Why is the pressure gauge on my airless sprayer only partially filled with liquid?

Recently, a concerned customer contacted us. They had purchased an airless sprayer from us that was equipped with a pressure gauge.

Inside the gauge, a visible air bubble could be seen, and the customer feared that liquid had leaked out or that the gauge might be defective.

This was the customer’s question:

Why is the pressure gauge only half full?
Is liquid missing or is the gauge defective?

The short answer: No — this is completely normal and technically intentional.
For those who would like a more detailed explanation, you’ll find it below.

What Is a Liquid-Filled Pressure Gauge?

Liquid-filled pressure gauges are widely used in paint spraying technology.
They are typically filled with glycerin or silicone oil and are used wherever:

• high pressures occur (e.g. on airless sprayers),
• pressure fluctuations or pulsations are present,
• a stable, easy-to-read display is required.

In addition to classic analog gauges, digital pressure display systems are also available.

Why Is the Pressure Gauge Not Completely Filled?

A liquid-filled pressure gauge is intentionally filled to only about 60–70%.
The remaining air space is not a defect, but serves several important technical functions.

1. Temperature Compensation

The filling liquid expands when heated.
During operation, the pressure gauge warms up, and ambient temperature or direct sunlight also has an effect.
If the housing were completely filled with liquid, dangerous internal pressure could build up as temperatures rise.

This could:

• damage the viewing window,
• destroy seals,
• distort measurement accuracy.

The air space acts as an expansion buffer and protects the gauge while ensuring reliable operation.

2. Vibration Damping on Airless Sprayers

Airless pumps generate pressure pulsations by design.
The liquid inside the gauge dampens these vibrations and prevents the needle from oscillating.
This results in a stable, easy-to-read pressure display — essential for precise pressure adjustment.

3. Pressure Equalization Inside the Gauge

The remaining air space allows controlled pressure equalization inside the gauge, for example during:

• rapid pressure changes in the system,
• temperature fluctuations,
• transport or changes in the position of the device.

Without this air space, the gauge would be exposed to unnecessary mechanical stress.

4. Extended Service Life & Higher Accuracy

Thanks to the combination of liquid and air space:

• springs and measuring mechanisms are protected,
• seals are subjected to less stress,
• measurement errors are reduced.

As a result, the pressure gauge remains accurate and reliable for a longer period of time.

Can the Liquid Level in the Pressure Gauge Change?

Yes — and this is normal.
The visible liquid level may vary due to:

• temperature differences,
• the mounting position of the gauge,
• transport conditions.

When Is a Pressure Gauge Damaged and Needs Replacement?

Pressure gauges are often damaged during transport, for example if the airless sprayer tips over or if the gauge is struck.
A cracked glass or visible mechanical damage indicates that the gauge should be replaced to ensure safe and accurate pressure readings.

Other signs of possible damage include:

• leaking liquid,
• cloudy or dull glass (often caused by aggressive cleaning agents),
• a stuck needle that no longer responds.

Common Misconception: Half-Filled = Defective

The opposite is true.
A completely filled pressure gauge would be technically problematic.
A partially filled gauge with a visible air bubble is correct and ensures optimal function.

The air space is necessary for:

• temperature compensation,
• vibration damping,
• pressure equalization,
• long service life and reliable accuracy.

Need advice?

If you have any questions about choosing the right pressure gauge for an airless device or any other airless-related topic, we are available to advise you.

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Questions or queries?

For any further questions or if you would like to get your hands on one of these products seen in this article, you can contact us on +49 (0) 30 220 15436, fill out our contact form or email us. If you wish to purchase one of the products mentioned above, you can check them out in our online store. You can also visit us at our service base during office hours.