A Practical Guide to Pressure Stability During Longer Calibration Sessions

Why Does Pressure Become Unstable During Long Calibrations?

Pressure doesn’t exist in isolation. In any closed system, pressure is directly related to temperature, system volume, and the amount of gas or liquid in the system. The Ideal Gas Law describes this connection. When one changes, the others change as well.

The key takeaway is simple. If pressure changes, something else in the system has changed too. In pressure calibration, that "something" is often temperature or system volume rather than a leak.

Fortunately, pressure behavior during calibration is not random and can be traced back to a small number of common causes.

Common Causes of Pressure Instability During Calibration

Once you understand that pressure changes do not automatically indicate a leak, it becomes easier to narrow down what is really happening. 

The following are some of the most common contributors to pressure instability during longer calibration sessions.

Temperature Changes and the Adiabatic Effect

Rapid pressurization can cause the system temperature to rise, a phenomenon known as the adiabatic effect. 

When pumping stops, the system cools back toward ambient temperature, and pressure drops as it stabilizes. This behavior is often mistaken for leakage, but it is a normal thermal response of the system.

Leaks and System Volume Changes

Poorly sealed connections, expanding hoses, trapped air in liquid or contamination at connection points can all cause real pressure loss. 

Even when no gas is escaping, very small elastic changes in the system can slightly change the effective volume, which can affect pressure stability while pressure is being held.

Hysteresis Can Be Mistaken for Pressure Instability

In some cases, the issue isn’t pressure instability at all. It’s the way pressure was applied to the device under test.

If a pressure test point is overshot or undershot with a hand pump and then corrected, the pressure itself may be stable, but the device output can differ depending on the direction of approach. This behavior is known as hysteresis.

Hysteresis occurs when a device produces slightly different readings at the same pressure depending on whether the pressure is approached from above or from below.

How to Identify the Real Source of Pressure Instability

Before making adjustments, it helps to observe how pressure behaves over time.

• A pressure change that slows and then stabilizes is often the result of the system temperature settling
• A steady, linear pressure drop is more likely caused by a leak
• Different readings when approaching pressure from above versus below suggest hysteresis, often caused by overshooting or undershooting a test point before correcting
• Pressure that will not hold steady during long dwell times may indicate thermal- or volume-related effects

Recognizing when a test point was overshot or undershot helps prevent hysteresis from being mistaken for pressure instability.

These patterns can be difficult to recognize by watching live numerical readings alone. Viewing pressure data graphically often makes the source of instability much clearer. Plotting pressure over time, or comparing increasing and decreasing pressure points, helps reveal whether pressure changes are caused by temperature effects, leakage, or hysteresis in the device under test. 

That’s where tools that capture and visualize pressure data become especially useful. For example, the Ralston FieldLab Pressure Calibrator with FieldLab desktop software and the Bluetooth-enabled Ralston Field Gauge LC20 with the FieldLab mobile app display pressure graphically during calibration. Seeing pressure patterns visually makes it easier to recognize hysteresis and separate device behavior from system-related pressure instability.

6 Practical Ways to Improve Pressure Stability

Once you understand what influences pressure behavior, there are several practical steps you can take to improve stability during longer calibration sessions.

1. Allow Time for Temperature Stabilization

Pressurization is often applied quickly, especially when using a hand pump. Rapid pressurization raises gas temperature.

After reaching a test point, allow the system temperature to stabilize before making fine adjustments.

As the gas cools toward ambient temperature, pressure changes will slow, and readings will become more consistent.

2. Use High-Quality, Low-Volume Hoses

Hoses play an important role in pressure stability, particularly during long holds and outdoor calibration, because their characteristics influence how the system responds to temperature changes and shifts in effective system volume under pressure.

Using low-volume hoses that limit heat absorption and resist elastic expansion helps pressure settle more predictably and remain stable during extended holds.

Ralston Quick-test™ hoses are designed to help reduce temperature- and volume-related pressure changes. A reflective outer cover helps reduce temperature effects from the sun, while a reinforced inner core and small internal diameter limit elastic expansion and volume fluctuations.