Test kits usually comprise pre-prepared test-tubes containing reagents suitable for measuring a specific parameter in a specific range (eg. ammonia 0-10mg/l). A precisely determined quantity of sample needs to be added to the test-tube and a photometer is generally needed to determine the resulting measurement
Each set of tests normally comes with instructions and reagents, some kits need special preparation such as heating to a set temperature for a set period of time. The photometer is used to precisely gauge the colouration, but simpler kits may come with a colour-chart for visual comparison.
Test kits are a relatively quick and easy way to perform a measurement, but over time may be expensive compared with some online measurement methods. They only provide a snap-shot reading which may or may not be representative of the average readings in a given sample stream. Online measurement displaying graphed results gives a true picture of peaks and troughs during the day or week.
Having made that point, it should be noted that the vast array of test kits available means that a single photometer can be used to carry out a whole array of ‘snap-shot’ tests whereas an online monitor is often only able to give a single parameter measurement, although a few can give multiple parameter readings. Another drawback which is common to test kits and the online measurement, is if there is a need to measure accurately across a large range for a given parameter. For instance, you may have an option of five or more test kits across the range 0-3.0mg/l up to maybe 50-200mg/l. Many online instruments are also limited by range, but some may be able to measure the complete range with reasonable accuracy. when preparing tests, it is important to be aware not only of the parameters, but also the range of measurement required.
Precise execution of the measurement process is vital for accurate results, care should be taken over collection of the sample, adding of the reagent and timing.
Other forms of simple test such as test-strips provide an even easier (but less accurate) method of liquid analysis.
More on Spectrophotometers used to ‘read’ test results