Instrument elevation in the instrument layout diagram
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When drawing instrument layout diagrams, some diagrams show both the sampling point (*P) and the transmitter point (*T), while others only show the transmitter point. Why is that? Where does the instrument elevation come from?
In some measurements, the process medium is sent from the sampling point to the transmitter’s measurement chamber through measurement pipelines; the sampling point is located on the process equipment, while the transmitter is situated at another location (usually on an instrument rack/instrument box). In such cases, it is necessary to indicate the elevation of the sampling point and that of the transmitter. For example, orifice plates combined with differential pressure transmitters are used to measure flow rate. For some measurements, the transmitter is installed directly on the process equipment, such as built-in orifice plate flow transmitters; in such cases, it is only necessary to indicate the transmitter’s elevation. The instrument elevation is the elevation of the installation location during design.