This page is intended to document the actual method to WBo2 calibration for Vems 3.x process in one location in an easy to understand format for use with a windows box and MegaTune
Instructions for calibrating WBO2
These instructions are for bench calibrating your WBO2
There are two steps.
- Calibrating the pump_zero
- Free air calibration
This portion is a non-issue if you purchased a v3.3 and got an email with the pump_zero value in the email from the webshop.
You will be changing this value[ Highlighted box in MegaTune ]
- Turn on VEMS unit.
- Leave trigger off ( no rotation of crank or no stimulation of box)
- Leave WBO2 off
- This is a fresh boot only power applied.
- hook up a Voltmeter between pins 5 & 6
This voltage is "as close to zero as possible"
- you ask what does this mean. Well the variable Pump_pwm_zero has a digital output that has 256 values over 5 volts. So each point adjusts the voltage 19mV == 0.019 Volts.
- Change in value direction. If you follow the picture above with the positive and negative leads for your VoltMeter?. Changing the value Up one point will move the pump_pwm_zero up 19mV, otherwise it will drop 19mV.
- IMPORTANT After you change the value in MegaTune for the pump_pwm_zero you have to "burn to eeprom" and reboot the VEMS box to make the changes take effect. You have to do this each time to see a change.
- Tolerance Until told otherwise the tolerance is +/-20mV or +/-0.020V
- All readings are taken in DC mode.
- Typical values are between 92 and 107
Free Air Calibration
Now that the pump_pwm_zero has been set you are ready to do the free air calibration. You will be changing the config.wbo2_calibration value [ TODO: Highlighted box in MegaTune ]
- a change is this decimal value equates to a change of about 0.06% free Oxygen
- Note: The bar across the bottom of the Megatune main page with green and red dots are for air fuel ratio NOT Free oxygen.
- get up and running
Manmlp07 shows output
changes for pump cal dont take effect until you reboot v3.x. The commanded current resolution is appr. Istep= 7mA / 256 = 27uA steps. Since we usually measure voltage (usually from pump- as base, not from GND), so the acceptable tolerance can be expressed in a voltage:
- with open pump+ output, that is 10kohm (onboard) resistor, 27uA * 10kOhm = 270mV steps
- or 340 Ohm testload setup where pump+ is connected to nernst (through 240 Ohm) and nernst connected to pump- (through 100 Ohm): 340 Ohm * Istep= 9 mV
- or 200 Ohm testload setup (as above, but both test-resistors are 100 Ohm): 200 * Istep = 5.4mV
5V / 256 = 0.019 V / bit +/-20mV is minimum 170 changed to 20.0 (3.824) 175 changed to 20.3 (3.843) 180 changed to 20.5 (3.863) 185 changed to 20.9 (3.882) 186 changed to 21.0 (3.882) 190 changed to 21.2 (3.902)
minicom in r30
Man moves to comm mode
mlp07 switches to WBO2 related LCD-page
mll displays content (in terminal, so you can see even without real LCD)
byebye ends terminal mode and returns to megatune
I guess we should recommend a starting value (RCal=115 Ohm or so), and get the user read the O2% in free air. We make an O2%-reading => wbo2_calibration table which the user can look up => voila: WBO2 is calibrated in 1 step.