# The Naked Scientists Forum

### Author Topic: How do you set a barometer?  (Read 19775 times)

#### colarris

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##### How do you set a barometer?
« on: 15/09/2009 21:15:16 »
Ive got hold of an old barometer but dont know how to set it. I know i have to know the current pressure of my area but don't under how this relates to my barometer. It has the number 5 printed at the 12, 9 and 3 o' clock postions on the dial, 29 at the 10 o'clock postion, 28 at the 7 o'clock position, 30 at 2 o' clock  and 31 at the 4 o'clock position. Can anyone help??
« Last Edit: 23/09/2009 03:37:16 by chris »

#### colarris

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##### Re: How do you set a barometer?
« Reply #1 on: 15/09/2009 23:42:18 »
Many thanks for the reply and info, it all makes a little more sense now. :)

#### colarris

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##### Re: How do you set a barometer?
« Reply #2 on: 15/09/2009 23:57:03 »
Just found out the following as well...

"...the one on the right is in 'pouces' (the old French equivilent to the English inch and is slightly longer, aproximately 1.066 inches)this one is showing 28 and 8/12, which equates to 30.55 English inches. Note that, as was the custom, each pouce is divided into 12ths and not 10ths as on the English one."

That explains the calibrated marks on it scale.

#### Don_1

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##### Re: How do you set a barometer?
« Reply #3 on: 16/09/2009 08:42:23 »
Here's a natural barometer. Needs no setting, costs nowt and favoured by the Met Office in Bracknell (UK)

#### Don_1

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##### Re: How do you set a barometer?
« Reply #4 on: 16/09/2009 15:51:51 »
Fucus vesiculosus Common name 'bladder wrack' aka: black tang, rockweed, bladder fucus, sea oak, black tany, cut weed, dyers fucus, red fucus, and rock wrack. Common throughout the North Atlantic.

Rich in Iodine and said to increase metabolic rate, so is used in many slimming potions.

Is used by the funny folk to forecast the weather. Dry shrivelled bladders = warm dry weather: damp swollen bladders, rain.
« Last Edit: 16/09/2009 15:54:45 by Don_1 »

#### Bored chemist

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##### Re: How do you set a barometer?
« Reply #5 on: 16/09/2009 19:10:23 »
I have a piece of seaweed outside my window as a guide to the weather.
It it's wet its raining.
If it's dry the weather is fine.
If the weed is moving about a lot it is because it's windy.
If I can't see it that's strong evidence of fog (except at night).
If it has blown away then we have had a gale.

BTW, is there the slightest evidence that the "storm glass" works?

#### colarris

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##### Re: How do you set a barometer?
« Reply #6 on: 16/09/2009 20:44:35 »

Some interesting replies there! Thanks

#### colarris

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##### Re: How do you set a barometer?
« Reply #7 on: 16/09/2009 20:46:23 »
Below is a huge (20’’ × 35’’) Louis XVI Antique French Barometer.  Notice how the dial runs from 27 in the lower left to 28 at the center top to 29 in the lower right.  And notice how, as you said, there are 12 divisions to the “pouce” (the French or Canadian inch), with major divisions at 4/12 and 8/12 indicated with 4’s and 8’s.  However, you mentioned the number 5 indicated between the inches on your barometer, which suggests 10 divisions in each pouce, meaning that you have a more modern barometer.  This may also mean that it uses a difference pouce.

According to Wikipedia:

Pouce, noun, masc, French for “inch” (literally “thumb”) as a unit of measurement.

In France, le pouce du Roi “the Royal inch” measured about 27.142 mm (1.0686 modern inches).

In Quebec, le pouce québécois “the Quebec inch” was assumed to be a Royal inch; however Canada legally defined it as 27.070 mm (1.0658 modern inches)

Both of these old measurements were about 1 1/15 modern inches.

In 1959, on a proposal by Canada, the US, Canada and other nations of the Commonwealth agreed to an inch as exactly 25.4 mm, which continues in the current usage.

