Measuring intermediate pressure

In this article we’ll focus on:

• What intermediate pressure is.
• How to measure this first stage parameter .
• Normal and abnormal patterns or behaviours during your evaluation of the intermediate pressure.
• A brief introduction to adjustment options.

 WHAT IS INTERMEDIATE PRESSURE?

Intermediate pressure can be described as the amount of pressure that builds up behind a piston or diaphragm to overcome (1) the spring force and (2) the ambient pressure force. This build-up of pressure is required to close the first stage valve assembly, i.o.w. in order for the soft and hard seat assembly in the first stage to seal airtight.  See illustration below.

Furthermore, when dealing with unbalanced first stages, the intermediate pressure either can be assisted by (unbalanced diaphragm first stage), or resisted by (unbalanced piston first stage) the downstream supply pressure. This downstream supply pressure is responsible for variations in the intermediate pressure as the supply pressure decreases throughout the dive. The downstream supply pressure force is not shown in the above illustration and omitted intentionally. A more detailed description of this occurrence is outside the scope of this article.

The magnitude of the intermediate pressure is primarily set by the strength of the first stage spring.

Furthermore, the intermediate pressure will increase or decrease proportionally to the increase or decrease of the water depth (ambient pressure). This is a very important relationship in order for the regulator to function properly at depth.  The explanation of the latter is outside the scope of this article.

In the unfortunate situation that (1) the soft and hard seat assembly would not seal airtight and (2) the second stage valve assembly remains closed, air would continue to escape at the sealing point resulting in a rise intermediate pressure and this pressure would keep rising until the intermediate pressure would be strong enough to push the soft and hard seat in a closed position (seal the leak) or if the previous does not happen, equalize with the supply pressure. The above of course implies that none of the components would fail at these elevated pressures.

HOW TO MEASURE THE INTERMEDIATE PRESSURE?

All that needs to happen is to connect a pressure gauge to a low pressure port of the first stage. Ideally the gauge would have a scale of 0-20 or 0-25 bar (0 – 300 or 0 – 350 bar) and be fitted with a safety relief valve to protect the gauge just in case the pressure spikes beyond the range of the gauge. If your pressure gauge is not fitted with a safety relief valve, make sure a (downstream) second stage is connected.

The most common way to measure the intermediate pressure is using an intermediate pressure gauge that is fitted with a male BCD coupler that can be connected directly to the low pressure inflator hose, also called the BCD hose.

Alternatively to the above method, a pressure gauge could be connected to a low pressure regulator hose, provided the gauge has a suitable inlet fitting or have a  gauge directly connected to the low pressure regulator port if you have a pressure gauge that is fitted with a low pressure port adaptor (i.o.w. a 3/8 UNF male thread).

With the gauge connected to the first stage, slowly open the cylinder valve or air supply (set to the recommended supply pressure range) and watch the needle quickly swing out. On average – depending on model and brand – the needle would come to a stop in the 8.5 to 10.3 bar range (125 to 150 psi) and remain stationary. Ideally there should be no further needle movement -.   The point where the needle stops and remains stationary is called the lock-up pressure.

After your initial reading, give the second stage a gentle purge – or a couple of gentle purges - and evaluate the reading (lock-up pressure) once more. During the purge, the intermediate pressure is expected to drop then to rise again once the purge is over.

Now that you have established the value of the intermediate pressure, you have to compare the reading with the recommended value by the manufacturer and decide if this reading is acceptable or not.

It is important to pay attention to the fact that the specifications given by the manufacturer are related to a set supply pressure and this supply pressure is most often high supple pressure or a high supply pressure range. So check the literature as not to get caught out.  Hence it is important to make sure that your supply pressure is within that specified range in order to make any proper observations. Example: for the Apeks Flight, the acceptable range of intermediate pressure is 9 to 10 bar with an inlet pressure of 150 – 232 bar.

Said the above, we personally like to evaluate the intermediate pressure both at low and high supply pressures in order to get a better picture of the changes in intermediate pressure  throughout the whole supply pressure range.  This also helps to establish a better understanding of the overall condition of the first stage and allows you to check if the intermediate pressure follows the expected pattern for that particular type of regulator.

Depending of the type of first stage, the intermediate pressure will vary depending on the supply pressure.

