Understanding Pressure Measurement in Dairy Industry

In any dairy processing facility, maintaining precise control over fluid pressure is not optional – it’s essential. Whether milk is being pasteurised, homogenised, or concentrated into powder, pressure directly influences product safety, quality, and equipment performance. Understanding how pressure is measured and which instruments are used for the job forms the foundation of reliable dairy operations.

What is pressure and why does it matter in dairy processing? 🔗

Pressure is defined as the force exerted per unit area by a fluid – whether that fluid is liquid milk flowing through a pipe or steam heating a pasteuriser. In dairy plants, pressure measurement serves multiple critical functions: it ensures milk flows at the correct rate through heat exchangers, confirms that pasteurisation systems maintain the required overpressure, and verifies that vacuum evaporators operate under the right conditions for gentle concentration.

Without accurate pressure data, operators cannot guarantee that treated and untreated milk remain separated, that homogenisers produce a consistent product, or that equipment is protected from dangerous pressure build-up. Every stage – from raw milk storage tanks to final filling – depends on continuous, reliable pressure monitoring.

Key pressure concepts every dairy professional should know 🔗

Atmospheric pressure 🔗

Atmospheric pressure is the pressure exerted by the weight of air in the Earth’s atmosphere. At sea level, it equals approximately 101.325 kPa (or 14.696 psi). This value acts as the baseline reference for most pressure measurements in dairy plants. Atmospheric pressure matters especially when working with vacuum systems used in milk evaporation and spray drying, where reducing pressure below atmospheric level lowers the boiling point of milk and enables concentration without excessive heat damage.

Absolute pressure 🔗

Absolute pressure is the total pressure measured from a starting point of perfect vacuum – meaning zero pressure. It is calculated as the sum of gauge pressure and atmospheric pressure:

Absolute Pressure = Gauge Pressure + Atmospheric Pressure

This measurement is particularly important in steam-based dairy systems. The relationship between steam pressure and temperature is defined in terms of absolute pressure, so accurate absolute pressure readings are critical for determining exact pasteurisation temperatures.

Gauge pressure 🔗

Gauge pressure measures pressure relative to atmospheric pressure. When a pressure gauge in a dairy plant reads zero, it doesn’t mean there is no pressure – it means the pressure inside the system equals the surrounding atmospheric pressure. Most industrial pressure instruments display gauge pressure because it is more practical for everyday monitoring. If a pump discharge gauge in an HTST pasteuriser reads 3 bar, it means the pressure at that point is 3 bar above atmospheric pressure.

Vacuum 🔗

Vacuum refers to any pressure below atmospheric pressure. In dairy processing, vacuum conditions are essential in evaporators and dryers. A vacuum evaporator, for instance, reduces the internal pressure so that milk boils at a much lower temperature (sometimes as low as 40-50 °C), preserving flavour and nutritional value. Vacuum is typically expressed as a negative gauge pressure value, such as −0.8 bar, indicating the system is 0.8 bar below atmospheric pressure.

The Bourdon tube pressure gauge 🔗

The Bourdon tube gauge is the most widely used mechanical pressure measuring instrument in dairy and other industries. Invented by Eugène Bourdon in 1849, this instrument has remained popular for over 170 years due to its simplicity, reliability, and cost-effectiveness.

Construction 🔗

A Bourdon tube gauge consists of the following main components:

Bourdon tube: This is the core sensing element – a hollow, C-shaped (or sometimes spiral or helical) metal tube with an elliptical or flattened cross-section. One end of the tube is fixed and open to the process fluid, while the other end is sealed and free to move. Common tube materials include phosphor bronze, brass, beryllium copper, and stainless steel, chosen based on chemical compatibility with the fluid being measured.

Socket (connector): This threaded fitting connects the gauge to the piping or vessel where pressure is to be measured. It provides the entry point for the pressurised fluid into the Bourdon tube.

Mechanical linkage and gear mechanism: A rack-and-pinion or lever system connects the free end of the Bourdon tube to the pointer. This mechanism amplifies the small movement of the tube tip and converts it into a rotational motion that sweeps the pointer across the dial.

Dial and pointer: The calibrated dial displays pressure values in units such as bar, psi, or kPa, while the pointer indicates the measured pressure.

Casing: A protective housing, usually made of stainless steel or ABS plastic, shields the internal components from environmental damage.

Working principle 🔗

The Bourdon tube operates on a straightforward elastic deformation principle. When pressurised fluid enters the fixed open end, it pushes against the inner walls of the flattened tube. Because the tube has an elliptical cross-section, the internal pressure tries to force it into a more circular shape. As the cross-section rounds out, the curved tube tends to straighten – this is the key mechanical action. The sealed free end of the tube moves outward, and this displacement (typically just a few millimetres) is transmitted through the linkage and gearing to the pointer, which rotates proportionally across the calibrated dial.

When pressure is removed, the elastic properties of the tube material cause it to return to its original shape, and the pointer returns to zero. This entire process is purely mechanical – no electrical power is required.

Types of Bourdon tubes 🔗

The shape of the Bourdon tube varies depending on the pressure range being measured. C-type tubes, bent at approximately 270°, are the most common and are suitable for low to medium pressures (up to about 60 bar). For higher pressure ranges, spiral tubes offer improved sensitivity, while helical tubes provide even greater accuracy and resistance to overpressure conditions, handling pressures up to several thousand bar.

Bourdon gauges are also available in dry and liquid-filled (glycerine-filled) variants. Liquid-filled gauges dampen pointer vibration and are better suited for applications where pulsating pressure or mechanical vibration is present – a common scenario near dairy pumps and homogenisers.

