CHAPTER 1 - GENERAL INFORMATION
FLUID COMPONENTS, INTL
Doc. No. 06EN003291 Rev. A
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Model ST98 Flow Meter
1. General Information
Description
The model ST98 is a thermal mass flowmeter for air or gas measuring applications. The ST98 consists of a flow
element, a flow transmitter, and an enclosure. An in-line flow element is used for smaller diameter pipe or tubing
sizes and, for pipe sizes greater than 2-1/2 inches (40 mm bore), an insertion flow element is used. The flow
element’s process connections can be threaded or flanged.
The ST98 flow transmitter accepts AC or DC input power and the output signal can be set for either a standard
range current or voltage. A display is optional. An RS-232C serial I/O port provides setup, monitoring and
troubleshooting access using either FCI’s model FC88 Programmer or a PC-compatible computer.
The ST98 enclosures provide environmental protection for the flow transmitter. The flow transmitter can be
integrally mounted with the flow element or remotely separated from it. Hazardous location local and remote
enclosures are optional.
Theory of Operation
The flow element of the model ST98 uses the thermal dispersion operating principle: A low-powered heater
produces a temperature differential between two resistance temperature detectors (RTDs) by heating one of the
RTDs. Mass flow rate changes cool the heated RTD and cause a proportional change in the temperature
differential between the RTDs. The instrument’s flow transmitter converts the RTD temperature differential into a
scaled output signal and an optional indicated display value.
The signal from the unheated RTD is used to provide an indication of the air or gas temperature on the optional
display.
Insertion 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. A heated RTD is inserted into the top thermowell. A reference RTD
(with no heater) is inserted into the bottom thermowell. In order to correctly orient the sensing element a flow
arrow has been etched onto a machined flat portion of the sensing element. See Figure 1-1 for a view of the
sensing element.
The element is inserted into the process media through a hole drilled into the process line.
Figure 1-1 View of the Sensing Element
ARROW PARALLEL TO FLOW
ACTIVE RTD (HEATED)
REFERENCE RTD (NOT HEATED)
