Have you ever wondered how a digital tyre pressure gauge works? You connect it to the tyre valve, and a number appears on the screen. But how does it turn the pressure of the air into an electrical reading?
Pressure sensors are used in more places than you might realise. They help monitor car tyres, take blood pressure readings and control equipment in factories. In hospitals, ventilators use pressure sensors to help monitor and control the air supplied to patients.
These measurements help us make decisions. Does a tyre need more air? Is a machine working within its safe pressure range? The sensor provides information that a person or an automatic system can act on.
Since starting work in sensor manufacturing, I’ve become more interested in how these devices work. Several ideas we study at GCSE come together inside them. Let’s follow those ideas from the pressure itself to the number on the screen.
First, a little GCSE physics
Pressure tells us how much force acts over a particular area:
p = F / A
pressure = force ÷ area
For the same force, a smaller area means a greater pressure. This helps explain why a sharp knife cuts more easily than a blunt one.
Gases and liquids exert pressure too. Inside a tyre, air particles continually collide with the inner walls, exerting a force on them.
To measure pressure, we need something that responds to that force in a predictable way.
A thin surface that bends
One way to build a pressure sensor is to use a thin surface called a diaphragm.
The diaphragm is held around its edge. If the pressure on one side is greater than on the other, it bends slightly. You can picture this by imagining a tightly stretched sheet being pushed in the middle.
The movement inside a sensor can be too small to see, but it is enough to affect tiny resistors on the diaphragm. These are called strain gauges. As the surface bends, they stretch or compress with it, changing their resistance.
Different pressure sensors work in different ways. We’re following a resistive pressure sensor, named after the resistance change it uses to detect pressure.
How does bending change resistance?
At GCSE, we learn that a wire’s resistance depends partly on its dimensions. For wires made from the same material at the same temperature:
- A longer wire has a higher resistance.
- A thicker wire has a lower resistance.
When a conductor stretches, it becomes longer and usually thinner. Both changes tend to increase its resistance.
The resistors on the diaphragm respond to its bending in a similar way. Their changes in shape produce changes in resistance. The material they are made from also affects how strongly they respond.
We now have a link between pressure and electricity:
Pressure makes the diaphragm bend, and that bending changes resistance.
The change might be tiny. Fortunately, an electrical circuit can detect it.
From resistance to voltage
The resistors form part of a circuit supplied with electrical energy. When their resistances change, the circuit’s output voltage changes too.
You may know voltage as potential difference, measured in volts. The sensor’s electronics can measure this changing voltage.
Many resistive pressure sensors use four resistors connected in a circuit called a Wheatstone bridge. Some are positioned so their resistance increases as the diaphragm bends, while others decrease. The circuit uses these changes together to produce a voltage signal.
But why use four resistors when one can already respond to bending? That arrangement helps the sensor detect very small changes, as we’ll explore in the next post.
But there is still a question to answer. If the electronics measure voltage, how do we get a pressure reading?
How does the device know the pressure?
Imagine making a thermometer with a liquid that rises as it warms. Seeing the liquid rise would tell you that the temperature had increased. It would not, by itself, tell you the temperature in degrees Celsius. You would need to establish a scale.
A pressure sensor needs something similar.
During testing, known pressures are applied and the sensor’s electrical output is recorded. This establishes the relationship between pressure and output. The process is called calibration.
For a simple example, imagine a complete measuring device set up to give these readings:
| Applied pressure | Output voltage |
|---|---|
| 0 bar | 1 volt |
| 5 bar | 3 volts |
| 10 bar | 5 volts |
These are illustrative values, rather than readings from a particular sensor.
If the output increases evenly between those points, a reading of 2 volts would represent 2.5 bar.
The electronics use the established relationship to calculate pressure from the measured voltage. That pressure can then appear on a screen or be used to control equipment.
Where do thin films fit in?
You might picture a resistor as a small component with a wire at each end. In a thin-film pressure sensor, the resistors are made from very thin layers of material deposited onto a prepared surface.
Those layers are patterned to form the sensing circuit on the diaphragm. As the diaphragm bends, the resistors deform with it.
Making these layers and shaping them accurately involves some interesting manufacturing techniques. We’ll return to those in another post. First, it helps to understand what the finished device is trying to measure.
Back to the number on the screen
We can now follow the whole process:
- A pressure difference makes the diaphragm bend.
- The bending changes the resistors’ resistance.
- The circuit’s output voltage changes.
- The electronics use calibration to calculate the pressure.
- The pressure reading appears on the screen.
Each step connects to the next. The diaphragm responds to pressure, the resistors respond to its movement, and the circuit produces a measurable electrical signal. Calibration gives that signal meaning.
So, next time you use a digital pressure gauge, consider what happens before the number appears. Inside the sensor, a tiny change in shape helps turn the push of air into a measurement you can use.
We’ve followed pressure all the way to a reading on a screen, but we’ve left one part unexplained: the circuit connecting those tiny resistors. Why are there four, and how does comparing them help us measure pressure? That’s the question we’ll tackle next.
