commonly used to measure the cracking effort  – you can choose either one to measure the cracking effort -:
o   Method 1: use of a differential pressure gauge only.
Inhale very slowly/gently through the mouthpiece adapter while watching the differential pressure gauge needle. When the first audible indication of airflow is heard, the position of the differential pressure gauge needle is recorded (this is the cracking effort).
o Method 2: use of a differential pressure gauge in combination with a intermediate pressure gauge.
This method requires in addition to a differential pressure gauge connected to the second stage an intermediate pressure gauge to be connected to the first stage. We recommend having both instruments side by side. As per method 1, inhale very slowly/gently through the mouthpiece adaptor whilst watching the intermediate pressure gauge. When the intermediate pressure on the intermediate pressure gauge drops, record the differential pressure gauge reading to determine the cracking effort of the 2nd stage. Compared to method 1, this method is more accurate and repeatable.
Note:  when the vacuum created by the technician reaches the point of poppet-orifice separation (iow. the valve cracks open), the intermediate pressure will drop slightly. The reaction of the intermediate pressure can be compared to slightly purging the second stage. As soon as an airflow is heard, the intermediate pressure drops.
- Gently inhaling/sipping air is crucial as cracking effort is by definition the minimum amount of inhalation required to start the airflow.
- The amount/value of the cracking/inhalation effort for a second stage regulator is specified by the manufacturer at a specific supply and intermediate pressure and will be different from model to model.
- How to adjust the cracking effort for a specific second stage is described in the manufacturer’s technical literature.
- mbar to “ (inches) of H2O: 1 mbar = 0.401865 “ (inches) H2O
- mm w.c. to “ (inches) of H2O: 1mm of water = 0.039370 “ (inches) H2O
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.

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