Monday, September 24, 2018

Barometer Use at Higher Elevations

Normally we work with maritime applications of barometers at sea level. But some mariners live inland at higher elevations and take their quality barometer home when they leave the vessel. And there are, of course, many folks who are not mariners at all, but still care about accurate pressure, and they live inland at higher elevations.

For those at higher elevations the issue arises immediately that aneroid barometer dials cover a fixed range of pressures, but the atmospheric pressure decreases notably with elevation above sea level. The global average pressure of 1013 mb at sea level will drop to about 920 mb when the barometer is taken to an elevation of 2,600 ft (800 m). This pressure is not even on most aneroid dials, so they would not work at all. The failure is even worse when a low pressure system goes by.

When considering the purchase of a barometer for use at higher elevations we must look into the dial range relative to the intended elevation of use.  The table below shows how pressure changes with elevation. The correction is Po - Pa.

Properties of the standard atmosphere from Modern Marine Weather, 3rd ed.

This is still an approximation because this conversion depends somewhat on the outside air temperature if much different from the values listed, but that is a small effect in normal temperature variations. In fact, for many applications you can estimate the pressure drop with our jingle "Point four four per floor," which means the pressure drops by 0.44 mb for each 12 feet of elevation above sea level.  At a 240 ft elevation, the correction would be (240/12) x 0.44 = 8.8 mb.

The Fischer Precision Aneroid Barometer is a good example to look at because this instrument offers a custom version for higher elevations. The standard unit can be used for elevations from sea level to 2,600 ft (800m); the high-elevation model is intended for elevations from 2,600 ft to 6,500 ft (2,000 m). We can see how this plays out by looking at the dials and thinking over expected pressures. This is shown in the figure below.

Figure 1. Fischer Precision Barometer dials

You may have to zoom the image to see the values, but we have marked three pressures common to all barometer use: the global average sea level pressure (SLP) at 1013 mb, plus the high-end limit we might expect on land at 1050 mb SLP. Actual land records vary from the all time extreme of 1080 (SLP) or so in the Siberian High to pressures in the1050s representing land records across the US. At sea we can see periodically mid 40's, but very rarely ever high 40s.

The low end on land is equally nebulous as hurricanes do come ashore, but away from that possibility record deep Lows on land are on the order of 970 mb (SLP). So for land use, we would want our barometer to cover the range of about 970 to 1050 (SLP).

Once we take the barometer to sea, however, we can expect deeper Lows... not that we want to ever get that near to them. A typical central pressure for a medium to large hurricane can be about 920 mb as shown below.

Again, we are not doing any tactics with pressure once we get into that sort of low pressure, or anywhere near it, but we do want our barometer to read properly at the deep low end of the scale.  One crucial feature of the Fischer instruments is they are guaranteed to have an accuracy of ≤ 0.7 mb at all pressures shown on the dial. This is important because these instruments are used on Navy vessels that do periodically end up in very low pressures.

The result is that for a barometer used at sea, the required pressure range is much larger than on land. We have illustrated that schematically with the red and yellow extension of the land-based range. Notice that the standard dial actually goes much below what we would ever expect at sea; this extended range is there to cover use at elevations above sea level. Going back to the basics, our instrument at higher elevations reads station pressure, but we need to interpret that in terms of sea level pressure, because all weather reports and weather maps use SLP.

Once we go to higher elevations, we are moving farther away from the coast where the likelihood of measuring hurricane pressures diminishes greatly. This is something that each user can judge from their regional history.

The top right side of Figure 1 shows a standard dial used at the upper limit of its elevation. We see that the active part of the dial (the yellow arc) is now on the bottom of the dial, but all expected atmospheric pressures are covered with equal accuracy.  This active range marked in yellow is what establishes the upper limit of elevations (2,600 ft) for the standard model.

Above 2,600 ft, the high-elevation version is needed, and that dial is shown in the bottom of Figure 1. The left side shows the lower elevation limit and the right shows the upper elevation limit.  Over the full elevation range, we see the active part of the dial change from one side to the other, while an intermediate value would occupy the center part of the dial.  The key point to note is that the full range of expected atmospheric pressures would show within the instrument's dial and accuracy range.

Electronic Barometers

A logical question that might arise at this point is why not just use an electronic barometer for pressures at high elevations? They don't even have a dial, just digital pressure output. The answer is accuracy.  Electronic barometers must also be adjusted and calibrated during manufacture just as aneroids must. And like most common aneroids, common electronic barometers are typically adjusted to read accurate pressure in the mid range at sea level.

It is true that even a common electronic barometer, such as the ones we have in our cellphones that we access with a barometer app, will read more accurately at the very low pressures experienced at high elevations than will a common aneroid, but they still lose accuracy at the very high and very low ends. A cellphone barometer, and indeed many electronic barometers that cost less than $2,500, could be off as much as 2 mb at 760 mb if they were exactly right at 1013 mb.  (The value $2,500 is not a random number; it is the cost of two popular laboratory instruments that indeed provide their guaranteed high accuracy over their full valid ranges.)

Since we are not going to know how any random barometer will actually behave in extreme pressures when we buy it, we have to rely on the published accuracy, reputation, and history of the instrument. We also do not want to be misled by "pressure precision" or "pressure change sensitivity" or "relative pressure" specifications listed under an accuracy specification. The cheapest electronic barometer is good at these. Likewise, even "sensor" accuracy is not a guarantee, because these sensors have to be soldered and incorporated into a circuit, all of which sensitively affects the final output accuracy. In short, we have to get to a pretty high price tag on an electronic unit to get guaranteed pressure accuracy specifications over a specified broad range of pressures.

Even compared to fairly expensive electronic barometers, it is difficult to beat the Fischer Precision Aneroid Barometer's guaranteed accuracy ≤ 0.7 mb over all pressures shown on the dials of both low and high-elevation models* After ten years of use at sea, there is no competition at all—which incidentally makes cellphones a good choice for an electronic barometer, because we are going to replace them in any event in this period!  A cellphone is very convenient for daily pressure work, with a quality aneroid in constant view to check it with.

* It is important to recognize that all barometers, aneroid and electronic, drift over some period of time, so they must be compared with a reliable source and set to the right pressure if needed. For a high-quality barometer, aneroid or electronic, this process of setting the instrument is not affecting or related to its calibration over the full pressure range; it is just moving the full calibration curve up or down a small amount. We recommend that all barometers be checked this way once a year—although we have seen some Fischer instruments and laboratory-grade electronic instruments not budge over 5 years.

If you look at the top of the standard dial in Figure 1, and indeed look at most brands of aneroid barometers (especially in the US and UK), you see the pressure of 1000 mb is at the center of the dial, whereas it is globally known that the average pressure is 1013 mb, which is well off to the right. We tracked down this mystery in our Barometer Handbook. By long established convention, these barometer dials are centered for an elevation of 365 ft, which happens to be the average elevation of England, where much of the early aneroid development and production occurred. This put their average pressure at the top of the dial. On the other hand, looking to more modern dials from some European makers, you will see 1010 or 1015 centered at the top of the dial.

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