ABSTRACT

In this report the measurement and calculation capability of ABB 267 CS has been evaluated. Several methods can be used to measure mass flow. A common one is to use a flow restrictor (like a venturi or orifice plate) in combination with a differential pressure transmitter. The mass flow is a function of the pressure drop over the restrictor and can be calculated if proper information about dimensions and fluid is available. The flow calculation can be performed on line in a separate device (a ""flow computer""), in a process computer (PLC) or similar. But it can also be performed internally in the differential pressure transmitter, if this functionality has been included. Because these devices have the possibility to measure not only differential pressure (?P), but also line pressure (P1 and P2), atmospheric pressure (P0) and temperature (T) they are sometimes called ""multivariable"". Today (July 2006) there are a small number of such transmitters available on the market. One of these is the transmitter evaluated in this report. As different types and models have different functionality it is important to check the calculation capability of a selected transmitter since it is not obvious to most users what the instrument is really calculating. Roughly the process can be divided in five parts; 1. Determine the relation between ?P and flow for the restrictor in use at ?Pmax. 2. Measure ?P and calculate flow rate. 3. Measure P and T. 4. Compensate (re-calculate) flow rate with respect to P and T. 5. Present result at standard temperature and pressure or as mass flow. ABB 267 CS (including PC software) can handle steps 2 to 5.