How to Specify a Digital Pressure Gauge

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Digital Pressure Gauge (Photo Credit: ASHCROFT)

With the advancement in gauge technology, digital pressure gauges are becoming more popular. With their  superior accuracy and functionality, most new plants are increasingly employing digital gauges especially in industrial processes where accuracy is paramount.

Although we probably all know the key parameters required to purchase a regular dial gauge, we may be at a loss when it comes to digital gauges. This is because in addition to the regular features of a typical pressure gauge, digital gauges are equipped with other functionalities that need to be specified. 

Digital pressure gauges are specified in much the same way a typical pressure gauge will be specified except for few specification items that are specific to digital pressure gauges. The table below shows the key parameters that must be taken into consideration for a general purpose digital pressure gauge to be accurately specified.

Parameters to Specify	Details of Specification
Accuracy	Typical accuracies include ±0.1% FS, ±0.25% FS, ±0.5% FS etc
Case Size	Standard case sizes include 3’’, 41/2’’,6’’ etc.
Case Material	Depending on your application, typical ones include 300 series stainless steel, fiberglass reinforced thermoplastic, black painted aluminium etc
Wetted Materials	Specify materials that will resist corrosion. Typical materials include 17 – 4 PH stainless steel sensor, stainless steel socket etc
Socket Size	Typical socket sizes include 1/8’’ , ¼’’ , ½’’ etc.
Connection	Specify whether process connection will be lower (6 o’clock), top or side
Measuring Range	Specify the measuring range of gauge in the desired unit. For example 15 psi through 20,000 psi, 0 – 100 bar etc
Power Source	For digital pressure gauges, they could be powered through: 1. Alkaline batteries (two pieces of size AA typically) 2. C Alkaline batteries (two pieces) 3. Loop powered by a 4 – 20mA source 4. Line powered (specify voltage and current) Specify the desired power source you desire for the digital gauge
Battery Life	Battery life is specified in hours. Typical battery life include 500hrs, 1000hrs, 2000hrs etc
Battery Indicator	Specify the battery indicator levels
Cycle Life	Specify the cyle life of the digital pressure gauge. e.g 5million cycles, 10 million cycles etc
Operating Temperature	Operating temperature of gauge is critical for accurate performance. Specify the operating temperature of gauge depending on temperature regime of the environment where the gauge is to be used.
Storage Temperature	Specify storage temperature of gauge
LCD Display	Specify the type of LCD display
Character Height	Specify the character height of the LCD display - specify height of upper character and that of lower character
Engineering Units	Specify the engineering units you want your digital gauge to display - psi, bar, mmHg etc
Backlight	Specify whether the LCD display should have a backlight especially in applications where illumination could be a problem especially at night
Enclosure Rating	Specify the enclosure rating. Common enclosure rating for gauges is IP 67
Keypad Functions	Specify the keypad functions of the gauge. Three key with multi press functionality are typical.

How to Specify a Pressure Gauge

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Pressure gauges are ubiquitous devices. They are everywhere you go. In a process plant, pressure gauges and sensors are the eyes of the plant. If there is a process irregularity, the first point is usually a pressure gauge somewhere in the plant that has indicated to us that something is not right with the process.

Given their importance and the fact that they are common place, we tend to take them for granted and in the process avoidable mistakes are made. When you need to replace a bad pressure gauge, most often we just buy a replacement with similar product and range. But this could be dangerous! How can you then specify the correct gauge to avoid mistakes? We intend to achieve that with this article.

How to Calibrate a Pressure Gauge with a Dead Weight Tester
How to Select a Pressure Gauge
How to Calibrate a Pressure Gauge

Parts of a Pressure Gauge
Before you can accurately specify a pressure gauge, you need to know the various parts that make up

How to Calibrate a Pressure Gauge With a Dead Weight Tester

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Basic Operating Principle

Deadweight Testers (DWT) are the primary standard for pressure measurement. There are three main components of this device: a fluid (oil) that transmits the pressure, a weight and piston used to apply the pressure, and a connection port for the gauge to be calibrated.

