The category of instrumentation includes a wide variety of measurement methods covering test kits for one-off measurements through to on-line measurement monitors which run constantly giving real-time results.
The relative merits of these alternative methods are discussed on the individual pages.
In brief, the online system generally is used to provide an accurate record of peaks, troughs and trends over time for a single parameter – although it is acknowledged that some instruments can provide multiple parameter readings. The systems are generally physically large and heavy and usually part of a permanent installation, commonly involving pumping systems. As well as giving an indication of trends over time, an online system is ideal for generating alarms for high or low level readings. In some cases, wastewater treatment can be controlled by online readings – for instance shutting off treatment when a low level reading is achieved, or opening pen-stocks to release treated water to a water course.
A test kit and spectrophotometer (to read the results) provides just a ‘snap-shot’ result with no reference to whether the readings are rising or falling. This may be an ideal way for an on site laboratory to collect samples for analysis, or for using in ‘the field’ to collect remote samples using portable equipment which can be read using a hand-held device. The real advantage of the test kit is that a wide range of parameters can be measured with a single system.
A test kit is also a useful way of checking and calibrating an online measurement system which may be subject to ‘drift’ over time. However it is worth mentioning, that test kits usually require very concise instructions to be followed in order to get accurate results, and to counteract any errors, it is recommended that three tests are carried out and compared with each other rather than simply trusting the first result to be accurate. This helps to rule out possible errors due to contamination or simple, mistakes during the measurement process.