Fluid Components International FR73B Complete Manual - Manual

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General Information
FR78B FAST RESPONSE FLOW SWITCH/MONITOR
Installation, Operation and Maintenance Guide
Description
This document explains the operating principle of the Model FR78 Basic Low-Flow Switch/Monitor (see Figure 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 ( ∆ 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 - FR78B Fast Response Flow Switch/Monitor