The dead weight tester also contains an oil reservoir and an adjusting piston or screw pump. The reservoir accumulates oil displaced by the vertical piston during calibration tests when a large range of accurately calibrated weights are used for a given gauge. The adjusting piston is used to make sure that the vertical piston is freely floating on the oil. Please see How a Dead Weight Tester Works for a detailed description of the working principle of the device.

Calibration Basics

To carry out tests or calibrate a pressure gauge with the dead weigh tester(DWT), accurately calibrated weight masses (Force) are loaded on the piston (Area), which rises freely within its cylinder. These weights balance the upward force created by the pressure within the system:

PRESSURE = FORCE/AREA = W/A

So for each weight added, the pressure transmitted within the oil in the dead weight tester is calculated with the above formula because the area of the piston of the tester is accurately known.

Note:
if weights are in pounds (lbs) units and the area in inch square, then the calculated pressure unit is in Pounds per square inch(PSI).

If the weights are in kilogram (kg) units and the area of piston in meters square, then the calculated pressure [P = (W*G)/A, G = gravity in m/s2] unit is in N/m2 or pascal.

During calibration, the system is primed with liquid from the reservoir, and the system pressure is increased by means of the adjusting piston. As liquids are considered incompressible, the displaced liquid causes the piston to rise within the cylinder to balance the downward force of the weights.

Calibrating a Pressure Gauge with the Dead Weight Tester:

To calibrate a pressure gauge with a dead weight tester, set up the device on a level, stable workbench or similar surface as shown in the diagram below:

Proceed with the calibration according to the following steps:
Step 1:
Connect the pressure gauge to the test port on the dead weight tester as shown in the diagram above. Ensure that the test gauge is reading zero, if not correct the zero error and ensure that the gauge is reading zero before proceeding with the calibration exercise.

Step 2:
Select a weight and place it on the vertical piston

Step 3:
Turn the handle of the adjusting piston or screw pump to ensure that the weight and piston are supported freely by oil.

Step 4:
Spin the vertical piston and ensure that it is floating freely.

Step 5:
Allow a few moments for the system to stabilize before taking any readings. After system has stabilized, record the gauge reading and the weight.

Step 6:
Repeat steps 2 through 5 for increasing weights until the full range or maximum pressure is applied to the gauge and then decreasing weights until the gauge reads zero pressure. Calculate the error at each gauge reading and ensure that it is within the acceptable accuracy limits.

If you are doing a five point calibration, then increasing weights should be added corresponding to 0%, 25%, 50%, 75%, and 100% of the full range pressure of the pressure gauge. And for decreasing pressure you proceed in the order 100%, 75%, 50%, 25%, 0%.

For pressure gauges with less accuracy specifications, calibration at the points: 0%, 50% and 100% will suffice.

After calibration, your data can be recorded in a table in this manner:

Upscale Readings:

% Input	Weights,W	DWT Pressure (W/A)*	Test Gauge Pressure	Error
0
25
50
75
100

*DWT Pressure = W/A, if W is in lbs, and A in square inch  then DWT Pressure is in PSI(pounds per square inch). However, if W is in kg and A in square meters, then :
DWT Pressure = (W*G)/A, Where G = gravity in meters per seconds square(m/s2) and the DWT Pressure is in N/m2 or Pascal

Downscale Readings:

% Input	Weights,W	DWT Pressure (W/A)*	Test Gauge Pressure	Error
100
75
50
25
0

At each pressure reading, the absolute error is calculated thus:

Absolute Error = DWT Pressure – Test Gauge Pressure

The absolute error at each point should be within the acceptable accuracy limits of the gauge.

If the gauge error is in % span proceed as follows to calculate the error:

Span = Maximum pressure – minimum pressure %Error = [(DWT Pressure – Test Gauge Pressure)/Span]*100  for each pressure gauge reading.

