Monday, November 22, 2021

Atmospheric Pressure: Look Close to See its Microscopic Pulse

We had an inquiry today about whether or not our Marine Barograph app would read and record pressures accurate to a few tenths of a mb on a per second basis and plot then showing sub mb precision. The answer is yes it will, but the question caught my attention. I have known for a long time that pressure oscillates on a small scale at all times, but this never effects our weather work, which looks for solid trends of over a few tenths of a mb—in a sense for just this reason, namely we do not want small oscillations interfering with our reading, no matter where they come from.

When we look at a plot of the pressure on a fine scale we see these oscillations, but when you look at just one plot the tendency is to consider this electronic noise. You might think you are just looking at the limits of the sensor and its circuits.  But that is actually not the case at all. These oscillations are real.

Below we see three old cell phones with our app loaded into them, converting them in a sense to about a $800 electronic barograph. Any iPhone model 6 or newer includes a pressure sensor—you may have one sitting around in a drawer.

The dip in the top unit on the left side of the graph of about 0.2 mb marks the time when, upon receiving the inquiry about the units, I picked this one up and raised it up to the ceiling, a distance of about 5 ft. This was to check how the scale responded to such a change.

The middle one did not have its sensor offset turned on so the value is off some, but this does not affect the topic at hand.  

Data are stored every second for 30 min, and then stored in different time bins for plotting longer periods, which is described in the help file.  You can export any of these time bins, which I did for each one covering the past 30 min.  We can see the plot of these in the phones themselves, and indeed pinch zoom in to see all details, but to make the point at hand, I exported them and then plotted them in Excel, as shown below.

Here we see all three plots; I shifted the gray one up to compare them, to compensate for its lack of calibration offset being turned on.  This whole note was pretty spontaneous!

The important thing to note here is these are three completely independent sensors, being run by three completely independent little computers, which are powered by batteries. I cannot imagine any common electrical properties of any of them—although the batteries were all on chargers at the time that does in  a sense link them.

In short, these oscillations in the pressure appear to be real variations in the physical pressure of this room. I do not know what causes it. Back when i was working in a laboratory, I would likely have stopped to figure out why, but now we just note this and move back to what we are now working on—a new course in electronic chart navigation.

Hallways in condo buildings are often kept at positive pressure to keep cooking smells indoors. Navy warships and first responder vessels keep their wheelhouse in relatively high positive pressure to protect from toxic gases, so these things could be studied with our app. Barometers in these ships have to be in sealed housings that have tubing leading outside to read atmospheric pressure.

There is some element of pattern present as very roughly outlined below, but this would take a serious mathematical analysis to understand.

This rough outline indicates that the oscillations have an amplitude of about 0.05 mb with a period of just over 1 minute. It is not clear what is causing this, but it seems it must be a real physical phenomena and not some artificial electronic anomaly—but that is not 100% clear. We have to rule out any link via the battery charger and investigate any way a wireless signal might induce this effect.

We will take these outside tomorrow for 15 minutes or so to see if that  makes a difference. They will also be on batteries without chargers for that and I will put each one in a metal can (Faraday cage.)  I don't think it will matter. I think we are seeing the pulse of the atmosphere here!

And I want to stress that this is no reflection on the actual accuracy of the pressure itself. With a good offset inserted, the reading is likely accurate to within a few tenths of a mb, which we know from our own calibration tank and comparison with other instruments, but we can't make comparisons any closer than that. But even though the accuracy on the full pressure readouts are not in this 0.01 mb range,  the oscillations themselves are likely real. That is the point of this note.

When we first started working on electronic barometers we also reported a very interesting physical affect of a small short pressure bump under a squall, which is indeed atmospheric science, but we have not learned more on this since then, although you see in that article a reference to an even earlier event we reported—we were not lucky enough to have our new app in these early days.  Maybe now that our app is available, more folks will study such events.

In short, a simple tool like our $15 marine barograph app is a powerful way to look into details of the atmosphere, besides its great value in helping shape a fast, safe course across the ocean... or predict when a front will cross our local lake

Let's finish with a bit more perspective on this. Here is the dial of a high precision Fisher aneroid barometer, which has the pressure scale marked off in 0.5 mb intervals.


Note added Nov 26.  We have now done the outdoor test and the oscillations noted above are likely artificial, but we did indeed see what are more likely real sub-mb oscillations. See Sub-millibar Pressure Patterns, Part 2.

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