Abstract
This paper describes the development and design of two frequency modulating instruments for the measurement of fluid density and pressure. The reasons for choosing frequency modulation for the signal transmission are discussed, and the advantages it offers to Process Control. Effects like decay in cable insulation resistance, and various requirements of Intrinsic-safety, are analysed in detail. Both instruments use a vibrating body as a sensing element and it is shown that in each case, by the appropriate design procedure, the response is restricted to the parameter to be measured, other effects such as temperature variations being ignored. A comparison is made with existing instruments, whose limitations are analysed and used to draw up the desired specification for the improved instrument. The advantages and disadvantages are discussed, and the design and methods used are described in detail.