Sex, lies, and water vapor

OK, don’t get your hopes up. This will be the last mention of sex in this blog entry. However, we will deal at length with lies and water vapor, especially lies (OK, OK, let's just call it simple misunderstandings) about the feasibility of using direct water vapor measurement to correct mathematically for the dilution effects of water vapor on respiratory gases.

Without getting into the philosophy of natural laws, we can say that there are certain ironclad physical principles from which we cannot escape, and many of them are governed by simple equations. One obvious example is the calculation of dilution effects in simple mixtures of gases. A principle at the root of metabolic measurement using flow-through respirometry, by the way.

Competition is healthy - at its best, it keeps our wits sharpened. At its worst, when the irrational is promoted, it can approach the ridiculous. When considering water vapor and how to correct for its presence in the air stream, it is important to acknowledge the physics and respect the mathematics - defy this and run the risk of appearing the fool. It has come to my attention that some instrumentation resellers to biomedical research labs are attempting to discount the value to respirometry of measuring water vapor. One wonders why they are tilting at the windmill of universal physical principles. Granting them the benefit of the doubt, perhaps they do not understand basic principles such as Dalton's law of partial pressures. Perhaps they do not have a water vapor analyzer for sale and therefore need to justify archaic methods that have become obsolete.

Whatever the reasons for that claim, here are the basics.

In the case of metabolic measurement, our aim is to remove water vapor from the gas stream prior to calculating VO2 and VCO2. If water vapor is not removed, huge errors may result, especially in VO2. More accurately, the dilution effect of water vapor must be removed. The only disagreement comes in how that dilution effect is removed. Legacy systems compensate for the dilution effect of water vapor by simply removing it from the air stream. Of course, this is difficult to do reliably, and every method of scrubbing water vapor from air streams suffers from major disadvantages. What are these?

  • Chemical scrubbers have significant volume associated with them, can interact with CO2, and have a limited lifetime. They may also pose significant disposal issues.
  • Thermal scrubbers reduce the temperature of the air stream to about 1°C in the hopes of condensing the water vapor out of the air stream. This approach has two main problems. First, thermal scrubbers use heat pumps and associated paraphernalia that are quite complex and prone to failure. Second, and more significant, thermal scrubbers do not in fact remove all of the water vapor from the air stream. A water vapor partial pressure of about 0.65 kPa is left behind. If incurrent water vapor pressures decline below this point, which in some locations can be quite common, oxygen is suddenly less diluted and its concentration rises. Thus, VO2 is falsely elevated, and RQ is falsely diminished.

In search of an intelligent solution to these dilemmas, I suggested in my 2008 respirometry textbook, “measuring metabolic rates: a manual for scientists”, that these antiquated approaches should be replaced by mathematical compensation for water vapor dilution. In that book, I gave the very simple equation required to do so. It is:

O2’ = O2 * BP / (BP - WVP)

Not exactly complex! O2’ is mathematically dried O2 concentration, and O2 is oxygen concentration diluted by water vapor with water vapor pressure WVP. Finally, BP is barometric pressure in the same units as WVP. The same equation holds for any other gas species, such as CO2.

Not long after my book was published, this technique was put to its ultimate and most grueling test. Using the gold standard of respirometry validation, the propane burn, the mathematical water vapor dilution compensation technique provided far more accurate results than legacy techniques such as thermal water vapor scrubbers. You can read the original paper here - its citation details are at the end of this post.

Here is proof. This is a section of figure 2 from the above paper. The mathematical water vapor dilution compensation technique yields the results shown in the solid lines, while removal of water vapor using a thermal scrubber yields the results shown in the dotted lines. The target RQ is exactly 0.6, which is the stoichiometric result of burning propane in our atmosphere.

 On the left, the two techniques yield equivalent results (click on the image to embiggen it). This is when the incurrent air stream contains a significant amount of moisture. But when the air stream is drier (dewpoint < 1°C), the thermal scrubber can no longer dry the air stream effectively, resulting in an overestimate of VO2 and thus a huge, honking underestimate of RQ (VCO2/VO2). This is clearly seen in the right of the figure.

So not only does the mathematical water vapor dilution compensation technique yields equivalent results to legacy methods under ideal conditions, its results are actually significantly superior under adverse conditions!

There are also manuscripts in preparation (full disclosure: I am a co-author on one) that show an almost perfect match between the respirometry quotient of the animals, and the food quotient of the food they are eating. These studies use the Promethion system, both multiplexed and continuous, and would be seriously in error if the mathematical water vapor dilution compensation technique did not work. Curiously, many studies that use legacy systems show no such match, calling the accuracy of their RQ measurements into serious doubt. No wonder, when they rely on antiquated, primitive, low-tech, failure-prone techniques to eliminate water vapor dilution. Promethion, in contrast, doesn't ban water vapor; it says to water vapor, "Welcome! Glad to have you, but we need to measure you just as we also measure O2 and CO2." And, measuring water vapor leads to a host of additional advantages. These include measurement of metabolic water production and, potentially, the mathematical drying of food intake.

Anyone concerned about the reception by scientific referees of the mathematical  water vapor dilution compensation technique, merely needs to refer to my textbook, and to the above paper, combined with the ever-growing  number of citations in the primary literature that use the Promethion system. If anyone tries to sow fear, uncertainty and doubt (FUD) about this technique, please put me in touch with them. I promise to treat them with the respect they deserve.

A list of publications that I know use the mathematical  water vapor dilution compensation technique, is appended. Now, I would like to address the salespeople who are trying to cast doubt on validity and scientific acceptance of this method: Are they calling the authors of these papers, some of the most careful scientists in their fields, incompetent? Are they saying that the American Journal of Physiology, PLoS One, the American Journal of Clinical Nutrition, Molecular Metabolism, and the Proceedings of the American Academy of Sciences, are disreputable rags that would publish flawed science? Who are they to imply such things? What relevant qualifications do these salespeople have? What is their record of contributions to scientific research? I thought so.

To sum up: In a mixture of gases, every gas has a partial pressure, and the sum of all of the partial pressures in that gas mixture is the total pressure. In the case of the atmosphere, the total pressure is equal to barometric pressure. If you measure all of the requisite pressures, both partial and total, then you can dry gas samples mathematically as shown in the above equation. A slight rearrangement of similar equations, adding in flow rate, allows the calculation of metabolic rates. You can't have one without the other!

I encourage you to be the judge, applying your trust to the principles of the behavior of biophysical compounds found in Handbook of Chemistry and Physics. Apply your skepticism to any who propose that you bypass those standards.


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