Ammonia comes in two forms. There is free ammonia, which is a gas and is chemically known as NH3. Then ammonium, or NH4, which is the ionized form of ammonia. The two combined makeup total ammonia. NH4 is not harmful to fish, but NH3, even in small amounts is very harmful to them.
Most reagent-based test kits measure total ammonia. Then you can use pH and temperature to calculate the amount of free ammonia in your system. This can be time consuming and also is very prone to user error. Thus a direct calculation of free ammonia is much more useful.
OsmoBot detects free ammonia using optical, colorimetric sensors. It can do so at very low concentrations and give early indications that your fish may be experiencing stress.