FLT
®
Series FlexSwitch
TM
GENERAL
Fluid Components International LLC
1
1 GENERAL
Description
The FLT Series models are multipurpose measurement instruments. The models included in this manual are FLT93B (Basic), FLT93C (Sanitary), FLT93F
(Fast Response), FLT93L (In-Line) and FLT93S (Heavy Duty). Each model is a single instrument that is capable of detecting fluid flow and temperature.
It is also able to detect liquid level or fluid media interfaces. The instrument has two field adjustable alarm setpoints, two buffered voltage outputs,
as well as a built-in calibration circuit. The output of the alarm setpoints are 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 conductiv-
ity 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 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 sens-
ing element signal and activates the relay alarm circuits if the signal exceeds an adjustable setpoint.
The control circuit contains field selectable jumpers that are used to configure the instrument for the application.
Figure 1-1 View of the Sensing Element
Summary of Contents for FLT93B
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