What are the main reasons for hysteresis in petrochemical PP Diaphragm Pressure Gauge measurements

In the complex fluid measurements of the petroleum and chemical industries, the accuracy and stability of pressure instrumentation are crucial. Polypropylene (PP) diaphragm pressure gauges stand out for their excellent corrosion resistance, making them ideal for handling acidic and alkaline corrosive media. However, professional users frequently focus on a key performance indicator: Hysteresis.

Hysteresis refers to the phenomenon where the pressure gauge's indicated value differs when reaching a specific set point from a low-pressure state (ascending pressure) versus reaching the same point from a high-pressure state (descending pressure). This discrepancy is not a random error but a systematic deviation resulting from the instrument's internal physical characteristics and structural limitations. For high-precision control in petrochemical processes, understanding and minimizing hysteresis is essential for ensuring product quality and operational safety.

I. Mechanical Hysteresis of Elastic Elements: Material Intrinsic Properties

The core components of a PP diaphragm pressure gauge are the diaphragm and the internal movement mechanism. The primary source of hysteresis stems from the mechanical imperfections of these elastic elements.

1. Elastic Hysteresis of Diaphragm Material

Although PP diaphragms are often enhanced with PTFE coatings or used as part of a composite structure, as an elastic element, the strain recovery path is not perfectly identical when stress is applied and subsequently released.

• As pressure increases, the diaphragm deforms.

• As pressure decreases, internal microstructural friction and molecular chain rearrangement within the diaphragm delay its complete return to the initial state.

• This energy dissipation causes the strain (or displacement) during the ascending pressure process to differ from that during the descending process at the same pressure value, manifesting directly as pointer hysteresis.

• Especially for polymeric material PP, its viscoelastic characteristics are more pronounced. Under long-term or cyclical pressure application, this mechanical hysteresis effect is often more significant than in metal diaphragms.

2. Minor Friction in the Movement Mechanism

The displacement of the diaphragm must be transmitted to the pointer via precision mechanical components such as linkage rods, sector gears, and central gears. Minute frictional forces between these moving pairs constitute the second major source of hysteresis.

• During the ascending pressure process, the frictional force opposes the direction of motion.

• During the descending pressure process, the direction of the frictional force reverses.

• At the moment pressure reverses, the mechanism must overcome static friction before movement recommences, causing a delay between the pressure change and the pointer response.

• Even micron-level friction is sufficient to cause observable deviation in the pressure indication.

II. Viscosity and Compressibility Effects in the Fluid Transfer System

PP diaphragm pressure gauges typically utilize a diaphragm seal system with a fill fluid to isolate corrosive media. The physical properties of this fluid transfer system are significant contributors to hysteresis.

1. Viscosity of the Fill Fluid

The fill fluid (such as silicone oil or fluorocarbon oil) possesses a certain degree of viscosity. When the diaphragm deforms under pressure and displaces the fluid:

• The liquid must flow through internal channels and capillaries.

• The liquid's internal friction (viscous drag) impedes the immediate transmission of energy.

• This is particularly relevant during rapid pressure changes or when low ambient temperatures increase viscosity, slowing the fluid's mobility and delaying pressure transmission, thereby exacerbating the hysteresis phenomenon.

2. Gas Dissolution and Residual Microbubbles

If the degassing process is incomplete during the filling of the fluid, residual microbubbles or gases dissolved in the liquid introduce compressibility upon pressure changes.

• This causes the initial displacement of the diaphragm to first compress these gas bubbles rather than immediately transmitting the pressure to the Bourdon tube or internal sensor.

• The gas compression and release process is non-linear and time-delayed, creating an "elastic buffer" effect that introduces measurement hysteresis.

III. Stress Release and Relaxation in Structural Components

Long-term operation or thermal cycling can lead to stress relaxation in the PP housing and connection system, which is another indirect factor contributing to hysteresis.

• The preload connection (e.g., bolted assembly) at the edges of the PP housing and diaphragm can experience creep relaxation over time and with temperature variations.

• The relaxation of the preload changes the fixed boundary conditions of the diaphragm, meaning the starting state and path for each pressure cycle may not be perfectly consistent.

• When pressure is repeatedly applied, the tiny movements and stress redistribution at the connection interface cause a slight drift in the elastic element's zero point, leading to the separation of the ascending and descending pressure paths.