History of the pH Meter
The first commercial pH testers were built around 1936 by Arnold Beckman in the United States and by Radiometer in Denmark. However, glass pH electrodes that have a potential dependent on activity of H+ ions were constructed already 1906. The main problems to construct a pH tester was to design a sensitive high-gain amplifier and pack it in a compact, portable unit.
Since then pH testers became one of the most popular and important instruments used by chemists and non-chemists in labs, field and industrial environment. The basic ideas behind the pH tester haven't changed. However, the reliability and precision of today's pH tester are of course much better. Also many of the modern pH instruments are digitized and able to communicate with computer.