Understanding Digital Manometer Accuracy and Resolution

Meriam digital manometer for process control

Digital Manometers and Their Many Uses
What is an inch of water? The correct answer to this question has taken on increased importance with the demand for better accuracy and the introduction of digital manometers. For example, lets assume that you have just received your new digital manometer (like a Meriam M200) and you decide to verify its accuracy. The manufacturer claims 0.1% accuracy at 100 inches of water. You set up a test using a deadweight tester and a water manometer. After placing a 100 inch of water weight on the deadweight tester, you record a reading of 99.8 inches of water on the digital manometer and 100.2 inches of water on the manometer. Which is right? The answer to this question will be evident after the following discussion of manometers and dead weight testers and their relationship to digital manometers. There will also be a discussion of digital manometer accuracy and resolution.

Some Digital Manometer Applications
• Pressure transmitter calibration for process instrumentation in position
• Industrial processes such as heat treating and quenching(cooling)
• Monitoring the flow rate of gases for flow measurement devices in industrial processes.
• Testing of pressure switches for plant processes
• Check Power plant condenser efficiency by monitoring the vacuum
• Measure the vacuum in resin reactors for plastics manufacturing
• Positive displacement meter testing for gas utilities
• Detect blockages in filters, condensers and chillers(water filter steam power plant)
• Cooling costs can increase 50% due to scaling in condenser tubes; monitor scaling with digital manometer

Manometers – The manometer is considered a primary standard since the pressure exerted by the liquid column can be accurately determined by the measurement of basic physical properties: the height of the column and the density of the liquid. For a manometer filled with water the pressure equation is:

pressure formula

gt = gravity at instrument location
go = standard gravity (980.665 cm/sec2)
ρa = density of air at observed temperature
ρw = density of water at observed temperature
ρo = density of water at standard temperature
h = height of water column in inches

Notice that in order to determine the pressure indicated by the height of a water column one needs to know more than the height of the column. Both the local gravity and the density of the manometer fluid and the fluid being measured, in this case water and air, need to be known. As can be seen, the pressure indicated by an inch of water varies with location and temperature. In order to make the readings comparable they must be reported at the same temperature and gravity. The three commonly used reference conditions for water columns are shown on the table below.

Standard      Temperature        Gravity
Scientific        4° C (39.2° F)       980.665
AGA              15.6° C (60° F)      980.665
Industrial       20° C (68° F)         980.665

There is only one commonly used reference condition for mercury columns: 0° C (32° F) and 980.665 cm/sec2.  The use of the wrong temperature reference for water columns can produce errors of 0.1% to 0.2%. The failure to correct a mercury column from an observed temperature of 68° F can produce an error of 0.36%. The figure below shows the errors at different latitudes caused by not correcting for gravity.

us gravity map percent error-W

Consider the example presented earlier where the manometer indicated 100.2 inches of water and the digital manometer indicated 99.8 inches of water. In order to correct the manometer to reference conditions, some additional information is needed. The test was conducted in Corpus Christi, Texas at an ambient temperature of 27o C (80.6o  F). The digital manometer displays water column referenced to 20o C.

The gravity correction factor is the gravity at 28° north latitude divided by the standard gravity (979.185/980.665 = 0.9985). The temperature correction factor is the specific gravity at the ambient temperature minus the specific gravity of air divided by the specific gravity at the reference temperature ((099654 – 0.0012)/0.9983) = 0.997). Therefore the manometer reading corrected to reference conditions is shown below.
(100.2)(0.9985)(0.997) = 99.85″ @20° C
Notice the close agreement between this and the digital manometer reading.

The use of a water manometer is not practical for field calibrations since a manometer with a range of 100 inches of water would be over 8 feet tall. A solution to this problem is to substitute a heavy liquid, such as mercury, for the water and to adjust the scale to read in inches of water. This reduces the scale by a factor of 13.59, the ratio of the specific gravity of mercury to that of water. Although this reduces the scale length to less than 8 inches, it also reduces the scale resolution and therefore its accuracy. Typical accuracy for this type of manometer is 0.25 inches of water. This type of manometer can be read directly only at the conditions for which the scale was calculated. A typical scale would read inches of water referenced to 4° C when observed at 22° C at a gravity of 980.665 cm/sec2. Corrections must be calculated if any of the conditions vary from those at which the scale was calculated. The manometer is an accurate pressure indicating device if the specific gravity of the manometer fluid is accurately known and the correction factors are properly applied. Read and Download Entire Article

Article used by permission from Meriam Process Technologies®

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