A Guide for Selecting a Pressure Gauge
Figure 1: A vacuum pressure gauge
A pressure gauge measures a system's media pressure. Pressure gauges are applicable in many industries where monitoring system pressure is essential. They allow an operator to closely monitor and troubleshoot a system's pressure. For example, pressure gauges can help an operator determine if there is a leak in the system. Depending on the industry and application (e.g., hydraulic, water, vacuum, swimming pool), there are a wide variety of pressure gauge designs & options available. This article covers the parameters to consider when selecting a pressure gauge. To learn more about how they work, read our pressure gauge overview article.
Selection Criteria
1. Digital vs analog reading
Various digital and analog pressure gauges are available, making each type suitable for most pressure reading tasks. Use the chart below for a general comparison between the two types.
Digital pressure gauge
Analog pressure gauge
- Easy installation
- Requires a power supply to operate
- More expensive
- Steady and precise reading
- Easier installation
- Does not require power to operate
- Less expensive
- Less accurate
Figure 2: A digital pressure gauge
2. Gauge type
There are many types of pressure gauges. The following are common types:
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Bourdon tube: A bourdon tube gauge is the most widely used pressure gauge with a wide range of designs and excellent sensitivity and accuracy. These gauges are suitable for the pressure of media that do not crystallize and are not highly viscous.
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Diaphragm: A diaphragm pressure gauge is suitable for corrosive, highly viscous, or crystallizing media because most of the gauge's components do not interact with the system media.
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Bellows: Bellows pressure gauges have the advantage of being able to measure absolute and differential pressure without extra components.
3. Pressure gauge material
When making decisions about the material, there are two parts of a pressure gauge to consider: the cabinet and the process connection.
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Process connection: The process connection includes any component directly interacting with the fluid. The specific components depend on the type of pressure gauge but typically include the threads and the sensing element. Standard materials are stainless steel or copper alloy. Material incompatibility can cause issues like corrosion, leading to gauge failure and increased safety risks.
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Cabinet: The cabinet, also called the housing material, has a broader material selection and depends more on the application's environment. Common housing materials for pressure gauges include copper alloys, stainless steel, Inconel, and aluminum.
The following table describes some common materials. Refer to our chemical compatibility chart for further information.
Stainless steel
Inconel
Aluminum
Copper alloy
- Excellent corrosion-resistance
- Vulnerable to high-temperature oxidation
- Suitable to use with corrosive media
- Internal components are not affected by magnetism
- High corrosion resistance, especially to high-temperature
- Inconel is a class of nickel-chrome-based super alloys characterized by high resistance to oxidation and oxalic acid
- Ability to maintain high tensile strength at high temperatures.
- More costly than other materials
- Excellent corrosion resistant
- Lightweight
- Not suitable for very high-temperature applications
- Suitable for operation with non-corrosive media
- Not suitable for high-temperature applications
4. Fluid temperature
The pressure gauge components must be able to withstand the media’s temperature. For gauges that work with hot media, it is recommended that the gauge be used along with a siphon, cooling tower, or diaphragm seal. The maximum recommended fluid temperatures for the pressure gauge materials are:
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Copper alloys: 60 °C (140 °F)
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Stainless steel: 100 °C (212 °F)
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Inconel: 315 °C (600 °F)
5. Pressure range
The pressure range is the maximum and minimum pressures the gauge can read. Ideally, the gauge has a scale that is twice the system’s working pressure. The working pressure should be at most 75% of the gauge’s scale.
6. Dry vs liquid-filled
Figure 3: A glycerine-filled pressure gauge
A liquid-filled gauge is more expensive than its dry counterpart, but has many benefits. Liquid-filled gauges steady the needle in high-vibration applications, keep internal components lubricated, and increase the life of the gauge. The most common type is glycerine, but silicone is also available. Silicone is suitable for a wider temperature range and doesn't discolor over time. Liquid-filled gauges have the added disadvantage of leakage. Therefore it’s important to choose high-quality gauges.
7. Environmental conditions
When working under extreme ambient temperatures, pressure gauges might need to be isolated. In such a case, a remotely mounted gauge with a diaphragm seal and flexible capillary line can be used. It is recommended to use a snubber in areas where pressure spikes and pulsations are normal. In a moist or humid environment, a liquid-filled pressure gauge is preferred to eliminate the risk of condensation build-up. A pressure gauge with stainless steel, plastic, or a brass cabinet is suitable for outdoor use.
8. Cabinet size
A typical pressure gauge dial ranges in diameter from 6.4 cm to 15.2 cm (2.5 in - 6 in). Choose the most appropriate size based on the space limitations and readability requirements for the application.
9. Accuracy class
The accuracy class indicates the maximum permissible margin of error for the pressure gauge with respect to the maximum scale reading. For example, when reading a 100 bar pressure gauge with accuracy class 1, the deviation from actual pressure is 1%, i.e.,1 bar. The degree of accuracy required may differ in different industries.
10. Mounting method
Pressure gauges typically have mounting either below, on the side, or behind the gauge. Verify the mounting method is suitable for the application by making the gauge easy to read.
Figure 4: Three different kinds of pressure gauge mounts: a bottom mount (left), a back mount (middle), and a side mount (right).
11. IP rating
IP rating indicates the degree of protection for the pressure gauge against foreign particles such as dust, liquids, moisture, or accidental contacts. Pressure gauges may need to operate in some harsh environmental conditions. For such applications, our pressure gauge provides IP 42, IP 54, and IP 65 protection options.
12. Connection size
Pressure gauge has a wide range of connection size options. Common options are: ⅛ inch, ¼ inch, ⅜ inch, ½ inch, ¾ inch, and M 12x1.5.
