Ok, second attempt, i managed to erase my message before sending it ....... duuuuuhhhhhhh.

Looking at the datasheet i find the following.
1. Sensitivity is 20mV/kPa(137.893mV/psi)
2. There is an offset of approximatley 10kPa(1.45psi)

First i'd convert the AD reading into millivolts. This is easy to do by using the */ operator, this command makes a multiplication and an "invisible" division by 256 .....

milliVolts = ADreading */ 1250

.... same as doing ......

milliVolts = ADreading * 1250/256

..... on your calculator.

Now it's time to convert to psi, it probably won't be good enough just to divide our millivolts with 137.894 since that would give a reading in whole psi. We want better, i suggest psi*100, 14.7 psi would be 1470. I do this by using the ** operator, this is the same as */ but it divides by 65536 instead. So...

psiTimes100 = milliVolts / 137.893 * 100

.... equals.....

psiTimes100 = milliVolts * (100/137.893)

.... equals ....

psiTimes100 = milliVolts * 0.7252

.... which is the same as .....

psiTimes100 = milliVolts ** 47527

This would be the result if we didn't have an offset, but we do, so let's take that into account. Easy enough, just add them ....

psiTimes100 = psiTimes100 + 145

So our complete calculation should(could) look like this ....

milliVolts = ADreading */ 1250 'convert to millivolts
psiTimes100 = milliVolts ** 47527 'convert to psi*100
psiTimes100 = psiTimes100 + 145 'add the offset

Using 1.80V we end up with 1447 which is pretty darn close to your estimated 14.7 psi. If you live above sealevel and/or it's cloudy that's quite reasonable. You could get even better accuracy by measuring the output at two known pressures. From this you could calculate both the sensitivity and offset.

To dispaly vacuum i a different unit you simply separate them at your desired 14.7 psi.

IF psiTimes100 >= 1470 THEN
displayvalue = psiTimes100 - 1470 'dispaly zero at sealevel
'LCDOUT or SEROUT or whatever you want to use
ELSE
'do necessary calculations to produce your desired result
'and display it
ENDIF

Clear as mud?????
/Ingvar