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Pressure Measurement

How do pressure gauges work?


Pressure is one of the most commonly measured industrial parameters.  This article seeks to help users understand the choices at their disposal and help illustrate these choices.
 
In many situations, a pressure transducer or transmitter is used to send a pressure readings into a SCADA or PLC but today we’re thinking about local display; a pressure gauge.
 
Gauges may be analogue or digital.  Both have their respective advantages
 
 
BourdonTube 200 Analogue gauges often utilise the Bourdon tube.  They are time-proven, mechanically simple and quality gauges are accurate and reliable.

Bourdon tubes expand like a garden hose when turned on, causing the mechanism to turn a pointer.  For some gauges such as low pressure gauges, a capsule is used instead of the Bourdon tube.

Your process medium (e.g. gas, oil, water, acid or milk) will determine what your bourdon tube should be made of. For inert gases and liquids phosphor bronze is fine and more affordable but for corrosive mediums you will require a gauge with stainless steel ‘wetted parts’.
 
If your process is thick, sludgy or needs to be sterile then you may need a diaphragm. (see below for more details on diaphragms or chemical seals)
 
When measuring the pressure of oxygen, acetylene or high pressure gases the NZ Standards say you should use a safety pattern gauge. These gauges are constructed with a baffle wall & a blowout back so that should they rupture it is less likely any failure of the gauge will cause personal injury because their design ensures any catastrophic release of pressure is from the rear of the instrument.
 
3063SP 200 SPG 100 200

Vibration and sudden pressure surges are the enemy of all mechanical gauges.  Connection by hose to a remote location or liquid filling the gauge are both methods for dealing with vibration whereas snubbers can help with smoothing out pressure surges.

The vast majority of gauges are ‘single ended’ i.e. they have one input.  Some gauges are differential to allow for comparison of 2 pressures.
 
Differential 200 Single Scale 200 Snubber 200
Differential Gauge Single Scale Liquid Filled Porous Snubber

A diaphragm separates the gauge from the process for hygiene or other reasons.  Diaphragm gauges consist of the gauge itself, a specially filled fluid chamber and the diaphragm.  They need to be thought of as a complete system and cannot be disassembled (except by the factory).  
chemical seals 200
ADT681a 200
HoneywellSSC 150
A digital gauge is based on an electronic pressure sensor rather than a mechanical assembly. 


How to select the correct gauge.

When selecting a gauge, several questions should be considered.  For example
 
  • From how far away must the display be visible?  (Goes to size)
  • What degree of accuracy is required?  (Goes to process and cost)
  • What degree of reliability is required?  E.g. ‘Commercial’ or ‘Industrial Gauges’ 
And then some process specific questions such as
  •  Must the ‘wetted’ parts be made of specific materials for material compatibility or food safe? (Stainless steel for potable water, acids or sea-water.  Diaphragms are available for hygienic uses such as dairy.)
  • Is the gauge likely to be exposed to pressure spikes and/or vibration? (Solutions such as liquid filling, snubbers or remote mounting are options to consider)
  • What scale is required? Which unit (kPa, psi etc) is required and is dual or single scale options preferred?
 
Below is an overview / selection guide for various types of instruments. 
 
3063D 200
 
63 mm Analogue; 3063/3025
  • The small diameter workhorse of the analogue gauge world, the 63 mm gauge.
  • Accuracy ±1.6 % of full scale
  • 304 stainless steel body with either phosphor bronze (Model 3063) or stainless steel internals (Model 3025)
  • This quality industrial gauge looks after the planet – its component parts are fully serviceable and replaceable ensuring long product life cycle
  • Click the above model numbers for more details and to download a full datasheet
3100HD 200
100 mm Analogue; 3400/3100
  • The medium workhorse of the analogue gauge world, the 100 mm gauge.
  • Accuracy ±1 % of full scale
  • Available in either Glass Filled Nylon case and copper alloy internals (Model 3400) or 304 stainless steel case and 316 stainless steel internals (Model 3100)
  • Model 3100 is liquid fillable & fully customisable – choose your own dial, orientation and thread.
  • This quality industrial gauge looks after the planet – its component parts are fully serviceable and replaceable ensuring long product life cycle
  • Click the above model numbers for more details and to download a full datasheet
  • Larger (dial diameter) gauges are also readily available.
DPGW
Digital Pressure Gauge; DPGW
  • ±1 % Full scale accuracy
  • Suitable for gases and liquids
  • User selectable units e.g. kPa, psi and many more
  • 316L stainless steel sensor and wetted parts
  • Affordable digital gauge
  • Click here for more details and to download a full datasheet
ADT680
Digital Pressure Gauge; ADT680
  • 0.25 % of full scale model is our standard stocked gauge
  • 0.025, 0.05,0.1 % accuracy models available to order
  • A top performing digital gauge
  • Pressure ranges up to 2,800 bar
  • IP67 Environmental
  • Stainless steel wetted surfaces
  • Click here for more details and to download a full datasheet
ADT680W
Wireless Logging DPG; ADT680W
  • Specifications same as 680 but with the added wireless &  logging features
  • Storage for 140,000 readings
  • Wireless communication  (free software downloadable from Additel)
  • Click here for more details and to download a full datasheet
ADT681a 200
Logging DPG; ADT681
  • Specifications same as 680 but with the added logging option
  • Gauge, absolute and differential models available
  • Panel Mount options
  • Intrinsically safe version (681IS)
  • Data logging version* (up to 21,843 records)
  • Click here for more details and to download a full datasheet 
*Data-logging requires additional software
 


 

 
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