08-16-2017, 09:42 PM
Keeping the measurements flowing
Billerica, ma When you buy gasoline for your
car or fuel oil or natural gas for your home, you
expect an accurate measurement of the quantity
you buy. The companies that supply those products
to your local gas station, oil-delivery company,
or gas company also expect accurate and reliable
measurements when they make purchases from their suppliers.
For all these transactions, the engineers at GE Measurement
& Control Solutions design and test meters that measure flow
rate and calculate purchase quantities. In addition, the engineers
also design and test moisture analyzers that ensure that
natural gas contains only trace amounts of water.
The engineers start at the board level where they test circuits
and software, then move on to test sensors and mechanical parts,
and then test the entire system with tests that resemble actual
customer use in oil and gas fields. Along the way, they make
EMC (electromagnetic compliance) and environmental tests to
ensure the products comply with regulations.
Measuring more than liquid
When GE engineers talk about a flow meter, they are referring
to the entire device that is inserted into a pipeline electronics,
sensors, pipes, and other parts. In addition to sensing flow and
water, flow meters and moisture analyzers measure the temperature
and pressure inside a pipeline and use those measurements
to calculate flow rates and moisture levels.
Flow meters and moisture analyzers contain several inputs
and outputs. Flow meters, for example, may produce pulse
trains in which the pulse frequency represents flow rate. They
may also produce 4 20-mA analog outputs that represent flow
or another value such as temperature. Flow meters and moisture
analyzers also have one or more communications buses
such as RS-232/422/485, Modbus, Fieldbus, and HART bus.
The instruments use these buses for receiving commands and
for transferring data to a host computer.
Engineering tests start on the bench with simulations, after
which design engineers build and evaluate digital control circuits,
signal-conditioning circuits for the sensors, and analog
and digital output circuits. The GE engineers typically build
several complete prototype units, on which they perform accuracy
tests, functional tests, EMC tests, and environmental
tests. Finally, the production department will use actual production
processes to build a larger quantity of units for a pilot
evaluation. Engineers then repeat all the tests on these units,
which represent the final design. If everything passes, full production
begins.
The test plans start as soon as design engineers begin designing
electronics, sensors, and mechanical parts. Michael
Gambuzza, the lead electrical engineer for the moisture and
gas group, designs and tests the analog and digital circuits that
go into a meter s controls. While Gambuzza is designing electronics,
other engineers are developing test schedules, readying
the EMC lab, and preparing for environmental tests and
flow-rate tests.
Although Gambuzza spends much of his time designing circuits,
he also tests them in complete systems. For example, after
he designed the electronics for GE s Aurora series moisture
analyzers, Gambuzza ran tests in the optical lab. Why an optical
lab? Because the Aurora moisture analyzers use the Beer-Lambert
Law of Absorption. They measure reflected light and,
through digital-signal processing, calculate the moisture content
in natural gas as it travels through a pipeline. (The Aurora
moisture analyzer can measure moisture as low as 5 ppm.) Analog
circuits, under microprocessor control, drive a laser diode
over a swept-frequency spectrum (Figure 1).
We generate a drive signal to a laser diode to sweep its
input current, said Gambuzza. That sweeps the tunable diode
laser through a spectrum of wavelengths. At a particular wavelength,
you get the most light absorption because of the presence
of water. The more moisture in a gas line, the more light