How to hijack the numbers an LCD is transmitting? Preferably into an iOS device?

I want a way to take an engine compression gauge with a built in LCD display (like the one on a digital watch) and convert it into actual data such as a text input that I can have go into my pc and log the data at its highest point. To explain my actual goal, I want to hack up a rotary engine compression test. I'm tired of doing it manually and writing down each peak then watching my recording to calculate the time interval between peaks. A store-bought compression tester is $350+ and I could easily do the same thing if I could just hijack this signal and get it into my pc. Even better if I had a way to use it in an Xcode application. I'm not a real programmer but I make iPhone apps for myself pretty frequently, typically just simplified spreadsheets as standalone apps and I want to find a way to do this but I would need some hardware first. A digital compression tester is

No you can't…
1. LCD is a DISPLAY tech not a signal tech, it does not sent data only interprets it…
2. There are 2 types of compression gauge, analog and digital…
digital ones are the expensive ones. Analog are the cheap ones.
3. Even if it has an LCD screen for output it is most likely just using picoelectric plates to detect the compression
4. You would spend more time and money on "modding" an analog compression tester… Its ALOT simpler and cheaper to get a digital one with logging capabilities
5. Its VERY unlikely you have any idea how difficult this would be… You are essentially creating a FULL computer from random microprocessors… At least 30,000 lines of code would be needed for this project

In the long run, it will be cheaper to spend the 350 and get a tester that has an I/O port that connects directly to your computer (via USB, RS-232, Bluetooth or whatever they're using these days.) There a lot of calibration work involved in bulding your own measuring device and/or reverse-engineering someone else's and doing all the impedance matching needed to tap the signals without affecting/biasing the existing calibration. At the end of the day, you should find that the cost of parts alone used in testing will exceed 350, and then you have to figure in the cost of your time. If the device is priced competitively, you should lose by at least a factor of 2.

If you're bent on tapping the display, have you considered and OCR approach? What about setting up a digital camera to take pictures of the LCD display, then processing the pixmap? That's a hella easier than f**king with the electronics directly.

since you can fix the position of the camera relative to the LCD display, and you also control the ambient lighting conditions, it's the simplest of all solved AI problems once you've partitioned the snapshot into individual pixmaps and are just trying to determine what letter/number is in each pixmap.