Charles Edwin - antique clocks and barometers


 
How to Read a Mercury Barometer

CAUTION:

This article answers the questions
  • "How do I read a barometer?"
  • "How can I tell if it's working?"
  • "How can I calibrate it to match the Weather Service?"

It contains instructions for handling, diagnosing, and altering mercury barometers. The reader does so at his or her own risk. We will accept no responsibility for damage to anyone's barometer resulting from the processes described below. If you don't feel comfortable following these instructions, then consider the well-being of the barometer and just don't handle it. Barometers are delicate and tricky to work with. 


READING THE BAROMETER

A barometer is a device for predicting weather changes, rather than one for giving you an instant readout of current conditions. A change in the level of the mercury indicates the passage of a high or low pressure front over your area, and a corresponding change in weather.DialFalling atmospheric pressure precedes stormy or unsettled conditions, and a rise indicates the approach of settled weather. Weather indications are written on the face of your barometer to guide you.

In the absence of violent weather conditions, the atmospheric pressure over the large land mass of the United States seldom varies more than half an inch above or below the 30-inch mark (at sea level). Movement of the mercury may take several hours or a day, and may be in very small increments. Also, your barometer can indicate FAIR even in a rain storm, because the narrow and fast-moving low pressure front that brought the rain into your area passed so quickly. 

Because a barometer's mercury level moves very slowly and by minute amounts, barometers all have a "set" indicator. This will be a hand or pointer, usually made of brass, and operated manually. You put this marker in place next to the top of the mercury on a stick barometer, or over the black indicator hand of a dial barometer, and it marks the height that the mercury was at in its tube when you last checked. This marker makes the direction of change easy to detect. 

NOTE: Your banjo barometer may need a small physical jolt before the main hand will settle into its final position. This is because the shaft connecting the brass pulley in back to the main indicator hand on the dial binds slightly in its tube. Just tap firmly on the wood case a couple of times, and the hand will drop into its proper reading. Stick barometers obviously do not require this.


BEING SURE THE BAROMETER IS WORKING

 The height of the column of mercury inside its glass tube is affected by many things that can cause the barometer to give "inaccurate" readings, but before you can determine what your barometer is trying to tell you, you will have to know if the barometer is working at all or if the mercury system has a fault, and precisely what sort of a reading you are comparing your barometer to. 

1. Is the barometer actually working?

There are two forms of mercury barometer; stick (you read the mercury height by looking directly at the top of the column inside the glass tube, and compare it to a scale of inches printed or engraved beside the column), and dial, also known as wheel or banjo (you read from a hand pointing to numbers on a dial, corresponding to the height of the mercury inside the glass tube in the rear of the barometer).

TiltThere is a quick test for each type to tell you if you have a functioning mercury system. With the barometer hanging on its hook on the wall, slide the bottom sideways in an arc, to about a 45 degree angle. Do this gently, don't slap it sideways or jerk it suddenly.

On a stick barometer, the mercury should rise smartly to the top of the glass tube and hit the end with a "tick" sharp enough to be heard and possibly felt through the wood of the case. The vacuum space should fill totally with mercury, with no air pocket at the very tip. On a dial barometer, the indicator hand (the longer, black one) should swing clockwise around the dial, past the end of the indicated scale. If either type of barometer fails this test, the mercury system will need to be put back in working order by a competent restorer before there is any point in going further. 

The most common problems that prevent either stick or dial barometers from working may be pockets of air, interspersed in the column of mercury (and probably in the vacuum space at the top as well); some mercury may have been lost from spillage; or the mercury itself has become so dirty and contaminated that it can no longer function. Air often shows up as gaps or bubbles somewhere in the length of the column, and just one small one can stop the mercury from moving up and down.

In general, any of these problems means that the glass tube of mercury needs to be emptied, cleaned and dried, and refilled with clean mercury before there is any chance of the barometer working again. This is a job for a specialist restorer (nitric acid is used to clean the inner bore of the glass) and there is no point in wasting your money just putting in more mercury.

Dial barometers may also have had the fiddly string linkage between the float in the mercury reservoir and the indicator hand come off the pulley, and this is a fault that you can correct yourself with some patience and a modicum of mechanical aptitude.

By the way, has the mercury system been opened up to the atmosphere so that it can work? Lots of barometer owners seem to have left the cork stopper in the open end of the mercury tube, or have failed to unscrew the closing screw at the bottom end of a stick barometer. See the article on this site regarding moving and handling of barometers. 


2. With what reading are you comparing your barometer? 

There are many sources of readings of barometric pressure. The most common these days is The Weather Channel on cable tv, or the evening weather news. Be sure the tv station or other source is relatively close to you; just a few miles can make a big difference. Also, a nearby airport control tower, or pilots' data service, will have the current atmospheric pressure; pilots of small planes use it to set their altimeters, which are actually barometers. Airport readings are sometimes given as "station" readings, which are uncorrected for elevation. Be sure to ask if you use this source, then ask for the corrected reading, or adjust it yourself with the table at the end of this article. 
As you become accustomed to using a barometer, you'll learn to trust the weather predictions on the register plate or the dial, and forget the actual numbers altogether. 