Traditionally, the pouce was divided into 12 sections (as you mentioned), but more recently into halves, quarters, eighths, sixteenths, etc (which is what I’ve always used here in America).  Your barometer has 0.1 inch divisions obviously because the scientific community divides atmopsheric pressure into decimals (tenths, hundreds, etc), but I don't know when they started doing this.

I can’t clearly see where your barometer fits into all this.  Perhaps your barometer is dated, even if it’s only a patent date.

I did find these instructions on the medfordclock.com website.  You have an aneroid barometer.  The term “aneroid” comes from the Greek  a + neros, “without liquid” because the sealed bellows contain air (actually, a partial vacuum), and not liquid as old old glass ones did (and they also made scientific ones using mercury.)

Quote
How do I adjust my aneroid barometer?
Aneroid barometers have a small screw on the back. With a flat blade screwdriver turn this screw in either direction slightly while looking at the indicator needle. It should move in one direction or the other, tap the barometer to see where the needle settles. Continue until proper pressure reading is obtained. Do not turn the screw counter-clockwise (to the left) too far since it could come out.

How can I relate those measurements to the modern millibar ones?

#### colarris

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##### Re: How do you set a barometer?
« Reply #8 on: 16/09/2009 21:16:25 »
This is an image of the one  I have...

#### colarris

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##### Re: How do you set a barometer?
« Reply #9 on: 16/09/2009 23:31:59 »
Thanks again for your time and research! I have noticed, looking at the baronmater I have, that it hasnt got that millibar scale underneath but at least now I have something to work with. Many thanks :)

#### Geezer

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##### Re: How do you set a barometer?
« Reply #10 on: 17/09/2009 05:36:36 »
It's possibly not too important to worry about the absolute barometric pressure (unless you are in the midst of a hurricane).

What may be more interesting is the direction of any change in barometric pressure. My granny used to give her barometer a good whack so that she could see whether the pressure was increasing or decreasing (the whack tends to eliminate mechanical friction in the mechanism).

#### Bored chemist

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##### Re: How do you set a barometer?
« Reply #11 on: 17/09/2009 06:57:00 »
The cork excludes an possible effect of the atmosphere (and I gave seen glass ones fused shut). At best this is an odd thermometer.

#### Geezer

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##### How do you set a barometer?
« Reply #12 on: 26/09/2009 19:55:21 »
The cork excludes an possible effect of the atmosphere (and I gave seen glass ones fused shut). At best this is an odd thermometer.
A cork will not isolate the enclosure from the effects of air pressure, and even if the enclosure is fused shut, air pressure will still have an effect on the total volume of the enclosure, or at least, the shape of the enclosure (assuming it's not a perfect sphere). The effect will be small, but if the material inside the enclosure is sufficiently sensitive to changes is pressure or shape, it can still react to changes in external pressure.

#### Geezer

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##### How do you set a barometer?
« Reply #13 on: 27/09/2009 05:33:07 »
Geezer, do you mean that sufficient paths exist in cork to allow for a timely equalization of the pressure gradients that weather produces?  I had wondered about this.  I wonder how an open container of Fitzroy's Storm Glass solution inside a vacuum chamber would react in the prescribed manner to pressure variations.
I must confess I have limited knowledge on the subject! I suspect that, to some extent, corks act like a diaphragm, and even glass vessels that are fused shut are not entirely immune from variations in atmospheric presssure.

#### Bored chemist

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##### How do you set a barometer?
« Reply #14 on: 27/09/2009 19:43:48 »
The volume of a sealed glass tube will change very slightly with pressure, but it will change a lot more with temperature.
Also the pressure inside the sealed tube will change much more with asmbient temperature temperature than with atmospheric pressure.
There's also the interesting point that if they were not sealed the solvents would evaporate- the one that my folks got as a wedding present is still pretty much as full as ever.

Is there any actual evidence that these things work?
Perhaps someone could set up a weatherglass with a webcam.

Hearsay of an anecdote from someone who sells them isn't scientific evidence.

