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Engineering Reference

Gauge Selection


  When selecting a pressure gauge consideration should be given to a number of parameters which have an effect on gauge accuracy, safety, and utility.


Accuracy required

Engineering Consideration

System Design Requirements Specification - initial system engineered design requirements will dictate the required pressure gauge accuracy.  Systems should be designed with cost and safety in mind.  Gauges which are more accurate generally cost more to build.

Selection Criteria

McDaniel Controls offers - pressure gauges with ranges of accuracy:  .25%, .5%, 1%, 1.5%, 2/1/2,  3/2/3% and choice of scale from inventory for many dual scale ranges, we offer more ranges and special calibrations than any other manufacturer of gauges.

Pressure Gauge Accuracy for newly designed systems - should be determined for each application and based on desired application system operational characteristics.  Operational parameters for many process environments are often fine tuned after the process system goes into production use.  Accurate gauge readings are required to maintain efficient process control.

Pressure Gauge Accuracy for replacement components - should be at least equal to the gauge being replaced unless otherwise determined by engineering review and in some cases with the component manufacturer's approval.   Selection of replacement gauges requires careful consideration of all the gauge selection parameters reviewed in this document and careful consideration and review of ASME B40.1-1991,  Gauges -- Pressure Indicating Dial Type -- Elastic Element.)
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Dial size

Engineering Consideration

Pressure Gauge Accuracy - determines dial size in two ways.  First, a requirement for higher accuracy dictates that a larger dial size be available for display of small and readable  pressure scale increments.   Second, by a requirement that the pressure gauge user/operator be able to read the scale from a distance that physical circumstances may dictate. 


Selection Criteria

Pressure Gauge Range Selection - The pressure gauge selected should have a dial size which permits the desired readable scale increments from the operator required viewing distance.  McDaniel Controls has a wide range of dial sizes from 1", 2", 2", 4", 4", and 6" dial sizes   (up to 20 inch dial size available).
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Operating pressure range

Engineering Consideration

Overpressure Failure -  is caused by the application of internal pressure greater then the rated limits of the elastic element (Bourdon Tube) and can occur when a gauge is subjected to pressures greater than design limits.

Loss of gauge accuracy - occurs when a gauge has been subjected to pressure exceeding design operation pressure limits.  Loss of gauge accuracy results from extending the elastic element (Bourdon Tube) past its limit for elasticity.


Selection Criteria

Pressure Gauge Range Selection - The pressure gauge selected should have a full scale pressure such that the operating pressure occurs in the middle half (25% to 75%) of the scale(12 o'clock position is best).  The full scale pressure of the gauge selected should be approximately two times the intended operating pressure. This will give the operator a visual indication of normal, ideal operating conditions.  This means a "green light" condition and everything is operating the way it should be.  If however, the "visual" of the pointer is way below or way above this 12 o'clock position, then we have a situation that needs attention.

McDaniel Pressure Gauges - are designed and manufactured to minimize failure and to protect personnel and property. 
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Chemical compatibility with gauge construction materials

Engineering Consideration

Corrosion Failure -  occurs when the elastic element (Bourdon tube) has been weakened through attack by the corrosive chemicals present in either the media inside or the environment outside it.  Failure may occur as a pinhole leakage  through the element walls or early fatigue failure due to stress cracking brought about by chemical deterioration or embrittlement of the material.


Selection Criteria

Chemical Compatibility Table - The Use of McDaniel Controls chemical compatibility table can assist in selection of both gauge internals and gauge case construction materials.

McDaniel Provided Diaphragm Seals - can be used to isolate gauge internals from process fluids.   McDaniel provided diaphragm seals are engineered for reliable use with bourdon tube type pressure gauges where separation of the medium and the control mechanism is necessary.  Seals are offered in a wide range of materials to withstand corrosive conditions. 

Diaphragm Seal Operation - The entire system above the diaphragm is solidly filled with a suitable liquid.  When line pressure is applied, the diaphragm displacement builds an equal and balanced pressure on the instrument side of the diaphragm, and the gauge then measures line pressure.   When properly filled, attached and calibrated it allows the attached gauge to function very close to its normal accuracy with only a slight accuracy loss through use of a seal.

McDaniel Gauge Case Style - a wide range of case styles available from McDaniel Controls provides opportunity to select a case that is liquid fillable and constructed with materials resistant to external attack by fumes and/or spillage of measured process media.
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Operating temperature range

Engineering Consideration

Loss of Gauge Accuracy - temperature change effects the stiffness of the elastic element (Bourdon Tube).  A change in the bourdon tube stiffness will effect gauge accuracy.

Explosive Failure  - for brass gauges in particular, solder is used to seal the ends of bourdon tubes.  Loss of solder seal can occur if the gauge is subjected to temperatures near the solder melting point.  Loss of solder seal would allow media to enter the pressure gauge case and possibly lead to explosive failure.