Bourdon gauges in dairy applications 🔗

In dairy plants, Bourdon tube gauges are commonly installed at pump discharge points, on pasteuriser heating sections, at homogeniser outlets, and on steam lines. They provide operators with quick visual readings of system pressure. For hygienic dairy applications, specialised sanitary diaphragm pressure gauges (such as those made by WIKA) feature flush-mounted diaphragms and food-grade materials that prevent milk residue from accumulating inside the gauge.

Bourdon gauges typically offer an accuracy of about ±1 to ±2% of full scale. While this is adequate for routine monitoring, more precise measurements require other instruments – which is where manometers come in.

Manometers 🔗

A manometer is one of the oldest and most fundamental pressure measurement devices, relying on the height of a liquid column to indicate pressure. Despite being centuries old in concept (the U-tube was developed by Christiaan Huygens in 1661), manometers remain valuable in dairy operations for calibration, low-pressure measurement, and differential pressure applications.

Working principle 🔗

All manometers operate on the hydrostatic balance principle: a pressure applied to a confined liquid causes that liquid to rise or fall in a tube, and the height difference directly corresponds to the pressure. The relationship is given by:

P = ρ × g × h

Where P is the pressure, ρ (rho) is the density of the manometer liquid, g is gravitational acceleration, and h is the height difference between the two liquid columns. The measurement depends only on fluid density and vertical height – not on the tube’s diameter or shape – which is why manometers are considered primary pressure standards.

Types of manometers 🔗

U-tube manometer: The most common type. It consists of a transparent U-shaped tube partially filled with a liquid – historically mercury, but now more commonly water or coloured oils due to health and environmental concerns. One end is connected to the system being measured, and the other is open to the atmosphere. The difference in liquid height between the two limbs gives the gauge pressure. When both ends are connected to two different pressure points, the device measures differential pressure.

Well-type (reservoir) manometer: One limb of the U-tube is replaced by a large-diameter reservoir. Because the reservoir’s cross-section is much larger, the liquid level change in it is negligible, and the operator only needs to read the scale on the narrow tube side. This simplifies reading and reduces errors.

Inclined-tube manometer: The measuring tube is tilted at an angle (typically 10° to 30°) from horizontal. This spreads the liquid movement over a longer length of tube, making it possible to detect very small pressure changes with greater precision. Inclined manometers are particularly useful for measuring draft pressures in HVAC and boiler systems, and for verifying low-pressure readings in dairy evaporators.

Manometers in dairy applications 🔗

In dairy processing, manometers serve several important roles. They are used for measuring small pressure differences across filters, heat exchangers, and membrane systems – helping operators determine when cleaning or replacement is needed. They are also used to monitor vacuum levels in evaporators where gentle concentration of milk takes place under reduced pressure.

Perhaps most importantly, manometers serve as calibration standards for other pressure instruments, including Bourdon gauges and electronic pressure transmitters. Because a manometer’s accuracy depends only on the known density of the liquid and the measured height difference – with no mechanical parts that can wear or drift – it provides a reliable reference against which other devices can be checked.

Bourdon gauge vs manometer: choosing the right instrument 🔗

Both instruments measure pressure, but they serve different purposes in a dairy facility. Bourdon tube gauges are robust, portable, easy to read, and suitable for a wide range of pressures – making them ideal for routine monitoring at various points in the processing line. However, they require periodic calibration and can be affected by vibration and temperature changes.

Manometers, on the other hand, excel at precision. With no moving mechanical parts, they are inherently accurate and reliable for low-pressure and differential pressure measurement. Their main limitations are bulkiness and unsuitability for high-pressure applications. In practice, dairy plants use both: Bourdon gauges for everyday operational readings and manometers for verification, calibration, and sensitive measurements.

Maintaining accuracy in dairy pressure measurement 🔗

Accurate pressure readings depend on more than just choosing the right instrument. In dairy environments, hygienic design is paramount. Pressure instruments must use food-grade materials, sanitary process connections (such as Tri-Clamp or DIN 11851 fittings), and surfaces with low roughness values to prevent bacterial growth.

Regular calibration schedules are essential. Bourdon gauges should be checked periodically against a known reference – often a manometer or a certified dead-weight tester. Environmental factors like temperature fluctuations and mechanical vibration can introduce measurement errors over time, so gauges installed near pumps or on vibrating pipelines benefit from glycerine-filled casings that dampen pointer movement.

For dairy plants pursuing digital transformation, modern electronic pressure transmitters are increasingly used alongside traditional mechanical instruments. These transmitters convert pressure into electrical signals that can be integrated into SCADA systems and programmable controllers, enabling real-time monitoring and automated control. However, traditional Bourdon gauges and manometers continue to serve as reliable backup instruments and local indicators.

What do you think? How important is it for dairy professionals to understand the fundamentals of manual pressure instruments like Bourdon gauges and manometers, even as plants move toward fully digital monitoring systems? Could a lack of this foundational knowledge create risks during equipment failures or power outages?

References

1. https://www.azosensors.com/article.aspx?ArticleID=2221
2. https://en.wikipedia.org/wiki/Pressure_measurement
3. https://tameson.com/pages/bourdon-tube-pressure-gauge
4. https://www.sciencedirect.com/topics/engineering/bourdon-tube
5. https://instrumentationtools.com/bourdon-tube-pressure-gauge-working-principle-animation/
6. https://www.elpro.si/en/the-importance-of-measurement-in-the-dairy-industry/
7. https://www.realpars.com/blog/manometer
8. https://www.dwyeromega.com/en-us/resources/measurement-of-pressure-with-the-manometer
9. https://www.anderson-negele.com/us/dairy-process/
10. https://cdsentec.com/process-instruments-for-dairy-processing/