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Our New Pressure Switch!

Lots of uses and as cheap as.

by Julian Edgar

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About 12 months ago we ran a series of articles on using two very low cost pressure switches that we'd sourced. The boost pressure switch triggered at just 1kPa (0.145 psi), and we covered some exceptional uses for it. We showed at "Siting Cold Air Intakes" how it could be used to sense aerodynamic pressures over the surface of a car (eg for finding the best site for a cold air engine intake), then in "Powering Up the Pressure Switch" we added an anti-bounce feature to the switch.

All that was fine and dandy - but the demand for the ultra-sensitive pressure switches was so high that we soon sold out of both the pressure and vacuum switches.

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So when we discovered another batch of very sensitive switches that can measure both vacuum and pressure, we jumped at the chance of getting a supply. But note that - just like those previous switches - numbers available in the AutoSpeed shop are quite limited.

The New Switch

The new switch is also very sensitive - in fact, after a minor modification, even more sensitive than the last switches! It is also easily adjustable for trip point. The following table shows the switch trigger points:

Switch Setting Trip Pressure
As delivered 5.8 kPa (0.84 psi)
Adjustment screw wound in 9 kPa (1.3 psi)
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Note that since the switch has both vacuum and pressure ports, these values apply to both ports. In other words, either applying a negative pressure to the vacuum port, or a positive pressure to the other port, will result in the switch tripping. The pressure port is the one nearer the adjustment screw.

By replacing the internal spring with a stiffer one, the trip pressure can be increased. However, the switch - as with the last pressure and vacuum switches we carried - is really designed as a very sensitive switch, rather than one designed to turn on at say 6 psi turbo boost.

And it's possible to make the switch extraordinarily sensitive. Undo the adjustment screw until it falls out, remove the internal spring, then place the adjustment screw back in the switch, just winding it in enough to seal the opening. Configured like this, the switch has the following characteristics:

Switch Setting Trip Pressure
Internal spring removed 0.3 kPa (0.04 psi)

This 0.3 kPa sensitivity is three times as sensitive as the last pressure switch we were able to source! Literally, blowing on the port can trip the switch...

The switch is a single pole, normally open design which uses silver-plated brass contacts. It is rated for operation in environments from -10 degrees to 85 degrees Celsius, and can cope with a maximum over-pressure of 1 Bar (14.5 psi). As with the previous switch, the new switch is designed to operate only very small electrical loads - so it needs to be connected to relay if you're going to be able to operate fans or other power-hungries. The new switch can be used with the relay module that we covered in detail at "Powering Up the Pressure Switch".

Using It

The uses that can be made of the new switch are very similar to those that we covered with the old switch. So rather than repeating those stories, here are just some summaries:

  1. Finding Aerodynamic High Pressures
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    It's easy using the switch to find areas of high aerodynamic pressure on the car. That's pretty useful because if you site the intake of a cold air duct (leading to the engine's airbox) in an area of high pressure, more air will flow into the duct, making more power.

    Finding these spots is easy. Remove the internal pressure switch spring to increase its sensitivity, and then connect the switch to a low powered buzzer and a battery. Run a tube from the pressure port of the switch to areas of the car's body where you want to investigate the aerodynamic pressure that's acting on the body.

    In our previous article on this topic ("Siting Cold Air Intakes") we measured the pressures acting on the front of a 1991 Lexus. By monitoring the speed with which you're travelling when the switch clicks over (and the buzzer sounds), you can get a good idea of the relative pressures acting on the car. If the switch clicks over at a lower speed, then the pressure there is higher.

    These results were found by using the previous switch:

    Probe Location Speed Needed to Trigger Switch
    (higher speed = lower pressure being found)
    Blocked grille in lower front bumper 85 km/h
    Radiator grille between headlights 110 km/h
    Surface of headlight 110 km/h
    Base of windscreen, centre 145 km/h
    Base of windscreen, off centre 150 km/h

    Therefore the area at the front of the car with the highest pressure was on the lower front bumper, where there's a sealed-off grille.

    Click for larger image

    However, note that because of the sensitivity of the previous switch, some testing required fairly fast road speeds. In contrast, the much greater sensitivity of the new switch means that the same sort of testing can be carried out, but at slower speeds.

  3. Switching Loads on and off Based on Airflow
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    In our previous article ["Aero Flow Switch!"] we showed how a sensitive pressure switch could be used to turn on (and off) devices based on the airflow speed over the vehicle. One good application of this is to turn on an intercooler fan when the car is slowing down. Taking this approach is really tricky, because it also takes into account whether the car is experiencing a headwind or tailwind!

    With the new switch set for its maximum sensitivity, it triggers at a road speed of about 40 km/h when the sensing hose is placed in a forward-facing open-mouthed small cup (I used the lower half of a 35mm film container). So it's easy to use the new switch (together with our relay module) to turn on an intercooler airflow fan whenever the speed of the car drops below 40 km/h.

  5. Blocked Filter Alarm
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    One very good use for a low pressure switch is as an indicator that the airfilter is starting to restrict flow. As explained in detail in our "Dirty Filter Warning" article, by plumbing a pressure switch to both sides of the filter, the pressure drop across it can be monitored. When the pressure drop becomes too high, the switch will trip. The vacuum switch that we used to perform this function tripped at 0.09 psi, so when its internal spring is removed, the new pressure switch (which has a vacuum port, remember) is nearly twice as sensitive!

    Click for larger image

    Because of its twin port design, the new switch is even easier to mount that the old one, and adjustment of the trip point can be made by turning the adjustment screw and/or replacing the internal spring.


We've got a limited number of switches in the shop at AUS$19.95. This represents very good value, with some similar switches selling for up to triple this price. And as we've covered, there're some very tricky things that can be done with them - read the original full articles for a much more in-depth coverage of the techniques.

Sensitive Pressure Switch Stories

"Siting Cold Air Intakes"
"Powering Up the Pressure Switch"
"Dirty Filter Warning"
"Aero Flow Switch!"

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