Leakage Past Gauge Case Seal - can result from exposing the gauge case to elevated temperatures which may bake the seal and cause the seal to loss its elasticity.

Selection Criteria

McDaniel provided guidelines - for high temperature gauge use can be referred to when high temperatures or extreme environments are expected.

Mounting Options - are important when consideration is given to use of a siphon or pigtail which can trap condensed steam or other vapor to isolate the gauge from the heat source.  Use of a siphon, or pigtail, makes a liquid seal in the line to the gauge and prevents the high temperature from reaching the gauge internals.

McDaniel Provided Diaphragm Seals - are another option to stem flow of measured media to the bourdon tube measuring element.
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Humidity

Engineering Consideration

Loss of Gauge Accuracy - condensation inside pressure gauge lens can make the gauge hard or impossible to read.

Selection Criteria

McDaniel Designed Liquid-Fillable gauges - All McDaniel Gauges are designed to avoid formation of condensation inside the gauge lens.    In environments where humidity and moisture are still a problem, McDaniel Controls liquid-fillable gauges are an excellent solution.  Glycerin or other choice of fillings prevent moisture from entering the case body and condensing on the inside of the case lens.  An added benefit of a liquid-filled pressure gauge is extended gauge life due to reduction of wear to the movable gauge internal parts.
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Vibration, pulsation, and shock

Engineering Consideration

Vibration Failure -  the most common mode of vibration failure is that where the movement parts wear because of high cyclic loading caused by vibration, resulting in gradual loss of accuracy, and, ultimately failure of the pointer to indicate and pressure change.

Vibration-Induced Fatigue Failure - vibration may in some instances, result in high loading of various parts of the pressure element assembly.  This loading could cause cracks in the element itself, or in joints.  Case pressure buildup may be slow, but it is possible that a large hole may suddenly develop, with a high rate of case pressure rise, which could result in a failure similar to an explosive failure.


Selection Criteria

McDaniel Designed Liquid-Fillable gauges - are used for applications involving vibration or pulsation.  Glycerin  is an excellent damper of bourdon tube movement.  Emersion of the entire assembly in a protective liquid envelope provides lubrication and shuts out corrosive or dirty atmospheres -- thereby minimizing wear of moving parts and prolonging gauge life significantly.  McDaniel Controls gauges can be filled with Glycerin, Silicone, depending on application needs.  See choice of fillings.

McDaniel provided pulsation snubbers - are used to minimize effects from pulsation and surges.  The snubber helps keep accurate data flowing by dampening pulsation and surges.  Gauge life is also extended by reducing gauge wear.   

Mounting Options - may become an important consideration for elimination of mechanical vibration between mechanical equipment and the pressure gauge.  In some cases it may be appropriate to connect the pressure gauge to the mechanical device using flexible hose and tubing so that equipment mechanical vibration is dampened by structural components.  McDaniel Controls offers a wide range of dial sizes, mounting options, and case styles to fit customer gauge mounting needs. 
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Pressure fluid composition

Engineering Consideration

Pressure Gauge Accuracy -  can be degraded when the measured media contains high solids or is such that the measured media changes between states of gas,  liquid, or solid, due to varying process temperature and media chemical compositions.  Measured media solids can restrict flow of the media to the elastic measuring element (Bourdon tube) or completely block flow.  Consequences of such loss of accuracy may be a pressure gauge which stops responding to pressure changes or is slow or sporadic in response to pressure changes in the measured media.

Selection Criteria

McDaniel provided diaphragm seals - are engineered to prevent mixing of the measured media with liquid used to transmit diaphragm seal pressure to the Bourdon tube.  Use of diaphragm seals are recommended where there is a possibility of problems with process media solids or process media change of state.
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Method of mounting

Engineering Consideration

Vibration Control -  can be achieved by remote mounting of gauges monitoring equipment with high levels of mechanical vibration.

Available Structure and Design -  available building structure and system design presents a need for variable mounting requirements and case styles.

Selection Criteria

McDaniel designed pressure gauges - offers a wide range of NPT sizes and case mounting styles as follows:

NPT - 1/8", 1/4", 1/2" NPT
           Other connection sizes available
           1/4"    MPF,       1/4"   MPM,         1/4" HPF,
               7/16   S.A.E.,    9/16"  MPM,     9/16" HPM

Mounting Styles

  • Standard Bottom Connection
  • Center Back or Lower Back Connections
  • Panel Mount (Center and Lower Back Connections)
  • U-Clamp Center Back
  • Front Flange (Center or Lower Back Connection)
  • Rear Flange (Bottom or Lower Back Connection)

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McDaniel Controls, Inc.,  P.O. Box 187 Luling, LA 70070
Street address: 14148 Highway 90 West, Boutte, LA 70039 U.S.A.
Tel: 985-758-2782  New Orleans: 504-467-1333 Fax: 985-758-1688

E-mail: Click HERE to send mail Web address:  http://www.mcdanielcontrols.com