The error in % span should be within the acceptable accuracy limits otherwise the calibration will have to be repeated to correct the errors.

If the pressure gauge error is in % FSD(Full Scale Deflection), proceed as follows to calculate the error:

% Error = [(DWT Pressure - Test Gauge Pressure)/FSD]*100

Correction Factors:

The deadweight tester has been calibrated to the Gravity, Temperature, and Air Density stated on the calibration certificate right from the laboratory.
Equations and factors are given on the certificate to adjust for any variations in these environmental conditions.
Always refer to the documentation for the Dead Weight Tester to ensure that for maximum accuracy, the necessary calibration correction factors are applied to any reading from the device.

Gravity Correction

Gravity varies greatly with geographic location, and so will the deadweight tester
reading. Due to the significant change in gravity throughout the world (about 0.5%), ensure that the tester in your possession has been manufactured with the specification of your local gravity, otherwise you  may have to apply the correction for the calibrated gravity.

To correct for gravity use:

True Pressure = [(Gravity (CS))/(Gravity(LS))]*P(Indicated)  Where: P(Indicated)  = Pressure indicated by gauge being calibrated Gravity(CS)  = Gravity at Calibration Site Gravity (LS) = Gravity at Laboratory Site

Temperature Correction

Temperature and Air Density variations are less significant than gravity. Variations should be corrected for when maximum accuracy is required.
To correct for Temperature variation use:

True Pressure = P(Indicated) [1+ {T(DWTCT) – T(OT)}*{ΔP/100}]  Where: P(Indicated)    = Pressure indicated by gauge being calibrated T(DWTCT)     = Dead Weight Tester calibrated temperature in the laboratory T(OT)              = Operating temperature at calibration site ΔP                    = Percentage pressure change per unit temperature change

How to Calibrate a Pressure Gauge

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In the plant, pressure gauge calibration is often taken for granted probably because they seem to be everywhere in the plant that one just assumes that some how the gauges are accurate even when they are out of calibration. A pressure gauge can be calibrated with a standard pneumatic calibrator, a dead weight tester or any other suitable calibrator.
There is no standard way to calibrate a pressure gauge.  The way a gauge is calibrated depends on the way the gauge is used. Here an outline is given on how a pressure gauge could be calibrated with any type of calibrator.

How to Select a Pressure Gauge

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Mechanical pressure gauges, which require no external power, provide an affordable and reliable source of accurate pressure measurement. The need to select the right pressure gauge requires that the device be accurately specified otherwise
numerous problems might surface in the course of using a wrongly selected gauge. The factors discussed here are by no means exhaustive but they are the main things that will assist you to select the right type of pressure gauge. For the right specification and selection process, consult the manufacturer of the particular gauge you intend to use.

Pressure gauges should be selected by considering the factors below to prevent misapplication. Improper application can be detrimental to the gauge, causing failure and possible personal injury or property damage.

To select the right pressure gauge for your application, the following factors should be considered:

Gauge Accuracy
Pressure gauge accuracy ranges from grade 4A to D according to ASME 40.1. For a mechanical pressure gauge, accuracy is defined either as a percentage of the full-scale range or a percent of the span. As the accuracy increases, so does the price of the gauge. Therefore, the application where the gauge is required should be carefully considered before deciding on the accuracy of the gauge. While requirements differ from one industry to another, the following are general guidelines:
• Test Gauges and Standards: 0.25% through 0.10% full scale accuracies.
• Critical Processes: 0.5% full scale accuracy.
• General Industrial Processes: 1.0% accuracy. Less Critical Commercial Uses: 2.0% accuracy.

An Introduction to Pressure Gauges

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A pressure gauge is a pressure sensor that is used to indicate the pressure of a given process or system. A pressure gauge usually refers to a self-contained indicator that converts the detected process pressure into the mechanical motion of a pointer. Depending on the reference pressure used, they could indicate absolute, gauge, and differential pressure. “Gauge” pressure is defined relative to