Pressure gauges can be found in virtually every industry. Depending on your application, these instruments play an important role in measuring the air, gas or liquid pressure in your system to keep it running safely and smoothly. However, for optimum efficiency, you will need to make sure the pressure range you choose for your gauge aligns with your application requirements and a few other factors.
Ashcroft has been in the pressure gauge business for more than a century and a half, so following the guidelines in this article can save you time and give you confidence in your purchasing decision. Read on to learn about the factors to consider when choosing the pressure range for your gauge. Then, when you are done, we will share additional information about pressure gauge selection that you can use as a reference.
Follow ASME's best practices for operating pressure.
The first step in selecting a pressure gauge range is to understand the operating pressures of your process. Ashcroft follows ASME B40.100 standards, which recommend the following:
Operating pressure should be 25% to 75% of scale, and the gauge range should be twice the operating pressure.
Ideally, the mid-scale on your dial should be around 12 o’clock, or the mid-scale of the full-scale gauge span.
If pulsation is present, the maximum operating gauge pressure should not exceed 50% of the full-scale range.
Know the maximum design pressure for your application.
After operating pressure, the next thing you need to confirm is the design pressure. The design pressure is the maximum pressure that your gauge will experience during your application at a given moment in time (not sustained). There can be a surge of pressure when the machine turns on, or surges or pressure spikes can happen at other times during the application.
Pressure gauges can handle 130% to 150% of the range, depending on the range, but if the pressure goes beyond that you will need to get a gauge with a higher range or protect the gauge from inaccuracy issues or possibly overpressure that can cause gauge failure. Both factors are safety issues that should not be ignored. Read, Why Did My Pressure Equipment Fail? 6 Instrument Killers to learn more.
A pressure limiting valve (PLV) can help protect the gauge from overpressure by shutting off the pressure going into the gauge. Consider this accessory when process pressure can at times exceed 130% to 150% of the range.
The PLV is usually set at the full-scale range of the gauge. When the pressure reaches the setpoint of the PLV, pressure into the gauge is shut off, protecting the gauge from overpressure. It will also contain the process media so nothing dangerous leaks out into the environment.
Other gauge factors to consider when choosing a pressure range.
After determining the operating and design pressure of your application, you will need to look at the other factors that can influence the final range you select, including:
Dial Sizes and Features
All pressure gauges have a variety of dial size options and features to consider. Depending on the range you need, dial sizes typically range from 1 ¼ in. to 16 in. Gauge size and readability are very important considerations. The larger the dial, the easier it will be for the operator to read. You can narrow down your options by asking yourself:
Dial features on the other hand also consider numeral type, major graduations and minor graduations.
Figure 1: Major and Minor Graduations
Minor graduations can be difficult to read if they’re too close together, so consider the readability of these smaller graduations when selecting the best dial size.
Other considerations:
Pointer width. This cannot be greater than the width of the dial graduations and may be a limiting factor when determining how many graduations are available with corresponding dial sizes.
- Space restrictions. If there is not a lot of space where the gauge will be installed, you may also be limited in your dial size selection.
- Dial arc. Typically, the dial arc of a gauge is 270 degrees in accordance with ASME B40.100 guidelines. However, some dial arcs are less than 270 degrees.
Low-light conditions
. If your gauge needs to be read in dimly lit areas, using a retroreflective dial material like the Ashcroft Duravis™ gauge dial is a good option. The retroreflective material is the same material used on stop signs.
Unlike a reflective material that disperses light in different directions, Duravis™ material reflects light back to the light source, making readability easier. See Figure 2 below. Customization
. Ashcroft offers a variety of printing options for your dials. We can add your logo, use your brand colors, create special dial markings, add color zones and more to accommodate and personalize the gauge for the application. Figure 3 below is an example of a colored zone dial using green to indicate a safe zone, yellow to indicate caution and red for danger.
Figure 2: Ashcroft® Duravis™ Gauge
Figure 3: Colored Zone Dial
Scale Options
When it comes to gauge scales, you will want to confirm the following which can also affect the range:
Which unit of measure is required
? In the U.S., a
pound of force per square inch or
PSI is common. Other popular units of measure include kPa, kg/cm2 and bar.How many units of measure are needed
? If your application requires a dial with multiple units of measure, you can select a single, dual, triple or quadruple scale option. Regardless of the one you choose the dominant scale that will be read most will be the inner scale where the numerals and graduations are larger than the outer scale. Whether you need a refrigerant scale. Refrigerant scales like ammonia or refrigerants R11, R12 and 134A can measure both pressure and a corresponding temperature scale. Refrigerant scales that measure both pressure and vacuum are common. Typical units of measure are inches of mercury or millimeters of mercury. When there is a need to measure both pressure and vacuum, a compound scale is available (see Figures 4 and 5 below).
Figure 4: Ammonia Scale
Figure 5: Compound Scale
Accuracy considerations
Generally speaking, the better the accuracy of the gauge, the more graduations are required. So, if your application requires ±0.5% accuracy or better, a dial size less than 100 mm is not recommended. That’s because the number of required graduations can make the gauge difficult to read.
Minor dial graduations generally do not exceed twice the accuracy of the gauge. As an example, a 100 psi range gauge with a required accuracy of ±0.5% will have minor graduations of 1 psi (refer to Figure 1).
In situations where you require a gauge with better accuracy, this usually means more graduations on your dial. In these cases, consider getting a dial that’s a larger size to accommodate the measurement increments.
Ready to expand your knowledge of pressure gauges?
A pressure gauge is a small but significant part of any application. That's why selecting the right pressure range for your gauge is critical for ensuring your instrument performs as it should. If your gauge range doesn't consider all of the factors stated above, your operation can be at risk of failure. For your reference, here are some related resources that may interest you.
Feel free to contact us today to talk to one of our industry experts and get all your pressure gauge questions answered.