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How should you specify a hydrocarbon dew point analyzer?

So, you are starting a new project and have been asked to specify a hydrocarbon dew point analyzer. What operational parameters should you specify?

First step is to realize that you cannot use a GC-based analysis and a software package to do this. Yes, in theory, one can use a high-end GC (up to at least C14 analysis) in a controlled laboratory environment to do this. But in practice, in the field, it is not possible. (look for technical note called "The Fallacy of Calculating Hydrocarbon Dew Point" in the Technical Resources page). There is a joke about theory and practice that would fit in well here, but we'll skip that for now.

Ok. Then what are the parameters you need to specify. For a full write up, check the Technical Resources page for a note called "How to specify a Hydrocarbon Dew Point Analyzer". But let's do a brief overview here. For an accurate specification you need to specify 10 different parameters as follows. Some are self explanatory, some need more explanation. These parameters in rough order of importance are:

1- Method of detection

2. Mirror material

3. Maximum cooling range without the use of external or addition coolers

4. Maximum operating temperature

5. Maximum operating pressure

6. Communication protocols

7. Communication Software

8. Data logging capabilities

9. Power requirements

10. Hazardous Area Certification

1- As you have probably guessed, the most important criteria is the method of detection of condensation. Although all HC dew point analyzers are based on automated chilled-mirror technology, not all of them are equal. In principle, all dew point analyzers chill a surface and look for condensation. But what is important, nay crucial, is how you detect the onset of condensation.

There are basically two methods for detection of condensation: 1) CEIRS™ technology which was invented and patented by ZEGAZ Instruments, and 2) the older methods based on light scattering. CEIRS™ (Chilled-mirror Evanescent IR Spectroscopy) is truly a 21st century advancement in chilled-mirror technology. It combines the fundamental chilled-mirror concept with IR spectroscopy to achieve unprecedented reliability and accuracy. The older, light-scattering method was invented more than 40 years ago. Particularly in the context of natural gas measurements, it suffers from a lot of problems. Of course, you can always opt to use a rotary phone if you prefer last century's technology, but moving on is probably a better option.

2- Mirror material should be specified. Some analyzers use a metallic mirror, some use optical quality ceramic glasses. Metallic mirrors tend to react with the components of natural gas and discolor and corrode, then need to cleaned or replaced within a relatively short time. It is best to specify an inert optical material. Metals should be excluded as they cause a lot of maintenance and drift problems.

3- Maximum cooling range should be specified. This is the temperature difference, with respect to the ambient, that the cooling system can achieve. Make sure that this is the native cooling capability without the use of additional cooling, such as vortex coolers, or cooled cabinets. Some suppliers misguide the customer by quoting a number that they can only achieve using additional cooling, but leave that in the fine print. A native cooling capability of 60C or higher would be optimal.

For a discussion of the other parameters, check the Technical Resources page and look for a note called "How to specify a Hydrocarbon Dew Point Analyzer".

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