Absolute vs Gauge Pressure

[Nickwinsor]

The dash is all configured and working really well, I just have a question regarding absolute vs gauge pressure.

I think I understand the concept - absolute being the sensor reads below atmospheric pressure levels, and gauge meaning the sensor reads from atmospheric pressure (approx 100kpa) up. So 0 kpa guage is really approx 100 kpa absolute.

The fuel and oil pressure sensors in my racecar are psi/kpa gauge sensors. At 0.5v, the pressure is 101.3kpa (absolute) and at 4.5v the pressure is 1135kpa (absolute). The ECU has this calibration data, and transmits this in SI units over the CAN (The ECU is a Haltech PS1000, using it's v2 CAN protocol.

I used the CAN 2.16 communication templates I found somewhere on this forum, and the dash is showing the correct absolute pressure.

I'm old school though and I want to display my pressures as psi guage. When the fuel pump is off, for example, I want it to read 0psi. Currently, however, it reads 14.7psi. I've tried selecting this in the display properties section, when selecting the channel I select psi g. The dash only displays psi absolute. When I select psi absolute, the display remains exactly the same. Always 14.7psi. I've tried changing the default value in the received channel section in communications to -100 kpa, +100 kpa etc, no effect. Also changed Adder to -100. No effect (pure guess work and playing around).

Is there a reason why this is happening? I know the dash is receiving the correct calibration, because if I set it to kpa absolute, the reading is accurate (101.3kpa). Note here, that if I set it to display kpa gauge, it also reads 101.3kpa, not 0 kpa.

Can anyone help me with this? Much appreciated!

[adrian]

There are two ways to do this:

1. In the comms template

You were close with the adder of -100 but it actually needs to be -1000 (or -1013 to be more accurate). This is because the base resolution of the channel is applied after the multiplier/divisor/adder calculation and in this case the resolution is 0.1kPa so the adder has to be 10x bigger.

2. Create a conversion table.

Go to Calculations>Tables, add you pressure channel to the input and create a new channel as the output (could call it "Fuel Pressure Gauge" for instance). Then configure the table like this:

0 : 0
101.3 : 0
1135 : 1033.7

Only real difference is you will get negative numbers in vacuum with the first option.

[Nickwinsor]

Hi Adrian,

I see, much like the Boost Gauge table. That is brilliant, thank you very much