August 18, 2010

A Practical Solution to Using Temperature Data to Compensate Conductivity

Greetings! Pete Anson here. I’m a Senior Product Manager here at Emerson Process Management and responsible for all liquid analytical instrumentation. A customer recently came to me asking how he could use the temperature data from an installed Rosemount 644 temperature transmitter for compensation of conductivity on a boiler feed water application. The customer really trusts the reliability and accuracy of the Rosemount transmitter and wants to integrate the output to support a liquid analytical measurement.  

Nice concept but the only thing that makes it practical is if you have the ability to receive and algorithmically integrate the temperature data for compensation of the conductivity measurement.    

Fortunately, Rosemount Analytical does have a solution for this “out-of-the-box” scenario. Using the Model 1056 installed with a current input signal board, any 4-20mA signal from any device can be received, integrated as compensation data for conductivity, and displayed locally. The user simply selects the programming option to use the 4-20mA data as temperature for conductivity measurement with the installed Model 400 Endurance line contacting conductivity sensor. The input of temperature data from the 644 transmitter is received at 1/second to provide immediate determination of conductivity, which is highly affected by temperature.

Further, the Model 1056 can power the transmitter (up to 36 volts DC) to avoid additional DC loop power leads of the temperature device. The temperature data and the measured conductivity can be output to a PLC or DCS if wired to the 4-20mA output. HART configurations of Model 1056 can report all live variables and diagnostics associated with the conductivity loop to the DCS including the temperature data converted from degrees to 4-20mA readings.

Using the versatile 4-20mA input capability of the Model 1056, many other inputs can be accepted and displayed.  Variables such as atmospheric pressure can be input to compensate dissolved oxygen measurement for partial pressure.  And, almost any type of measurement can be input and displayed for the convenience of field technicians and I&E. This is especially useful in the field if the 4-20mA device does not have a local display, aka “blind transmitter.”  

The Model 1056 offers an innovative solution for this unconventional but reasonable request.  The solution described using the Model 1056 with the current input board will elevate the customer’s confidence in the process measurement, allow real-time local display and reporting of live temperature readings from the adjacent transmitter, and potentially save wiring of leads to power the temperature transmitter.

Please feel free to check out our Model 1056 page for more information, including product data, features and benefits.

August 5, 2010

The Combustion Geek: Part Deux

Doug Simmers here again. And this week, I’d like to talk about one of the most pervasive analytical applications — the measurement of oxygen in combustion flue gases.  A burner is inherently an inhospitable environment, so measuring the flue gas composition flowing out of this violent process has come to be a good way of establishing the optimum fuel/air ratio for best combustion efficiency. It is also beneficial in minimizing pollutants such as NOx, and CO2. As climate change concerns increase, carbon dioxide’s status is changing from that of a natural result of combustion to one of a new polluting gas.

Some of the earliest measurements of combustion gases were:

  • The Orsat – a lab technique that was taken out to the field.  It was not a continuous measurement, and there was no signal to send to the control room.
  • Another type of measurement uses electrochemical cells that generate a small electrical signal when O2 is reacted with a specific chemical in the cell, but service in continuous operation usually results in significant drift of the O2 reading over time.
  • Paramagnetic analyzers use the magnetic properties of Oxygen to get a reading of O2, but usually require that the flue gases be cleaned of ash and condensing moisture prior to measuring.

Flue gas analysis took a decided step change (read improvement) with the development of the zirconium oxide (ZrO2) sensing technology.  This was developed as a technology transfer from the Apollo moon program in the early 1970’s.  The ZrO2 technology had significant advantages over past measuring techniques. 

  1. The sensor is heated, so it could be placed onto the end of a probe, and inserted right into the hot flue gas stream. 
  2. The sensor works on the partial pressure difference of O2 on the reference side of the cell and that on the process side. 
  3. The cell is non-depleting, and lasts for many years.
  4. The sensing cell is like a thermocouple in that it develops it’s own millivolt signal, or electromotive force (emf). 
  5. Further, this emf is inverse, and logarithmic.  The raw millivolt signal gets stronger at the typical O2 levels found in flue gases (2-5% O2).   This is all represented mathematically in the Nernst equation, EMF = KT log10 (P1/P2) + C.

We have a good description of how it works in this PowerPoint. If you’d rather have a copy e-mailed to you, please just let me know and we’ll send you one.

The ZrO2 technology spread quickly throughout industry, and into automotive applications.  Every car manufactured since the late 1980’s has one or more of these O2 sensors in the exhaust, and we will soon be seeing them applied to the small engines used in lawn mowers, chain saws, etc. Little wonder this technology is still the standard for industries that have a smoke stack in the process.

How much money can you save by controlling your fuel/air ratio to the optimum level?

Tune into my next post to find out.

Future Blog Subjects

History of combustion flue gas analysis

Operators are the real customer

Rethinking Hazardous areas

Furnace diagnostics

Deep dives into how analyzers work

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