Pressure Guage Sizing & Ranges
Gauge Size
When selecting a pressure gauge, the first factor to consider is the size of the gauge. The dial size should be large enough to ensure that you can easily read the measurement with the required accuracy. Pressure gauges typically come in standard sizes such as 63mm, 100mm, and 150mm, but other sizes are available for specific applications. For environments where visibility is a concern, such as in low-light or distant locations, larger dials may be preferable. Keep in mind that a larger gauge doesn’t necessarily mean more accuracy, but it does allow for easier reading.
Mounting
The mounting method for the pressure gauge is another crucial consideration. Pressure gauges can be mounted in various ways, including bottom, rear, panel, and flange mounts. The mounting type should align with your installation requirements to ensure stability and accurate readings. For instance:
- Bottom Mount: Ideal for direct mounting on top of the process.
- Back Mount: Best for panel or surface mounting with a head-on view.
- Panel Mount: Perfect for control panels with limited space.
- Flange Mount: Provides extra support in high-vibration areas.
When choosing a mounting type, also consider the thread type and size. In Australia, 1/4” BSPT (British Standard Pipe Taper) threads are common for 63mm gauges, while 3/8” BSPT and 1/2” NPT (National Pipe Taper) threads are standard for 100mm gauges. The correct thread type ensures a secure and leak-free connection to your process.
Construction
Pressure gauges are composed of two main components: the case (exterior housing) and the wetted parts (internal components exposed to the process fluid or gas). The case protects the gauge’s internals from environmental conditions, while the wetted parts come into direct contact with the process medium.
Case Materials:
- Plastic: Light and budget-friendly, but not ideal for tough environments.
- Stainless Steel: Highly resistant to corrosion, making it perfect for industrial use, especially in corrosive or high-temperature settings.
- Other Materials: Special cases may be made from aluminium or thermoplastics for added impact resistance or to keep things lightweight.
Wetted Part Materials:
- Brass/Copper Alloy: Best for non-corrosive fluids and gases.
- Stainless Steel: Highly resistant to corrosion, suitable for various liquids and gases, including corrosive ones.
- Exotic Materials: For challenging environments like high temperatures or corrosive conditions, materials like Monel, Hastelloy, or Inconel are ideal.
Range
The pressure range of the gauge is one of the most critical factors to consider. Gauges are available in a variety of predefined pressure ranges, which are standardised to accommodate most applications. Selecting a gauge with a range appropriate to your process ensures accuracy and longevity. A general rule is to select a gauge where the process pressure is at 50-75% of the gauge’s full scale. This provides a buffer for overpressure conditions and ensures the gauge isn’t operating at the extreme ends of its range.
800kPa gauges are not ‘officially’ a standard size but are usually available ex-stock from most manufacturers. For gauges with a range under 60kpa, a Capsule gauge is usually required. This gauge has a large diaphragm above the connection to allow measurement of such low pressures.
If your application requires a specific range that isn’t available in standard models, custom scaling and labelling options are available. However, these customisations may increase costs and lead times, so sticking to standard ranges is advisable when possible.
Accuracy
Pressure gauge accuracy is expressed as a percentage of the full-scale range, typically denoted as ±0.25%, ±0.5%, or ±1%. The required accuracy depends on the application. Critical processes may require higher accuracy gauges, while less critical applications can use gauges with lower accuracy. Ensure that the gauge’s accuracy meets or exceeds the requirements of your process to avoid operational issues.
Environmental Considerations
Consider the environment where the gauge will be installed. Factors such as ambient temperature, humidity, vibration, and exposure to corrosive elements can all impact the performance and longevity of the gauge. For instance:
- Temperature: Ensure the gauge is rated for the ambient and process temperatures it will encounter. Some gauges are filled with glycerine or silicone to dampen the effects of vibration and temperature fluctuations.
- Vibration: In high-vibration environments, a liquid-filled gauge can help dampen needle fluctuations and extend the life of the gauge.
- Corrosion: In corrosive environments, selecting materials like stainless steel for the case and wetted parts can prevent premature failure.
Special Features
Modern pressure gauges may include additional features to enhance functionality and reliability:
- Liquid-Filled Gauges: Filled with glycerine or silicone to reduce the effects of vibration and pulsation, and to lubricate internal parts, extending the life of the gauge.
- Overpressure Protection: Some gauges are equipped with a blowout back or pressure relief device to protect against overpressure conditions.
- Diaphragm Seals: Used in applications where the process medium is corrosive, highly viscous, or tends to crystallise. The diaphragm seal isolates the gauge from the process while still transmitting pressure accurately.
- Electric Output: Some gauges include electrical contacts or transmitters that can send signals to control systems or trigger alarms when pressure reaches a certain threshold.
Calibration and Maintenance
Regular calibration and maintenance of pressure gauges are essential to ensure they continue to provide accurate readings. Over time, gauges can drift out of calibration due to wear, environmental factors, or process conditions. Implementing a regular calibration schedule helps maintain the accuracy of your gauges and ensures they meet industry standards and regulations. Some industries may also require certification of calibration for compliance purposes.
Safety Considerations
When dealing with high-pressure systems, safety is paramount. Ensure that the selected gauge can handle the maximum pressure and conditions of your process to prevent accidents or equipment failure. In some cases, pressure relief valves or rupture discs may be installed alongside pressure gauges to provide additional protection.
By considering these factors, you can select the right pressure gauge for your specific needs, ensuring reliable performance and safety in your operations.