Fluid Components International FLT Series - Manual (Page 9)

CHAPTER 1 - GENERAL INFORMATION FLUID COMPONENTS, INTL
Doc. No. 06EN003246 Rev. B 1 - 1 FLTÔ Series FlexSwitchÔ
1. General Information
Description
The FLT Series models are multipurpose measurement instruments. The FLT Series models that are included in
this manual are FLT93-C, FLT93-F, FLT93-L and FLT93-S. Each model is a single instrument that is capable of
detecting air flow, fluid flow and temperature. It is also able to detect liquid level or fluid media interfaces. The
instrument has two field adjustable alarm set points, two buffered voltage outputs, as well as a built-in calibration
circuit. The output of the alarm set points are 6 amp relay contacts that can be used to control customer process
applications. One buffered voltage output is available for flow or level monitoring and the second buffered voltage
output is available for temperature monitoring.
Theory of Operation
The flow switch is a fixed position, single-point flow, level, interface and temperature switch. The operation of the
sensing element is based upon the thermal dispersion principle: A low-powered heater is used to produce a
temperature differential between two Resistance Temperature Detectors (RTDs). The RTD temperature differential
varies as a function of forced convection for flow measurement and as a function of fluid thermal conductivity for
level and interface measurement. The measurement of the fluid's temperature is obtained from the non-heated
RTD.
Sensing Element
The sensing element consists of two thermowells (hollow tubes) that when inserted into the flow process allows an
unimpeded flow inside the process line. The top thermowell has a self-heated RTD inserted into it. The bottom
thermowell has a reference RTD inserted into it. In order to correctly orient the sensing element a flow arrow has
been etched onto the threaded portion of the sensing element. See Figure 1-1 for a view of the sensing element.
Control Circuit
The control circuit converts the sensing element's RTD temperature differential into an analog DC voltage signal.
Dual comparators monitor the sensing element signal and activates the relay alarm circuits if the signal exceeds an
adjustable set point.
The control circuit contains all the removable jumpers that configure the instrument to perform as needed by the
customer.
Figure 1-1 View of the Sensing Element