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Pressure Gauges

The importance of pressure gauges are often overlooked.  A well designed and properly sized pressure gauge should provide accurate service for years.  The performance of spray nozzles, pumps and other fluid components are often judged in part by pressure gauges in the fluid system.  A quality pressure gauge will pay for itself within a short period of time by increasing the accuracy and control of your process.

In an effort to make it as easy as possible for you to select the right gauge for your application, we've dedicated this page to describing the key aspects that you should consider before selecting a pressure gauge.  The following table of links will automatically "jump" to that section of our page, or you can just scroll down and read everything in its entirety. 

Table of Contents

1. How to Select a Pressure Gauge

2. Accuracy

3. Dimensional Drawings

4. How Gauges Work

How To Select a Pressure Gauge
Since the accuracy of most pressure gauges is better in the middle portion of a gauge, you should always select a gauge with a range that is about double your maximum anticipated pressure. The maximum operating pressure should not exceed 80% of the full pressure range of the gauge.

Standard pressure ranges are measured in PSI and most of the gauges you can purchase in our web store have dual PSI/metric scales in BAR or kPA. Very low pressure gauges have scales that measure in Inches of H₂O, mm H₂O, and Oz./Inch².  Vacuum gauges have scales in inches of mercury, while compound gauges have scales that measure in both vacuum and pressure.

Our pressure gauges are available in 1-1/2" to 6" sizes (gauge size refers to the diameter of the dial, which is the viewing area of the gauge that shows the pressure scale and the pointer).  Select a gauge size that is easily viewed where you'll have it installed.  Obviously it has to fit within the physical space that you have.  The dimensional drawings for our gauges are found here.

Standard gauge connections are male NPT and they are located at the bottom or back of the gauge case.  Our gauges are also available in panel mount configurations using either a U-Clamp or 3-Hole Flange around the case.

Accuracy
Pressure gauges are manufactured to different accuracy's and are rated according to ANSI specifications.  ANSI is an acronym for American National Standards Institute and has served as administrator and coordinator of the United States' private sector voluntary standardization system for more than 80 years.  Founded in 1918 by five engineering societies and three government agencies, the Institute remains a private, nonprofit membership organization supported by a diverse constituency of private and public sector organizations.  To purchase copies of ANSI specifications please visit their website at www.ansi.org/. 

Pressure gauges are classified by their precision as it relates to the scale of the gauge.  The scale of the gauge, commonly referred to as the pressure range, is divided into three equal portions when characterizing its' accuracy.

Using a 0-300 PSI pressure gauge as an example, the accuracy would be referring to the 0-100, 100-200 and 200-300 portions of the dial.  The following pressure gauge is considered a "Grade B", which means that the accuracy for the 0-100 and 200-300 portions must fall to within 3% of the actual pressure being measured.  The middle portion is the most accurate and in this case would be accurate to within 2% of the pressure being measured.  On our site we categorize the accuracy by its "ANSI B40.1 Grade" and then list the percentages (ex. 3-2-3%).  You could also describe this gauge as having an accuracy of 2.5% full scale.

Continuing with the example above, the portions of the dial highlighted in yellow represent the ranges in which the accuracy is 3%.  The middle portion has an accuracy of 2%. You will notice that some companies describe their gauges as having an accuracy of  1.5% full scale.  This is the same as an ANSI Grade A 2-1-2% gauge, but it sounds better saying 1.5% full scale.

The higher the quality of the gauge, the more accurate it is.  We sell pressure gauges with accuracy's as high as .5% full scale (that means that at any point on the entire scale the pressure indicated by the gauge will fall within .5% of the actual pressure being measured).   Gauges that are more accurate tend to be more complex and cost more to manufacture - an example would be using machined gears instead of stamped gears (see pictures below).  Higher quality gauges last longer and some can even be repaired or recalibrated.

Since the accuracy is typically better in the middle portion of a gauge, you should always select a gauge with a range that is about double your maximum anticipated pressure.  Continuing with our example above, a 0-300 PSI gauge is ideal for applications having a maximum pressure range within 100-200 PSI.

Other factors to consider include:

1. material of construction (the connection, bourdon tube and window need to be compatible with your process)

2. configuration (bottom mount, back mount, panel mount or flange mount)

3. dial size (larger dials are easier to read from a distance)

We offer gauges with the following accuracy ranges (see dimensional drawings)

Dimensional Drawings
(click on a description to download/view the PDF file that describes that model gauge in detail)
(if you don't have a PDF viewer, click here to download one)

• Gauges, Dry, Grade B, 3-2-3%
• Gauges, Dry, Grade A, 2-1-2%
• Gauges, Filled, Grade A, 2-1-2%
• Gauges, Filled, Grade B, 3-2-3% (the dimensions are the same as the Grade A gauges above)
• Gauges, 316SS, Filled, Grade A, 2-1-2%
• Gauges, 316SS, Filled, Grade 1A, 1-1-1%
• Gauges, Low Pressure, Dry, Grade A 2-1-2%
• Gauges, Dry or Filled, Grade 2A, .5-.5-.5%
• Gauges, Ammonia, Dry or Filled, Grade A, 2-1-2%

How Pressure Gauges Work Most of our gauges are constructed with bourdon tubes to measure pressure and vacuum. The bourdon tube, which is a hollow metallic tube sealed at one end, flexes when pressure is applied.  It flexes because it naturally wants to straighten out, but cannot because it is linked to a geared movement.  As it tries to flex, this linear movement is changed to a rotational one by means of small gears.  They in-turn cause the pointer to indicate the measured pressure.  Gauges like this are designed for clean, non-clogging liquids and gases.

The pictures above, right, are of a less accurate, Grade B gauge (3-2-3%).  Gauges rated for higher accuracy will have higher quality parts (perhaps machined instead of stamped, heavier coils or adjustable links). 

To the left you can see the two most popular types of pressure gauges - dry and filled. The two most common causes for gauge failure are pipe vibration and water condensation (which can lead to freezing in colder environments). The delicate links, pivots and pinions of a regular gauge are sensitive to both condensation and vibration.  Filled gauges last longer because they have fewer moving parts and the housing is filled with a viscous glycol or silicon fluid.  The fill in a gauge helps dampen pointer vibration and eliminates corrosion resulting from condensed water when installed in areas that have humid air.

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