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Instrumentation options? Choosing the instrumentation that is suitable for you

Posted by Pia Limpiyasrisakul on

We often get asked the question “What gauges should we purchase?”, “What do you prefer, analogue or digital?”, “Which one is better, analogue or digital?”…. These questions give us enough food for thought to make it an interesting topic but not an easy one to answer. In our opinion, what you choose really is the technicians’ personal choice. Some people like the colour blue, others like pink or turquoise. Sure you get the idea by now. However, there are a few points to consider, so there we go…


When servicing regulators, 2 basic types of instruments are required:

  • A pressure gauge to measure the intermediate or first stage pressure,
  • A differential pressure gauge to measure the cracking effort of the second stage (often referred to as Magnehelic® gauge, which actually is a trade name for differential pressure gauges of the Dwyer® brand).

In some cases, a flow meter could be recommended.


Gauges are manufactured to have certain accuracy. In Europe analogue gauges are manufactured in accordance with EN 837 and for the US, they are manufactured in accordance to ASME B40.1 for analogue gauges and ASME B40.7 for digital gauges. For European gauges, accuracy goes hand in hand with the nominal size of the gauge: the larger the analogue display, the greater the accuracy can be. For US gauges, accuracy is put in classes depending on application (e.g. laboratory precision test gauges, process gauges, test gauges, industrial gauges and so on)

It is important to understand that accuracy is specified for specific environmental conditions of temperature and humidity; meaning that if the conditions are other than specified ones, the adverse conditions could have an effect on the gauge accuracy.

Gauge accuracy is most often expressed as a percentage of the span: +/- xx % F.S, where F.S. stands for full scale.

Example for a metric gauge: accuracy of +/- 1% F.S. for a 20 bar gauge means that this gauge would be accurate within +/- .2 bar whether the gauge is reading 1 or 20 bar.

Example for an imperial gauge: accuracy of +/- 1% F.S. for a 200 PSI gauge means that this gauge would be accurate within +/- 2 PSI whether the gauge is reading 2 or 200 PSI.

Alternatively, accuracy could be expressed as a percentage of the reading. For example 0.1 % accuracy of the reading would mean that if your gauge reads 10 bar the accuracy would be 0.1 bar but if the reading would be 5 bar, the accuracy would be 0.05 bar (which is actually twice as accurate).

How do you know how accurate your gauge is? There’s only one way to find out: have your pressure gauge calibrated by a recognised calibration laboratory. This procedure is fairly inexpensive. For professional repair centres or dive shops, we recommend having your gauge calibrated annually, every time a significant event happens to the gauge (e.g. the instrument being dropped) or the gauge shows suspicious / erratic behaviour.


Resolution refers to the displaying properties of the gauge and is hence related to your major and minor scale divisions of analogue gauges. You could compare resolution to pixels of a screen: the more pixels, the more resolution will be available for the same area.  In other words resolution is related to the smallest change that can be measured.

An example for a digital gauge: a gauge with a range of 0-10.0 bar is said to have a resolution of 0.1 bar. This means 1 decimal place or 0.1 bar. Important note, this doesn’t mean that the gauge is accurate to 0.1 bar. Accuracy and resolution are not the same.


For the pro-user, we recommend using analogue pressure gauges with a display of minimum 63 mm (2.5”) and ideally 100 mm (4”) or greater. We consider analogue gauges smaller than 63mm (2.5”) only to be suitable for “in the field testing” as they likely will lack any accuracy or resolution required for set-up / adjustment work.

For analogue and digital pressure gauges, a range of 0-20 or 0-25 bar / 0-300 or 0-350 PSI is ideal as this would put the commonly encountered intermediate pressures mid-range of the gauge. It also gives the user a bit of a margin in case the intermediate pressure is on the high side – we still recommend the presence of a safety valve to protect your gauge -.

For measuring basic cracking effort a differential pressure gauge with a range of 0 - 3” of H2O is ideal. A 0 - 3” range gives the user a much better resolution compared to a 5 – 0 – 5” gauge. For most second stage regulators, cracking efforts values are within a 1” to 2” range.

If a differential pressure gauge is used in combination with a flow bench a range of 5 – 0 – 5” of H2O is recommended in order to visualize positive pressures (associated with venturi effect overrides / free flows) that might occur at higher flow rates.


