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Old 20-09-2007, 14:58   #1
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Barometers

We have had two 'bad eggs' with a weems and plath barometer and were wondering if anyone recommended a particular barometer, or digital vs. classic style instruments in regards to accuracy, function, reliability, etc.

Thanks,
Graham and Sue
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Old 20-09-2007, 15:15   #2
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I'm interested in the same problem, I have a sestrel barometer that turned out to have a real cheap movement inside a nice brass exterior that just didn't work. How do the electronic ones shape up?
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Old 20-09-2007, 17:43   #3
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The barameter on my Kestral seems to work well although my altimeter seems to think the C&C is a submarine. I can't recall which brand of brass mechinical barameter is on the boat but it and the Kestral always match the NOAA report fairly closely.
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Old 20-09-2007, 21:27   #4
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I assume you all know that they have to be "set" to the correct pressure first. No baromater is "factory set" for say. Especially an Altimeter. These have to be constantly reset using the Barometer as a gauge. Air pressure is a difficult one to pin down. About the only time it is truely close to accurate over a distance, is when you are some distance from land and have a large high over top. Then you can be reasonably sure of the setting you make being the same as the Met service is saying it is. Over land, there are many variances that will change air pressure in the imediate vacinity, that make accurate pressure readings difficult. Unless you are right close by to the place that the reading was made at.

As for the actual movements themselves. There are certainly cheap ones that are not worth the brass they are made from. But the two names above I thought were of good reputation. Maybe not it seems.
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Old 20-09-2007, 23:29   #5
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Hi Alan, Had the Sestrel (bought from Hutchwilco when they had the agency) checked out by a compass / barometer expert on the north shore and he was shocked at the (lack of) quality of the movement, there was no way he could get it to respond accurately at all.
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Old 21-09-2007, 00:08   #6
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All measuring instruments need calibration. Laboratory calibration requires a pressure vessel. However fairly accurate calibration can be done using close local weather station information. Low readings are used to set the zero and high readings are used to set the span. Don't just change the zero for the location or the overall accuracy will be compremised. As Alan stated locations over land and over coastal waters can vary with gravitational differences caused by the moon, sun and the planets. Same things that cause regional tide differences. We use a good laboratory grade mercury barometer as a field calibration standard. The old analogue barometer used for a long time used horse hair as an element , the new digital type use a variety of pressure elements to do the same job. Not more accurate but maybe more user friendly and the information can be sent to onboard networked devices. If you need it.
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Old 21-09-2007, 01:18   #7
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Quote:
barometer expert on the north shore and he was shocked at the (lack of) quality
Wow, that's a concern. I had the same thing with those cheap Tawainese barometers a few years ago. Just not worth the money even though it was cheap. But you would certainly expect better from the ones stated.
On the subject of barometers however, I don't see the point. They can give you a trend which is good, but in my opinion, that's about all it's good for. No more use than the weather Rock hanging on a line. You know the one? if the rock is dry, it's not raining, if the rock is wet, it is raining, if you can't see the rock, it is foggy and so on. A barometer is telling you no more than what you can see out the window all ready. Even a trend of rising or falling tells you little, unless you can pull data from several different locations of several miles apart. I find that very usefull when listening to our maritime weather report. You can follow the fronts coming and passing by the barametric pressure readings at variouse measuring points around the country.
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Old 21-09-2007, 08:37   #8
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The barometer which Sue and I have had simply did not change readings. Even when hurricane Dean passed through Jamaica (we're in Miami, FL at the moment) I am sure there are better instruments out there. Maybe just go with mercury?

Graham and Sue
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Old 21-09-2007, 09:10   #9
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I don’t think most of us will want a 32" +/- tall Mercury barometer, which don’t like movement, and do prefer ambient temperature compensation, aboard.

All barometers should be tapped slightly before taking a reading. While watching the indicator needle, gently tap the glass and observe which way the needle moves.

Your main concern, in reading a barometer, is the rate & magnitude of change (whether the pressure is rising or falling).

If the needle does not move it would mean that the barometer is "holding steady" and similar weather conditions (persistence) should continue.