The relationship of the intermediate pressure to the supply pressure can be summarised as follows:

• For unbalanced piston first stages, the intermediate pressure will drop as the supply pressure drops.
• For unbalanced diaphragm first stages, the intermediate pressure will rise as the supply pressure drops.
• For balanced first stages: the intermediate pressure is stable and doesn’t change regardless of the supply pressure. However, experience will show you that you can expect a few psi of change between low and high supply pressures.
• However, for the over balanced types of balanced first stages (example: Scubapro MK25), the intermediate pressure will rise as the supply pressure drops. This increase is noticeable but not as pronounced as unbalanced piston first stages. This type of overbalancing is NOT to be confused with over balancing terminology used to describe a faster increase in intermediate pressure as the depth increases due to the use of environmental seals compared to a ‘classic balanced’ model.

(*) Word of caution: as the needle swings out, observe if the needle would sneak past the expected range. If this happens at a fast speed, there is likely to be a problem with the first stage and most likely the safety valve is about to blow off. Under all circumstances it is advisable to avoid that the intermediate pressure would exceed the pressure rating of the gauge. Therefore the use of a pressure relief / safety valve is highly recommended. However, if your gauge is not fitted with a pressure relief valve, you can control this by gently purging the second stage as you open the cylinder valve or supply pressure and then slowly decrease the pressure on the purge button till the air flow subsides. If the pressure keeps rising well above the expected values, shut down the cylinder valve or air supply immediately. If you’re not trained in equipment servicing, this would be a good time to get in touch with a trained technician to have your regulator looked after. Failure to observe this safety advice could lead to failure of the equipment or personal injury.

If you’re looking for some advice to find yourself a suitable intermediate pressure gauge, check out this article: ‘Instrumentation options? Choosing the instrumentation that is suitable for you’ or you can have a look at our instrumentation page.

ABNORMAL PATTERNS OR BEHAVIOURS DURING YOUR EVALUATION OF THE INTERMEDIATE PRESSURE

Now that we understand the ‘normal pattern’, let’s have a look at some abnormalities that could occur.

CASE 1: DRIFT

What is drift? Drift is a slow and minimal increase of the intermediate pressure once the needle appears to have reached it maximum. We are talking here in magnitudes of a few psi or a few tenths of a bar before the needle locks-up.

As long as this drift is within the manufacturers prescribed range this is considered acceptable. Example: for the Apeks Flight, the acceptable range is 0.25 bar maximum for 15 seconds after purging the regulator and with an inlet pressure of 150 – 232 bar.

Positive drift occurs because the hard and soft seats (i.o.w. the first stage valve assembly) are not entirely sealed airtight and air is allowed to leak by the sealing point. The increase in pressure will come to a halt once additional pressure behind the piston or diaphragm is powerful enough to seal the valve assembly airtight.

Some common causes for positive drift are:

• Minor defect (flaw) in the soft and hard seat combination (example: tiny scratch)
• Inadequate cycling of the regulator after a rebuild (the break-in period is to be extended)

The opposite needle movement could also occur and we like to describe this as ‘negative drift’. This happens because of minor leaks in the low pressure side of the regulator or regulator set. If the drop is significant enough, eventually the first stage valve assembly would open and re-establish the intermediate pressure to see it drop again. This would keep cycling until the leak is found and corrected. Leaks are not acceptable and should be corrected by a competent individual.

The causes of these leaks could be numerous and be located either in the first, second stage or low pressure inflator hoses.

Some examples:

• Miniature punctures in any low pressure hoses
• Leak from a balance chamber in a second stage or any leaks at the second stage for that matter
• Minor defects in the O-ring gland of the piston stem or head for unbalanced piston first stages
• Minor defects in the O-ring gland of the piston head for balanced piston first stages
• Loose low pressure port plug
• Minor leak at the diaphragm of the first stage
• Minor leaks at a turret assembly

CASE 2: CREEP

What is creep? Creep is a rise of the intermediate pressure well above the expected range or an extreme manifestation of positive drift. The pressure would rise and rise and keep rising until: (1) a safety valve blows or, (2) a downstream second stage free-flows or, (3) the intermediate pressure rises enough to seal soft and hard seat airtight or, (4) some component fails (pressure gauge, hose…).

Creep is caused by the inability of the hard and soft seat to seal of airtight, i.o.w. you could consider this a high pressure leak caused by a very poor seating between soft and hard seat.

