What type of sealing fluid is usually selected for PP diaphragm pressure gauges

The core function of a PP diaphragm pressure gauge, particularly one used in corrosive environments such as the petrochemical and chemical industries, is to isolate the process media from the pressure measuring instrument (typically a Bourdon tube) using a diaphragm. The key media for achieving this pressure transmission and isolation are the sealing fluid (also known as isolating fluid) and the fill fluid. The choice of sealing fluid directly determines the instrument's measurement accuracy, response speed, operating temperature range, and safety.

Common Sealing Fluid Types for PP Diaphragm Pressure Gauges

In PP diaphragm pressure gauge systems, the sealing fluid must possess excellent pressure transmission performance, good temperature stability, and compatibility with both the instrument's internal components and the external process media. Common professional sealing fluid types include:

1. Glycerin and Water-Glycerin Mixtures

Characteristics and Applications: Glycerin is one of the most basic and widely used fill fluids. It offers low cost and excellent temperature characteristics. The applicable temperature range for pure glycerin is generally around -20°C to 80°C.

Compatibility: Suitable for general water-based or neutral media.

Limitations: Glycerin is not suitable for vacuum applications due to its high vapor pressure, which can lead to measurement errors. Furthermore, glycerin exhibits poor stability in oxidizing or highly corrosive environments and has limited compatibility with materials such as PP housings and Viton diaphragms. For PP diaphragm gauges, glycerin should only be used in less corrosive conditions.

2. Silicone Oil

Properties and Applications: Silicone oil is the most commonly used and most adaptable sealing fluid in PP diaphragm pressure gauges. Depending on the model and viscosity, silicone oil can cover an extremely wide temperature range.

Low-Temperature Silicone: Suitable for extremely low temperature conditions, such as refrigeration or polar environments, due to its extremely low freezing point.

Standard Silicone: Suitable for use under most common temperature and pressure conditions.

High-Temperature Silicone: Suitable for harsh, high-temperature environments exceeding 200°C or even 300°C, ensuring stable viscosity and volume at high temperatures.

Advantages: Excellent temperature stability and low vapor pressure make it suitable for high vacuum and absolute pressure measurements. It also offers good compatibility with PP and most PTFE and Viton diaphragm materials.

Type Differentiation: When selecting a silicone oil, customers should clearly determine whether to choose a low-viscosity silicone oil for improved response time or a high-temperature type to cope with process temperatures.

3. Fluorinated Oil (Halocarbon)

Characteristics and Applications: Fluorinated oil (such as Halocarbon and Krytox) is a high-performance fill fluid.

Advantages: Their greatest strengths are their extremely high chemical inertness and oxygen compatibility. This makes them the preferred choice for ensuring safety when measuring highly oxidizing media such as oxygen, chlorine, and fluorine.

Applications: They are particularly suitable for chlor-alkali processes in the petrochemical industry and processes involving highly reactive chemicals. While more expensive than silicone oil, they are irreplaceable for applications requiring the highest safety standards.

Key Principles for Selecting Sealing Fluids for PP Diaphragm Pressure Gauges

Choosing a sealing fluid for a PP diaphragm pressure gauge is not a single factor, but rather a result of a multi-faceted trade-off.

1. Process Media Compatibility

This is the primary consideration when selecting a fill fluid. Although the diaphragm physically isolates the process media, it is still important to consider whether the fill fluid will react violently with the process media (such as explosion, combustion, or the generation of toxic gases) in the event of a diaphragm rupture. For example, in oxygen applications, fluorinated oil is essential, as silicone oil or glycerin can ignite when in contact with pure oxygen.

2. Operating Temperature Range

The sealing fluid must remain liquid and maintain a stable volume throughout the entire process temperature range.

Boiling Point: The boiling point of the sealing fluid must be higher than the maximum operating temperature. Boiling will cause measured pressure distortion and instrument damage.

Freezing Point: The freezing point of the sealing fluid must be lower than the minimum ambient temperature. If it freezes, pressure transmission will be lost and the instrument will fail.

Thermal Expansion: Thermal expansion of the fill fluid is one of the main causes of temperature errors. In extreme temperature differences, it is necessary to select a fluid with a low thermal expansion coefficient or use capillary tubes for remote installation and add a volume compensator.

3. Measurement Characteristics and Viscosity

The viscosity of the sealing fluid directly affects the instrument's response time.

Low viscosity: Faster transmission speed and shorter response time make it more suitable for measurements requiring a fast response.

High viscosity: This results in slower transmission speeds and longer response times, but it is more suitable for providing some damping under high vibration or pulse pressure conditions, stabilizing the needle. High-viscosity liquids are also preferred for high vacuum measurements.

4. Pressure Type Considerations

Vacuum and Absolute Pressure: When measuring vacuum or absolute pressure below atmospheric pressure, silicone oil or fluorinated oil with extremely low vapor pressure must be used to prevent vaporization of the sealing fluid from affecting measurement accuracy. Glycerin or water-based solutions are generally not suitable.

Hydrostatic Pressure Influence: For remote installations (with capillary tubes), the density of the fill fluid can introduce hydrostatic errors, requiring professional calibration to compensate.