Is it a leak? Understanding the adiabatic process in pressure calibration

The adiabatic process is a physical phenomenon that can make us think our pressure calibration system is leaking.

The adiabatic process is something we have all encountered if we have been working with pressure calibration. Often we don’t realise it, and we think there is a leak in the system.

In short, the adiabatic process is a physical phenomenon that causes the pressure medium’s temperature to increase when we increase the pressure in a closed system. When we stop pumping, the medium’s temperature cools down, and it will cause the pressure to drop — so it does indeed look like a leak in the system.

You can find many in-depth, complicated physical or mathematical explanations of the adiabatic process on the internet. But hey, we are not physicists or mathematicians, we are calibration professionals! Lucky for you, you’ve got me to simplify this for you!

In this article we take a closer look at the adiabatic process, and how to recognise and avoid it. A little bit of compulsory theory to start with and then diving into practical things.

What is the adiabatic process?

An adiabatic process is a thermodynamic change whereby no heat is exchanged between a system and its surroundings.

For an ideal gas undergoing an adiabatic process, the first law of thermodynamics applies. This is the law of the conservation of energy, which states that, although energy can change form, it can’t be created or destroyed.

We might remember from our school physics the formula with pressure, volume and temperature, and how they depend on each other. The combined gas law says that the relationship between pressure, volume and absolute temperature is constant:

where:

    P = pressure
    V = volume
    T = absolute temperature
    k = a constant

When using the above formula and comparing the same pressure system under two different conditions (different pressure), the law can be written as the following formula:

We can think of this formula as representing our normal pressure calibration system, having a closed, fixed volume. The two sides of the above formula represent two different stages in our system — one with a lower pressure and the second one with a higher pressure. For example, the left side can be our system with no pressure, and the right side the same system with high pressure applied.

Looking at the formula, we can conclude that as the volume of a pressure calibration system remains the same, and if the pressure changes, then the temperature must also change. Or the other way around, if the temperature changes, then the pressure will also change.

Figure 1 shows a typical pressure calibration system, where we have a pressure pump, pressure T-hose, pressure instrument to be calibrated (1) and pressure calibrator (2).

Figure 1: A typical pressure calibration system.

Typically, the volume of our pressure calibration system remains the same, and we change the pressure going through the calibration points. When we change the pressure (and the volume remains the same) the temperature of the medium will change.

We can most commonly see the adiabatic process when we raise the pressure quickly with our calibration hand pump, causing the medium (air) to get warmer. Once we stop pumping, the medium starts to cool down causing the pressure to drop — at first quickly, but then slowing down and finally stabilising. This pressure drop looks like a leak in the system.

The same also happens with decreasing pressure — if we decrease the pressure quickly, the medium gets colder. When we stop decreasing, the media will start to warm up, causing the pressure to rise. This may seem odd at first — how can the pressure rise by itself? Of course, the pressure does not increase a lot, but enough for you to see it and wonder what’s going on.

So, the adiabatic process works in both ways, with increasing and decreasing pressure. The faster you change the pressure, the more the medium temperature will change, and the bigger effect you can see.

If you wait a while, the temperature of the pressure medium will stabilise to the surrounding temperature and the effects of the adiabatic process will no longer be visible. This is the essential learning from the adiabatic effect.

How do you know when it’s the adiabatic process and when it’s a leak?

The main difference between the adiabatic process and a leak is that the pressure drop caused by the adiabatic process is bigger in the beginning, then slows down and disappears as the system stabilises. In contrast, the pressure drop caused by a leak is linear and continues at the same rate. Figure 2 demonstrates the difference.

Figure 2: The pressure drop caused by the adiabatic process is fast at first, but then slows down and eventually stabilises, while the pressure drop caused by a leak is linear.

How to avoid the adiabatic process

Pressurise slowly

One of the easiest ways to minimise the adiabatic effects is to change the pressure slowly. By doing so, you allow the medium more time to reach the same temperature as its surroundings, minimising any temporary temperature changes. In practice, if you increase the pressure with a hand pump, and you step through several increasing calibration points, this may already be slow enough to avoid seeing the adiabatic process.

If you pump as quickly as you can up to 300 psi (20 bar), then you will most certainly see the effect of the adiabatic process.

Wait after pressurising

After adjusting the pressure, give it some time to stabilise. A minute or two should do the trick. This allows any temperature changes in the medium to reach equilibrium with the ambient conditions, and the pressure will stabilise accordingly.

Pressure media

You can also affect the adiabatic process with your choice of pressure medium. In practice it is of course not always possible to change the medium. Your normal hand pump uses the air as media, but for higher pressure, you may use a hydraulic pump with water or oil as the medium.

The effects of the adiabatic process are generally more prominent in air- or gas-operated calibration pumps than in hydraulic (water or oil) ones. This is mainly due to gas being much more compressible, so the pressure increase will push gas molecules closer together, and this work done in gas is transformed into energy, causing heat. In addition, gas or air has lower thermal conductivity than liquids, so less heat is conducted away from gas.

Conclusion

In our service department, we regularly get questions about pressure pumps having leaks, while in most cases it turns out to be the adiabatic process that has made the customer think that there is a leak.

Understanding the adiabatic process and its impact on calibration pressure pumps is crucial for users to avoid misdiagnosing issues. By changing pressure at a moderate pace and allowing adequate time for stabilisation, you can achieve more accurate and consistent results.

This article was originally published on the Beamex blog at https://blog.beamex.com/adiabatic-process-in-pressure-calibration. Beamex products are distributed in Australia by AMS Instrumentation & Calibration Pty Ltd.

Top image credit: iStock.com/Andrey Radchenko