IM-215 Rev-A.1 Se ries Innova-Switch™ Page 5 of 42
1.0 INTRODUCTION
The SIERRA INSTRUMENTS Innova-S witch™ Switch is the state-of-t he-art i n
gaseous and liquid flow sw itching or liquid level control. Flow or level detection is
accomplished by us ing a high resolut ion thermal differenti al technique. The sensor
wetted parts are of durable 31 6L series stainless steel, all weld ed construction with
no moving parts. The switch is easy to install and adjust, giving reliable, low
maintenance performance in t he most demanding applications.
2.0 DESCRI PTION
The Innova-Switch™ uses a thermal differential techniqu e 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 Temperat ure
Detectors ( RTD's) encase d in twin 316L series stainless steel tubes. One RTD is
self-heated using a constant DC curr ent. The other RTD is unhe ated to prov ide an
accurate process temperature refere nce. The thermal differential created betwe en
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 phys ical properti es may have a
secondary effect as well. The differential is gre atest at a no flow (or dry) conditi on
and decreases as the rate of flow inc reases (or as a liquid qu enches the sens or/wet
condition).
The SIERRA INSTRUMENTS sensor excitation metho d relies on constant current to
the heated and reference sensors. Thus pow er to the heat ed sensor is not constant
but changes linearly with temp erature as the sensor resistance changes.
Temperature compens ation is accomplis hed by using the amplified reference sensor
voltage that also changes linearly with temperature, as a dynamic reference. During
calibration dry/no flo w and wet/full flow conditions are i mpressed across the trip point
potentiometer. Sinc e this reference is not fixed but is set with respect to the
reference sensor voltage, as temperatu re changes the trip point potentiometer
voltage changes with t emperature 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 po wer.