If you are not sure if your aneroid barometer is working it can be checked very easily. Place the instrument is a tightly sealing clear plastic bag (ziploc). Get as much air in the bag as you can before you seal it up. While observing the dial of the barometer squeeze the bag and watch for the needle to move (increase). If the needle does not move there is a problem with the linkage in the instrument or with the bellows.

Electronic barometers* use detection cells whose resistance or capacitance level changes sufficiently with air pressure to react to the small changes found in the atmosphere. The detection cell can be connected to electronic circuits or microprocessors in various ways to provide a wide range of possible applications. The reading outputs on some units can be downloaded to a computer for storage and analyses.

* Kestrel 4000 (about $240) measures
Barometric pressure
Pressure trend
Altitude
Relative humidity
Heat stress index
Dewpoint
Wet bulb temperature
Density altitude
Wind chill
Air, water, and snow temperature
Current wind speed
Average wind speed
Maximum wind gust
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Old 21-09-2007, 09:48   #10
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Quote:
Originally Posted by GordMay View Post
Your main concern, in reading a barometer, is the rate & magnitude of change (whether the pressure is rising or falling).
I have to agree with Gord. I will admit to being unpracticed when it comes to weather forcasting but the trend is more important to me than the barometer reading exactly matching the weather service reports.
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Old 21-09-2007, 17:56   #11
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Personally I watch the movement of my barometer, the actual reading is generally useless to me. Its watching it dive or hold steady that is important.

Neat hint about the plastic bag, I will try it for sure
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Old 21-09-2007, 23:04   #12
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I'm sure if I put the Sestrel onboard It would fly past cat 1 just for being there and I probably will use it for that purpose, my surprise / beef is with the quality (lack of) of a reasonably upmarket brand.
Though if I did upgrade I would look at the electronic ones as they record the trends rather that you having to log them.
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Old 22-09-2007, 22:39   #13
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Quote:
Originally Posted by graham2 View Post
We have had two 'bad eggs' with a weems and plath barometer and were wondering if anyone recommended a particular barometer, or digital vs. classic style instruments in regards to accuracy, function, reliability, etc.
I had an expensive Weems and Plath barometer too. It was junk. It would go out of calibration in a day or two, so you could never know if the pressure was falling, or if it was just displaying an incorrect value that happened to be lower than the incorrect value it displayed yesterday.

When I left to go cruising, I bought a Barigo barograph from Robert White Instruments in Boston MA. If I remember correctly, it was about US$500. The price included a year's worth of your choice of US or metric recording paper.

It was correctly calibrated when it arrived (I checked) and it has worked great for 3 years. The only problem I've had is some AA batteries are a little too big and will pop out of the battery clip when you aren't looking, but a twist-tie around the battery clip solved that problem. The battery just drives a quartz clock mechanism. If you don't put a battery in, you can still read the air pressure, but the drum doesn't turn.

It holds up well under way. You can see rough seas on the barometer strip because the needle bounces around, but it has the effect that the line is a few mm wide instead of just the size of the pen tip. It still gives readings good enough to use.
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Old 22-09-2007, 22:54   #14
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I'll add my recommendation about the Kestrel 4000 - I have one, and love it. The graphing function works well and is easy to browse; here's a shot of a nasty low that went through last year.

Cheers,
Steve
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Old 23-09-2007, 03:48   #15
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Are you a human barometer? Do your joints or muscles ache at the approach of a storm or foul weather?

I am. As with many others, my joint pain (osteoarthritis) increases dramatically with low atmospheric pressure (particularly with more rapid changes).

Most storm fronts are preceded by a drop in atmospheric pressure. Precipitation is only possible under certain conditions. One condition which facilitates precipitation is reduction of atmospheric pressure. As a storm front approaches an area, the atmospheric pressure in the area preceding the front drops considerably. My swelling, aching joints give me a relatively accurate warning of any approaching low pressure fronts. These low pressure fronts usually indicate a storm or precipitation is approaching the area. Any bad weather usually arrives within 24 hours or less.

My Unsubstantiated Speculation:
Take a balloon and put it into a vacuum. As the pressure is reduced around that balloon, it expands. And so, the same thing occurs within the tissues and arterial vessels around the joints. If there's already swelling, inflammation, abnormal mechanics in the joint, as the pressure goes down, the gas and tissue expand (vascular expansion), pressing on (and activating) the nerve receptors. This is perceived as more pain.
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