I have a Pfeiffer cold cathode vacuum gauge just off the turbo pump on my RGA system. Simple to use, 24v in and it gives a signal out depending on pressure. I want to build a display to connect to it. Heres the thing, the output is linear, in a way. Anything over 8.5v is over pressure. 8.5means 1 Pa, 7.5 is 1e-1, 6.5 is 1e-2. Simple, every volt is a decade down to 1.8v where it peters out at 2x10^-7 Pa. If the output is below .5v that means there is something wrong with the gauge.

Now the problem is that between each decade the voltage is logarithmic to the pressure. So at 6.5v the pressure is 1e-2, 6.8 is 2e-2, 6.97v is 3e-2, 7.2v is 4e-2, and so on.

Any ideas how to deal with this? I am figuring using the analog input on something like an Arduino or Teensy++. On the arduino it will give me 10 bit resolution between 0-5v. Use a voltage divider to scale it and output the info on a LCD.

But how do I process the data on something like this. I suppose I could do a lookup table where it subtracts the number that determines the decade and then does and inverse log of the resulting number to determine the pressure in that range. So if it saw a voltage of 5.92v it would set the range to e-3, subtract 5.5 to get .42v, calculate 10^.42 to get 2.63 and then display 2.63e-3Pa on the LCD. Also do an option to show mbar or torr.

I really have no clue how to program something like this. Or even if I am doing it close to the right way. I do not want to do this on a PC!

Any ideas?

-Jerry

Now the problem is that between each decade the voltage is logarithmic to the pressure. So at 6.5v the pressure is 1e-2, 6.8 is 2e-2, 6.97v is 3e-2, 7.2v is 4e-2, and so on.

Any ideas how to deal with this? I am figuring using the analog input on something like an Arduino or Teensy++. On the arduino it will give me 10 bit resolution between 0-5v. Use a voltage divider to scale it and output the info on a LCD.

But how do I process the data on something like this. I suppose I could do a lookup table where it subtracts the number that determines the decade and then does and inverse log of the resulting number to determine the pressure in that range. So if it saw a voltage of 5.92v it would set the range to e-3, subtract 5.5 to get .42v, calculate 10^.42 to get 2.63 and then display 2.63e-3Pa on the LCD. Also do an option to show mbar or torr.

I really have no clue how to program something like this. Or even if I am doing it close to the right way. I do not want to do this on a PC!

Any ideas?

-Jerry

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