Watchdog
The AMI Watchdog provides a set of desirable features
that are needed for trace oxygen analysis at a remarkably low cost.
On the face of it, you might think that all you
really need is the oxygen sensor, an off-the-shelf cell block and a
single range amplifier so that you can plug it into your SCADA system or
flow computer. What could possibly go wrong?
As many people have discovered, there is more to it
than just that. A trace oxygen analyzer is an excellent leak detector.
The tiniest leak, due to improper assembly or incorrect choice of
sample system components, will render trace oxygen readings meaningless.
The sample system has to be able to provide a controlled flow both of
the measurement sample and a span gas and also allow the sensor to be
replaced rapidly (excessive exposure to air can damage or destroy a
trace oxygen sensor). The electronics has to be able to deal with the
wide range of possible oxygen readings from under 1ppm to 20.9% and
accurately compensate for temperature variations. The Watchdog provides
all of this – and extensive electronic capabilities - at a very low
cost.
Trace oxygen analyzers like the Watchdog are
essentially leak detectors. AMI’s patented cell block allows the
analyzer to be built with almost no possibility of internal leaking,
with minimal wetted volume and with front-panel sensor access. All
sample handling components – the flow meter, needle valve and a four-way
sample/span/off selection valve – are integrated into a solid metal
block. Connections between them are drilled passages. The result is a
highly reliable sample system with all necessary components provided,
and a very fast “come-down” time – when a sensor is replaced, or
calibrated on air with a 1 minute exposure (at 70°F), it takes about 15
minutes for it to come down to below 10ppm. Competitive analyzers
typically take hours.
The AMI oxygen sensor is inherently faster than
competitive sensors due to its metallic body and large active electrode
area. The standard T2 sensor is also naturally resistive to hydrogen
sulfide up to 10ppm H2S over its warrantied life, while the T4 is
capable of resisting up to 500ppm H2S, with no scrubber material
required, and hence no maintenance or leakage issues.
The sensor is immediately accessible on the front
panel of the analyzer, and can be replaced in seconds. It is not
necessary to expose the sensor to air unshorted while installing it as
is the case with most competitive analyzers (which significantly
increases their come-down time). As a result, although a span gas port
is provided, it is practical to accurately calibrate this analyzer on
air in less than a minute.
The electronic and software package provides a
standard set of features including ten analog output ranges. The output
may be field selected as either 4-20mA or 1-5V. The analyzer is
capable of operating off a battery/solar panel, and it logs power brown
outs and failures should they occur. It tracks the life of the sensor,
indicates the remaining life, logs the calibration history and logs the
oxygen readings over its operational (output) range, and also tracks
excursions of up to ten times that range.
The analyzer can be wall-mounted with 4 externally
accessible screws, or bolted to a 2” pipe using standard U bolts.