#### Geezer

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##### How do you set a barometer?
« Reply #15 on: 27/09/2009 21:06:24 »
The volume of a sealed glass tube will change very slightly with pressure, but it will change a lot more with temperature.
Also the pressure inside the sealed tube will change much more with asmbient temperature temperature than with atmospheric pressure.
There's also the interesting point that if they were not sealed the solvents would evaporate- the one that my folks got as a wedding present is still pretty much as full as ever.

Is there any actual evidence that these things work?
Perhaps someone could set up a weatherglass with a webcam.

Hearsay of an anecdote from someone who sells them isn't scientific evidence.

Good point. Changes in temperature would seem to have a much greater effect. You'd have to keep the thing at constant temperature to see if there was any change with pressure.

BTW, my daughter gave me one of those glass barometers (see above) that you put coloured water in. I've had it for about ten years. It always ends up pumping out the water until the enclosure is exposed to the air. I think the pumping action is caused by changes in temperature more than anything else. I gave up refilling it some time ago!

#### laeed

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##### How do you set a barometer?
« Reply #16 on: 20/05/2010 05:00:19 »
Ive got hold of an old barometer but dont know how to set it. I know i have to know the current pressure of my area but don't under how this relates to my barometer. It has the number 5 printed at the 12, 9 and 3 o' clock postions on the dial, 29 at the 10 o'clock postion, 28 at the 7 o'clock position, 30 at 2 o' clock  and 31 at the 4 o'clock position. Can anyone help??
Hey, here are 9 steps about How to Set and Read a Barometer,

Things You'll Need:

* Humidity Gauge
* Rain Gauge
* Barometer
* Environmental Thermometer
* Wind Gauge

Step 1
Identify your type of barometer. Most personal barometers are "aneroid" while scientific ones are "mercury."

Step 2
Check to see whether there is an adjustment screw on the back of an aneroid barometer. Read your instructions to learn whether it is necessary to turn this screw to set your barometer to the current pressure reading.

Step 3

Step 4
Call the weather service, the airport or a news outlet to obtain a current barometric reading.

Step 5
Turn the adjustment screw, if you need to do so, to set the barometer to the weather service barometer reading.

Step 6
Check to see whether your personal barometer has two needles. Most have one that automatically follows the changes in atmospheric pressure while the other remains fixed until moved by hand.

Step 7
Set the movable dial to coincide with the needle marking the current pressure.

Step 8
Watch for the needle that follows pressure changes to move up or down from that fixed point. Tap the barometer lightly before taking a reading.

Step 9
Compare readings to weather reports and other reports of barometric pressure to confirm the accuracy of your unit.

Tips & Warnings

*
The normal, average sea-level pressure is about 29.92 inches, which means the barometer's needle will rest near 30. Pressures seldom increase or decrease 1 inch above or below the 30-inch mark unless weather conditions are extreme.
*
Generally, low or decreasing barometic pressure means cloudy, unsettled or wet weather, and high pressure or increasing pressure means calm and clear weather.
*
Nonmercury, aneroid barometers work through a small, sealed metal chamber with most of the air inside drawn out. The chamber reacts to the rise and fall of outside atmospheric pressure by contracting or expanding, and a needle is used to display a reading.
*
Calibrate your barometer only during a period of fair weather, not during stormy weather. Check it every year or so.
*
Read the instructions accompanying your barometer. You may need to make a special adjustment if you live at an elevation greater than 1,000 feet.
*
Remember, changes in pressure are more important than exact readings - a downward trend is a general predictor of stormy weather and an upward trend suggests fair weather.
*
Scientific-grade mercury barometers are expensive and require training and skill to operate properly. They are used mostly in laboratories and weather offices. Do not open a mercury barometer - mercury is poisonous.

« Last Edit: 09/08/2012 13:28:39 by peppercorn »

#### The Naked Scientists Forum

##### How do you set a barometer?
« Reply #16 on: 20/05/2010 05:00:19 »