Some common causes of this to happen:

• Major defect (flaw) in soft and hard seat (example: big scratch)
• Removable hard seat installed the wrong way
• Missing O-ring in regulator with a removable hard seat
• Wrong installation of the components other than the removable hard seat (e.g. the smaller spring for diaphragm regulators)
• Non original parts (e.g. non-manufacturers soft seat)
• Misalignment of the components

Creep is not acceptable; the cause should be traced and corrected by a competent individual.

 A video of a regulator suffering from creep can be found here. 

CASE 3: SIGNIFICANT PRESSURE DROP WITH FULL PURGE

Up till now we have evaluated the intermediate pressure under ‘static’ conditions, meaning that there was no flow through the regulator. Dynamic intermediate pressure is the intermediate pressure under flow conditions. You already know that with a gentle purge, the intermediate pressure will drop. If you go for a full purge, you can expect this intermediate pressure to drop even more. To the latter there are designs out there that will reduce this significant reduction in intermediate pressure, an example hereof would be a Mares regulator with the DFC feature.

There are certain conditions that would make this intermediate pressure drop so low that this extreme drop in pressure would interfere with the proper operation of the regulator. The main cause of this to happen is a flow restriction.

Some common causes are:

• Cylinder or supply pressure valve not fully open (likely just cracked open)
• A flow restriction or blockage in a cylinder valve
• Extremely low supply pressure (a few bar or psi’s)
• A significant flow reduction through the first stage filter due to a blockage in the filter or an extremely contaminated filter
• Extremely worn (and thus deeply grooved) soft seat in the first stage

The condition of an extremely low intermediate pressure under full purge is not acceptable; the cause should be traced and corrected by a competent individual.

ADJUSTING THE INTERMEDIATE PRESSURE

In order to understand how adjustments are made and how those affect the intermediate pressure, it is important to have a look at (1) the characteristics of the main spring and (2) the working range of a regulator.

Previously mentioned was that “the magnitude of the intermediate pressure is primarily set by the strength of the first stage spring”.

Springs are unique because as long as they are compressed (or elongated) below the point of permanent deformation (yield point of the material) they are very predictable with regards to the force they can exert. This relationship is expressed by Hooke’s law and is linear. The more a spring gets compressed, the greater the force it exerts. See illustration below. Inside first stage regulators, springs are compressed.

Let’s take the example of a unbalanced piston first stage spring. Before assembly, the spring is in its most relaxed state (uncompressed) and the force the spring exerts is 0.  As we install the spring, the spring will be tightened in the first stage body by the spring retaining cap. This compression puts a preload on the spring. With the regulator unpressurised, there is a gap between the tip of the piston and the soft seat. This gap is defined as the working range.  See second illustration below. As the regulator now is pressurized, the piston moves and compresses the spring (moving to the right in the graph below), gradually increasing the spring force. This action continues till the soft seat and hard seat seal airtight or i.o.w. the piston has reached the end of its working range. The intermediate pressure has now been established.

The first adjustment option is to increase or decrease the preload on the spring. We call this direct spring tension adjustment. This adjustment does not affect the working range of the first stage. The below graph illustrates that with an increase in the preload and the same working range, the overall force of the spring is increased.

The second adjustment option is to increase or decrease the working range of the spring. We call this indirect spring tension adjustment. This adjustment does affect the spring force that can be exerted by the main spring. The below graph illustrates that with an increase in working range, the overall force of the spring is increased.

Adjusting the intermediate pressure is the domain of the service technician and how to adjust is outside the scope of this article. However, we reserve our right to mention the most common possibilities:

• No options: if the intermediate pressure is out of range the only corrective action is to replace components, e.g. the main spring (example: Aqualung Calypso)
• Direct spring tension adjustment:
• The use of shims on the main spring to increase or decrease the preload on the main spring (example: Scubapro MK2).
• The use of and adjustment screw to increase or decrease the preload on the main spring (example: Scubapro MK17).
• The use of an adjustment ring to increase or decrease the preload on the main spring (example: Cressi AC2).
• Indirect spring tension adjustment:
• The use of an adjusting screw that affects the positioning of the soft seat in relationship to the hard seat in order to alter the working range of the regulator (Scubapro MK 25).

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. accepts 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, practices and procedures prior to commencing work.