TV screen Atmospheric pressure readings given out to the public by the U.S. Weather Service, and repeated by most television weather broadcasts,are mathematically corrected for elevation above sea level, so that weather reporting across the continent can use standard nomenclature and also so that the weather stations don't have to have custom-built barometers for their readings. The standard elevation used is sea level, even if it is reported in Denver, Colorado, a mile above the sea. This is done by taking an actual reading, then adding a factor to it to correct for the known elevation. You will be doing the same thing, after you read the next section.


MAKING YOUR READING MATCH THE WEATHER SERVICE

  If you are lucky enough to live on the ocean front, or on a boat in the ocean, you know your elevation is zero, or sea level (lakes don't count; the Great Lakes for instance, are about 650 feet above sea level). The rest of us will need to find our elevation above sea level. One of the best sources is the local city or county engineer's office. The resident engineer needs to know elevations all over his area for such things as water services. Just call and ask. The office may be able to give it to you right down to the block where you live. With that information in hand, you can apply it as follows:

Stick barometers: There is no physical alteration you can make to the barometer to correct its reading. Not to any of them, even the ones with adjustable cistern capacities. They are all made to operate at sea level, period. At elevations up to 1,000 feet, just use the table below to make corrections to the reading on your stick barometer and enjoy it for the lovely antique it is.Closing Screw At elevations above about 1,000 feet, the common household variety just flat won't work. Short of cutting a section out of the case and shortening the glass tube as well, you can't make them read in synch with the corrected broadcasts. Many owners mistakenly use the closing screw at the bottom of the case to push the mercury up to the "right" reading, a futile exercise. Doing this just limits the capacity of the barometer's mercury system to accommodate high changes in volume. 

You should be aware that many stick barometers are constructed with built-in errors in the relationship between the surface of the mercury in the cistern and the placement of the scale on the case. Fitzroys are notorious for this; I've worked on dozens and I've yet to find an accurate one. You can check yours with a tape measure. Measure from any inches number on the register plate to the center of the cistern. Twenty-nine inches on the scale should be placed 29 inches from the center of the cistern.29 Inches 

Dial barometers: With the more-or-less standard 35 inch long mercury tube that is generally used in most of these, there is latitude for some change to adjust your reading up to elevations of a maximum of 1,000 feet. With the barometer hanging on the wall, remove the bezel that holds the glass over the dial. Two or three screws around the perimeter are usually standard. Take the barometer off the wall, keeping it upright, and open the back door.Setting the hand Grasp the pulley in the back between thumb and forefinger with one hand, and with the other turn the indicator hand on its shaft to the right reading. Put the barometer back in its normal position on the wall and check it. A few adjustments may be required to get the right setting of the hand on the shaft. If you don't feel comfortable doing this, don't try it. There is considerable potential for messing up the workings of the instrument. Read our disclaimer. 

If you reside above 1,000 feet and want your dial barometer to be accurate, a shorter mercury tube can be fabricated and installed. We've done several and they have all worked, up to 6,800 feet in one case. It isn't particularly expensive, and nothing is damaged by installing a shorter tube in the case. The chances of the tube you have now of being original, or even over 100 years old, are slim to none. Old glass corroded away in contact with mercury, and the tubes were replaced wholesale in order to keep the barometers working. Yours has probably been done several times, so replacement has no effect on the antique value of the instrument as a whole. 

In conclusion, a scientifically precise reading from your barometer doesn't really matter, in the grander scheme of things. As long as the mercury level drops when a storm is coming and rises for fair weather, that is really enough of an indicator to make it a good predicting device. That's all it did one or two hundred years ago, and it was good enough then. 

ALTITUDE CORRECTION TABLE
FOR STICK BAROMETERS 

 

50

Add 0.06 
 

550

Add 0.59 

100

Add 0.12
 

600

Add 0.64 

150

Add 0.17
 

650

Add 0.69 

200

Add 0.22
 

700

Add 0.74 

250

Add 0.27
 

750

Add 0.79 

300

Add 0.33
 

800

Add 0.84 

350

Add 0.38
 

850

Add 0.89 

400

Add 0.43
 

900

Add 0.94 

450

Add 0.48
 

950

Add 0.99 

500

Add 0.54
 

1000

Add 1.04 

Use this table to correct your reading. For instance, if you live at 600 feet elevation and your barometer reads about 29.31, and the weather news says the current barometric pressure is 29.95 inches of mercury, just find your elevation in the table above and add .64 to 29.31. 







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