Sierra INNOVA 215 - Manual (Page 5)

IM-215 Rev-A.1 Series Innova-Switch™ Page 5 of 42
1.0 INTRODUCTION
The SIERRA INSTRUMENTS Innova-Switch
™
Switch is the state-of-the-art in
gaseous and liquid flow switching or liquid level control. Flow or level detection is
accomplished by using a high resolution thermal differential technique. The sensor
wetted parts are of durable 316L series stainless steel, all welded construction with
no moving parts. The switch is easy to install and adjust, giving reliable, low
maintenance performance in the most demanding applications.
2.0
DESCRIPTION
The Innova-Switch
™
uses a thermal differential technique to sense changes in the
heat transfer characteristics of a media. Figures 1A and 1B show the outline of the
Innova-Switch
™
. The sensor consists of a pair of matched Resistance Temperature
Detectors (RTD's) encased in twin 316L series stainless steel tubes. One RTD is
self-heated using a constant DC current. The other RTD is unheated to provide an
accurate process temperature reference. The thermal differential created between
the heated and reference RTD pair is a function of the density and/or velocity of the
media with which the sensor is in contact. Other physical properties may have a
secondary effect as well. The differential is greatest at a no flow (or dry) condition
and decreases as the rate of flow increases (or as a liquid quenches the sensor/wet
condition).
The SIERRA INSTRUMENTS sensor excitation method relies on constant current to
the heated and reference sensors. Thus power to the heated sensor is not constant
but changes linearly with temperature as the sensor resistance changes.
Temperature compensation is accomplished by using the amplified reference sensor
voltage that also changes linearly with temperature, as a dynamic reference. During
calibration dry/no flow and wet/full flow conditions are impressed across the trip point
potentiometer. Since this reference is not fixed but is set with respect to the
reference sensor voltage, as temperature changes the trip point potentiometer
voltage changes with temperature exactly the same as that of the heated sensor
voltage with which it is being compared. Thus full temperature compensation is
achieved with non-constant power.