  • Analogue gauges display the value immediately and the needle gives a visual clue to the investigator as to the value and where that value is located in the range. For digital gauges, the investigator actually has to read the value and process the data, no visual clue is present.
  • Analogue gauges allow the user to mark the display for an allowable pressure range, giving the user a visual clue as to go, no-go. Example, if the allowable intermediate pressure range is 9 to 10 bar, you could put a mark on the 9 and 10 bar scale, doing so will immediately give you a visual clue whether the pressure value is within the allowable range. This is not possible for digital gauges and interpretation is required.
  • Digital gauges can have a tendency to jump around. This is related to the sensitivity of the instrument and the sampling rate. More expensive units give the user the opportunity to adjust this to suit their needs (dampening function). Due to this tendency, novices will find digital differential pressure gauges more cumbersome in order to obtain a reading. We personally prefer analogue differential pressure gauges to measure cracking effort.
  • Digital gauges might be more sensitive as to detect positive or negative creep/drift. A negative creep or drift (dropping intermediate pressure) is an indication of leak on the low pressure side of the regulator whilst a positive creep or drift (raising intermediate pressure) indicates most likely a seating issue within the first stage (problem with the hard and soft seat).
  • Analogue gauges most often only display 2 units (e.g. bar and psi) whilst digital gauges allow the user to select or jog between many more units (e.g. PSI, kgf/cm2, bar, MPa and mH2O).
  • Digital gauges are not prone to parallax errors compared to analogue gauges.


  • If at all possible, protect your intermediate pressure gauge with a safety or over-pressure-relief valve so that in case the intermediate pressure of the first stage is greater than the range of your instrument, excess pressure is vented and your intermediate pressure gauge is protected. This could be especially important for unknown regulators that come in for service of freshly rebuilt regulators (we all occasionally make a mistake).
  • If no safety valve is present we recommend that a downstream second stage is connected to the first stage so that the second stage could act as a safety valve by initiating a free-flow in case of excessive pressures. Furthermore it would be good practice to slightly depress the second stage whilst gently pressurising the first stage so you can observe the initial intermediate pressure response and take appropriate action to avoid gauge damage.
  • Prior to taking any readings, with the gauge disconnected or not-pressurised, make sure the gauge reads 0. If this is not the case, many digital gauges have a zero-function to zero-out the gauge. For analogue differential pressure gauges, most likely there is an adjustment screw to zero-out the gauge. For analogue pressure gauges, unfortunately, most likely your gauge is damaged and will require repair prior to use.
  • Remove the batteries from your digital gauge when the gauge will not be used for a longer period of time: batteries could leak and cause damage to your instrument.
  • When reading analogue gauges, have you field of vision in line with the instrument to be read in order to avoid parallax error. Parallax is an apparent change in the position of an object when the investigator looking at the object changes position or line of sight. When taking a gauge readings, this error occurs when the investigators’ line of sight is not perpendicular to the plane of the gauge dial.
  • When measuring intermediate pressures make sure that the supply pressure to the first stage is in line with the recommended supply pressure as indicated by the manufacturer.
  • When measuring cracking effort ensure that the supply pressure to the first stage and the set intermediate pressure of the first stage are in line with the recommended values indicated by the manufacturer.
  • When adjusting the pressure of first stages, after each adjustment, slightly purge the second stage a few times to ensure that the change has been effective. Failure to do so could result in false readings. This is especially true if the intermediate pressure is adjusted to a lower level.
  • When taking measurements, allow the gauge to stabilize prior to recording the reading.
  • Analogue gauges are fitted with a case gauge vent or a vent valve. Normally once the gauge is installed set this compensating mechanism to the open position. Failure to have this mechanism in the open position could give you inaccurate readings.
  • Gauges are accurate precision instruments, rough handling, rough transport, dropping them could damage your instrument and affect the accuracy.
  • Analogue pressure gauges can be liquid filled. This liquid serves as an shock absorber for vibration and pressure peaks, acts as a lubricant, prevents moisture from entering the instrument…
  • Only use the gauge for the medium it is approved for. A gauge suitable for air use is totally not suitable for an enriched air or oxygen environment. Using an air gauge in these environments could lead to serious accidents.
  • The investigator performing the test and taking the readings should understand what he or she is measuring (i.e. understand the purpose or goal of the test) and know how to interpret or analyse the test results.


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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