Tools Used for Measuring Pressure?

MeyerFire University | G301.06

By Edward Henderson, Sr., PE

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RESOURCES

G121 SERIES

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NOTES & SUMMARY

• One-Page Summary [PDF]
• Notes Page [PDF]

G121 SERIES

1. What properties of fluids do we quantify?
2. What are the basic principles with hydrostatics?
3. The relationship of elevation and pressure?
4. Difference between pressure and head?
5. Exercise: Water Tower Height
6. Exercise: Maximum Standpipe Height
7. Exercise: Minimum Fire Pump Size
8. What is potential energy?
9. ​What tools do we use to measure pressure?

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TRANSCRIPT

What Tools Measure Pressure? 

INTRODUCTION

In this series on hydrostatics, we’ve covered fluid properties, principles of hydrostatics, elevation pressures, head, and potential energy. 

… but how are we going to measure this? And does “where” we measure it matter? 

… Do we want absolute pressure relative to a complete vacuum?, or gauge pressure, or differential pressure? 

it depends on what information is needed … and only then we can choose the right tool. 

Let’s wrap it up this series with the tools we use to actually measure pressure.  

Here are some tools:

TYPES OF GAUGES

A Barometer is used If we need to measure atmospheric pressure.   

Atmospheric pressure, is also known as barometric pressure (after the barometer). It is the pressure within the atmosphere of Earth as measured by a barometer.  This can change with both altitude and weather patterns. 

A piezometer tube is a device used to measure liquid pressure in a system by measuring the height to which a column of the liquid rises against gravity.   

Since the tube is open at the top, the liquid height is in excess of the local atmospheric pressure, so this equals gauge pressure.  These are common on radon mitigation systems you might have in the basement of your home. It’s U-shape shows a difference in height on the column, which indicates that there is a pressure difference between what is in the pipe and atmosphere. 

A differential manometer, as its name suggests, is used to measure the difference is pressure between two points in a pipe or between two pipes. This works in a similar way to the piezometer, except it’s not open to the atmosphere. 

A bourdon tube is part of a Bourdon pressure gauge.   

This closed tube is mechanically attached to linkage that rotates causing the gauge needle to indicate the pressure seen by the gauge.  These are known for their high accuracy and are the most common of all gauges within fire protection systems.  By the way, these are often calibrated at sea level (at one atmosphere) so altitude adjustment may be required. 

Electronic gauges make use of pressure sensors in order to convert pressure to an electronic signal.  

Different types of pressure sensors exist, but the most commonly use a piezoresistive pressure sensor. This sensor consists of a diaphragm that is equipped with piezoresistive elements. Changes in pressure cause the diaphragm to deflect, this deflection causes a change in cross-sectional area of the piezoresistive elements that is directly coupled to the electrical resistance.

Sooner or later, you will come across all of these gauges.   

The most common used in fire protection are the Bourdon Gauges.  As part of a fire riser, they always read static gauge pressure until water begins to flow.
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SUMMARY

The life expectancy of a pressure gauge is typically 10 to 15 years. Since many of a fire sprinkler system tests and inspections rely on the accuracy of the system’s pressure gauge(s), they must be periodically tested or replaced.  

According to NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, gauges must be replaced every 5 years or tested every 5 years by comparison with a calibrated gauge. Gauges not accurate to within ±3% of the maximum (full scale) reading are recalibrated or replaced.

I’m Ed Henderson; this is MeyerFire University.