How to Use Field Instruments Effectively

摘要

Most field instruments are made up of two parts: a primary element and a transmitter. Primary elements include flow tubes, orifice plates, temperature probes, pressure sensors, wet chemistry sensors such as pH, ORP and conductivity probes, and level gauges of all types. Primary elements typically produce a signal - usually voltage, current or resistance - that is proportional to the variable they are designed to measure (temperature, level, flow, pressure or chemistry). Primary elements are connected to the input of field transmitters. Field transmitters include temperature, pressure and flow devices. They process the signal generated by the primary element. First, they characterize the signal in linear format and apply engineering unit coefficients to it. Second, they transmit it in analog (usually 4 to 20 mA DC) or digital format (usually some variety of fieldbus).