101364 Model FT2
1. Introduction
1.1. Scope
This Instruction Manual describes the electrical and mechanical considerations involved with installing
and maintaining the hardware associated with the Fox FT2 Thermal Mass Flowmeter and Temperature
Transmitter.
This manual is divided into the following sections:
Introduction:
Installation:
Start up:
Programming:
Preventive maintenance:
Troubleshooting:
1.2. Product Description
Theory of Operation
The Model FT2 is an advanced thermal mass flowmeter with temperature transmitter. It is
microprocessor-based and field programmable. The FT2 thermal sensor operates on the law that fluids
absorb heat. Therefore, a heated sensor placed in an air or gas stream transfers heat in proportion to
the stream’s mass velocity. There are two sensors in different legs of a balanced bridge circuit, one
sensor detects the fluid’s temperature and a second sensor is maintained at a constant temperature
(constant Δ T) above the fluid’s temperature. The energy applied to the heated sensor to keep this
constant temperature difference is directly proportional to the mass of the flow velocity. The bridge
circuit maintains accurate flow measurement over a large temperature and pressure range.
Mass Flow
The Model FT2 measures mass flow, an advantage over most flowmeters, which measure volumetric
flow rate. Volumetric flow is incomplete because temperature and pressure are unknown and must be
measured separately. For example, the volume of a gas depends on its temperature and pressure. As
temperature and pressure changes, the gas volume changes but not it’s mass. Therefore a device
measuring mass flow is independent of temperature and pressure. By defining a Standard at 70° F and
1 Atmosphere (14.7 psia), 1 Standard cubic foot of gas equals the mass that is in 1 cubic foot of this
gas at 70° F and 1 Atmosphere.
The Model FT2 provides a direct measurement of Standard or Mass units with no additional
temperature and pressure measurements required.
I/O Description
The FT2 comes with two industry standard 4-20 mA isolated analog outputs, one isolated digital
output that can be used for frequency or alarm output, one programmable discrete input and a RS232
communication channel that can interface to a Palm™ handheld or a PC. The two 4 -20 mA outputs
are scalable for the 4 and 20 mA values and are assigned to flow rate and temperature. The frequency
output is programmable to represent flow rate and can be programmed using span/maximum
frequency, unit per pulse or pulse per units. The maximum frequency is 100 Hz. An optional Modbus
protocol is available in a configurable full or half duplex RS485 interface. In addition, a local display
Fox Thermal Instruments Inc., 399 Reservation Road, Marina, CA 93933 Page 1
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