An analyser can also be an instrument or device which conducts chemical analysis (or similar analysis) on samples or sample streams. Such samples consist of some type of matter such as solid, liquid, or gas. Many analysers perform such analyses automatically or mostly automatically, such as autoanalysers.
The analysis can be done on samples which the operator brings to the analyser or the analyser can be connected to the source of the samples and the sampling be done automatically. The source of samples for automatic sampling is commonly some kind of process such as a chemical process. Analysers connected to a such a process which conduct automatic sampling can be called online (or on-line) analysers or sometimes inline (or in-line) analysers. For inline analysis, a sensor can be placed in a process vessel or stream of flowing material to conduct the analysis. Another method of online analysis is allowing a sample stream to flow from the process equipment into an analyser, sometimes conditioning the sample stream in between such as reducing pressure or changing the sample temperature. Such sampling is typically for fluids (either liquids or gases). If the sample stream is not substantially modified by the analyser, it can be returned to the process. Otherwise, the sample stream is not returned; for example, if any reagents have been added for the analysis.
The pressure of a sample stream can be lowered by a pressure reducing valve, particularly since many analysers are not designed to withstand high pressure. Such pressure reducing or similar valves may be used to control the flow rate to the online analyser. If the process pressure is insufficient to allow a sample stream to flow by itself to the analyser, a small pump may be used to move it there. The temperature of a hot sample may be lowered by use of an online sample cooler. The sampling and analysis can be done at the command of an operator, periodically (for example, every 15 minutes), or continuously - providing an analysis result vs. time graph on a chart recorder, computer, or other device. For periodic sampling, valves (or other devices) can be switched open to allow a fluid sample stream to flow to the analyser and shut when not sampling.
Some methods of inline analysis are so simple, such as electrical conductivity or pH, the instruments are usually not even called analysers. Salinity determined from simple online analysis is often determined from a conductivity measurement where the output signal is calibrated in terms of salinity concentration (for example ppm of NaCl). Various types of other analyses can be devised. Physical properties can include electrical conductivity (or effectively electrical resistivity), refractive index, and radioactivity measurement. Rather simple processes which use inline electrical conductivity determination are water purification processes which test how effectively salts have been removed from the output water. Variations of electrical conductivity measurement of solutions include cation and anion conductivity. Chromatography such as ion chromatography or HPLC often tests the output stream continuously by determination of electrical conductivity, particularly cation or anion conductivity, refractive index, colorimetry or ultraviolet/visible absorbance at a certain wavelength. Inline sensors for dissolved oxygen in water are available. There are online and offline analysers available for various other types of analytes. Many of these add reagents to the samples or sample streams.