Fluid Components International FR73B Manual General Information - Manual

Doc. No. 06EN003281 Rev. - 1 - 1 Model FR78B Flow Switch/Monitor
CHAPTER 1 - GENERAL INFORMATION FLUID COMPONENTS INTL
1. General Information
Description
This document explains the operating principle of the Model FR78 Basic Low-Flow Switch/Monitor (see
Figure 1-1). The following pages also present the recommended procedures for the installation, operation,
maintenance, and troubleshooting of the Model FR78B.
The Model FR78B is an instrument that is capable of detecting liquid or gaseous environments. The instrument
has field adjustable alarm set points for control of the media.
Flow Element
The operational part of the flow element consists of two Resistance Temperature Detectors (RTD's) and one low
power heater. The heater is positioned adjacent to the Active RTD and will raise it's temperature above the process
ambient while the other RTD (Reference) will stabilize near the process temperature. This arrangement produces
a temperature differential between the two RTDs that is measured and converted into an analog voltage signal.
The temperature differential is greatest when the media is not flowing (no flow). As the process media begins to
flow, forced convection cools the Active RTD and reduces the temperature differential between the two RTDs.
Control Circuit
The basic functions of the control circuit are to provide power to the flow element, measure the resistance
differential (
D R) between the two RTDs, condition the sensing point signals, and provide relay alarm contacts for
customer uses.
A Double Pole Double Throw (DPDT) relay is available in the instrument for connections to the customer alarm
systems. The relay outputs can be set to be either open or closed when the sensing point is at low or no flow and
closed or open when the sensing point is at high flow.
The place where the relays change state will vary depending on the type of media, as well as gas or liquid
turbulence. Therefore the instrument has field adjustable alarm set points.
Figure 1-1. FR78B Fast Response Flow Switch/Monitor