Which Pressure sensor type for boost gauge

[tron9000]

I'm thinking of building a simple boost pressure gauge to measure boost pressure for a turbo.

I'm going to base it around an INA126 instrumentation amp and some LM3914's to display pressure as a bar graph. I'll be running it off the vehicles supply (11.8-14V typically)

typical pressure I'm looking at (according to the vehicles workshop manual) should be 1 bar maximum between 2000 & 3000rpm whilst driving (approx. 14.5psi). So I've scrounged a Honeywell SDX30A2 which should work within the range.

However I want to see vacuum pressure too (if any). So I require a gauge sensor not absolute, correct?

[max_torque]

If the point the pressure is measured is downstream of the throttle, then yes, there will be both positive and negative (relative to atmospheric) pressure.

Typically, on a trailing throttle at high rpm, minimum pressure will be around 20kPa(abs) and maximum pressure depends on how hard your turbo boosts, but most road cars will be running around 1bar boost (200kPa(abs)).  High performance or diesel engines can run up to (and sometimes over!) 2bar boost (300kPa(abs)).

I would suggest that you need as minimum a 2bar range sensor (either 0 to 2bar(abs), or -1 to 1 bar(g))

Depending on the display you are going to use, there are plenty of pcb mounted pressure sensors that output a 0-5v signal that make interfacing to a microcontrollers ADC very easy indeed:





0-5v means you only need a tiny amount of analogue input filtering to read the value, rather than messing around with op-amps etc.  (for high accuracy, you could calibrate the device in software)

If you mount all the electronics in the gauge itself, then you have to run a pressure taping pipe through the bulkhead to the engine, or you use a remote sensor and run the wires through.  However, you'll find that suitable rated (environmental protected, vibration and temperature proof etc) remote sensors are a LOT more expensive.

[tron9000]

Thanks for that. I'm just building one out of parts I have collected, but if I pursue will go the uC route. Would definitely make calibration easy 

It's nothing high powered, 200TDi in a landrover, its mostly for dianostics, see if I have a boost leak. I'll be measuring it at the manifold.

[max_torque]

Ok for a LR TDI then you are unthrottled, so will only ever have a small amount of negative pressure in the manifold (at high rpm, light throttle, where the intake system losses are not overcome by the turbo)


(iirc old TDi doesn't have throttle and uses vac pump for brake booster)

[tron9000]

yes the accelerator controls the fuel injector rather than throttle and the brake booster is supplied vacuum from a pump on the engine.