When you are researching pressure gauges for your pressure measurement needs, consider these factors: What is the application? How much pressure is involved? What size dial should you buy? What is the gauge accuracy requirement?
These are just some of the questions to ask that can help you find the proper range for your pressure gauge. It’s easy to be overwhelmed by choices, so this article will offer some tips for selecting the right pressure gauge range for your application.
Dial Sizes and Features
There are many dial options to consider when shopping for a gauge, depending on the range you need. Dial sizes can range from 1 ¼ in. to 16 in.
Why is that? Scales include numerals, major graduations and minor graduations (see Figure 1).
Figure 1: Major and Minor Graduations
Those minor graduations can be difficult to read if they’re too close together, so consider readability when selecting a dial size. How far away would it be from sight? It should be large enough to be easily read from distances and angles.
Note that the pointer width cannot be greater than the width of the dial graduations. Pointer width may be a limiting factor when determining how many graduations are available with corresponding dial sizes.
A space restriction where the gauge will be installed is another consideration when selecting dial size.
Does the gauge need to be read in low-light conditions? If so, a good choice is the use of a retroreflective dial material like the Ashcroft Duravis™ gauge dial. 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).
Figure 2: Duravis™ Gauge
Gauge dial printing is offered to accommodate and personalize the gauge for the application.
Offerings can include colored dials, dials with colored zones, special dial markings, customer names on the dial and false reading dials, just to name a few options.
An example of a colored zone dial could be the colors green (a safe to operate zone), yellow (a caution to operate zone) and red (a danger to operate zone). A personalized name on the dial can include the customer’s name, URL, phone number and customer logo (see Figure 3).
Figure 3: Colored Zone Dial
Typically, the dial arc of a gauge is 270 degrees in accordance with ASME B40.100 guidelines. There are, however, some dial arcs that are less than 270 degrees.
Scale Options
What unit of measure is required? Convention in the U.S. is typically a pound of force per square inch or PSI. Some of the more popular units of measure include kPa, kg and bar.
Does your application require a dial with two units of measure? If so, typically the dominant scale that will be read most is the inner scale where the numerals and graduations are larger than the outer scale.
In fact, there are various scale options to consider when selecting a gauge range. Aside from single scale and dual scale, your gauge can measure triple or even quadruple scale depending on how many measurements you need.
There are also refrigerant scales like ammonia or refrigerants R11, R12 and 134A. These scales measure both pressure and a corresponding temperature scale. Refrigerant scales that measure both pressure and vacuum are common.
Also offered are gauges that measure vacuum. 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 Figure 4).
Figure 4: Ammonia Scale
Accuracy
Generally, the better the accuracy of the gauge, the more graduations are required. Typically, a size less than 100 mm is not recommended for accuracies of ±0.5% or better. That’s because the number of required graduations can render the gauge difficult or even impossible 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. Consider getting a dial that’s a larger size to accommodate the measurement increments.
Mid-scale on your dial should ideally be around 12 o’clock, or mid-scale of the full-scale gauge span. ASME recommends that operating pressure should be 25% to 75% of scale, and the gauge range should be twice the operating pressure.
Design Pressure
The design pressure is the maximum pressure that your gauge will experience during your application. This doesn’t mean sustained pressure, either. 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.
Beware of gauge killers! 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.
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.
At Ashcroft, we publish rupture or burst pressures for our process gauges. Not many other manufacturers do.
We don’t like to pressure you, but we have more information.
You should be clearer now on how to decide what range gauge you need for your application to ensure accurate and safe pressure measurements for your application.
If you want to learn more about pressure gauges, feel free to visit our website and view any of our helpful white papers, webinars or guides in our resource center.
You can also check out some of our other articles about pressure gauges:
- Choosing the Right Pressure Gauge Dial Size
- How To Properly Store your Pressure Gauge
- How To Identify Your Pressure Gauge and Get the Proper Replacement
- How Often Should I Check the Calibration of My Pressure Gauge?
- How Much Do Mechanical Pressure Gauges Cost?
Feel free to contact us today to talk to one of our industry experts and get all your pressure gauge questions answered.