How to Specify a Pressure Gauge ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

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


 

Minor, Figure and Total Graduations
In the Dial of a pressure gauge, you have minor graduation, figure graduation and total graduation as shown in the diagram overleaf. When specifying a pressure gauge, the minor and figure graduation are required to be specified.


How to Specify a Pressure Gauge
To specify a pressure gauge accurately, the common parts of the gauge and other parameters will need to be specified. These common parts of the pressure gauge and other parameters required are tabulated below for a guide should you need to specify a gauge for your application:
Parts of Pressure Gauge to Specify Descriptions
Process FluidSpecify the process fluid where the gauge will be used e.g Natural gas, air , water etc
Temperature Specify maximum process temperature and ambient temperature where the pressure gauge will be installed
Pressure Range Specify the pressure range e.g 0 - 100 bar, 0 - 300 psi etc
Figure Interval Specify the figure interval required for the pressure gauge
Minor Graduation Specify the minor graduation required for the pressure gauge
Nominal Accuracy Specify the expected accuracy performance of gauge. 0.1% of full scale for test gauges: 0.5 to 1% of full scale for industrial gauges; 1 to 3% of full scale for utility quality gauges.
Dial Size Specify Dial size of gauge. Common dial size of pressure gauges are : 1 in. (25 mm), 1.5 in. (40 mm), 2 in. (50 mm), 2.5 in. (63.5 mm), 3.5 in. (89 mm), 4 in. (100 mm), 4.5 in. (114 mm), 6 in. (150 mm), 10 in. (250 mm).
Case Material Specify case material for the pressure gauge. Case materials are available in drawn steel, cast iron, aluminum, brass, polypropylene,  phenolic resin materials or stainless steel.
Drawn steel is the most economical selection.
Cast iron case is desired  where  ruggedness is the goal
Brass is selected for decorative value and for anti-magnetic applications
Aluminum is utilized on limited-weight installations.
Phenolic resin or polypropylene case materials are used in corrosive atmospheres.
Stainless steel cases are used  for special applications such as the sanitary industry. Common stainless steel (SS) variants are 304SS and 316SS
Case Style Specify the pressure gauge case style in accordance with your application. Common case styles includes open front, solid front, cylindrical case, rear blowout disk, rear blowout back
Ring Material Specify the ring material for the pressure gauge. In most cases case material and ring material are same for a pressure gauge but differences could occur. Common ring materials 304 SS and 316 SS
Window/Lens Specify the lens or window material of the gauge. Pressure gauge lens are available in a variety of materials, including shatterproof, flat, and beveled glass or cellulose acetate, acrylic, and non-electrostatic plastics.
Dial Material/Colour Specify the dial material and colour of the pressure gauge. Gauge dials are made of steel, brass, aluminum, solid plastic, or laminated plastic, with black graduations on white background or in other color combinations
Gauge Pointer Specify the gauge pointer material and style. The most common pointer material is Aluminium black. Adjustable pointers and maximum pointer pressure gauges are also available in certain applications.
Bourdon Tube (Sensing Element) Specify the materials of the sensing element for the pressure gauge. Most pressure gauge has the C-shaped Bourdon tube as the movement or sensing element. e.g Phosphor bronze, alloy steel, 316 or 403 stainless steel, beryllium copper, Monel, or Ni-Span C.
Socket Material Specify the socket materials. Typical socket material is the 316L SS
Connection Size Specify the NPT connection size of the pressure gauge. Typical connection sizes for gauges include 1/8'', 1/4'' , 1/2'' , 1'' etc.
Connection Location Specify whether it is going to be lower or back of gauge case
Weather Protection Will the pressure gauge have weather protection? Is the gauge being installed in a harsh environment? Specify the weather protection class e.g IP 54, IP 65, NEMA 4X etc
Mounting Specify the mounting position of the pressure gauge. Typical mounting positions include stem , panel mounting, flush or surface.
Liquid Fill Specify the type of liquid fill required for the pressure gauge. In applications where pulsation or vibrations are significant, liquid filled pressure gauges are the desired application choice. Typical liquid fill include Glycerin (most common), Silicone and Halocarbon