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Old 17-03-2006, 12:48   #1
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Barometers?

You know those barometer and clock sets every sailboat seems to have?

Anybody ever seen the barometer in one of those sets work???

I can't recall ever seeing one of those read the correct pressure. The one I have now goes up and down properly, but is off... needs some kind of calibration. Does anyone know how to calibrate a "Boston Shipstrike" barometer manufactured by Chealsea Clock?
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Old 17-03-2006, 14:02   #2
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Aneroid Barometers can be 'adjusted' by turning the small screw on the back of the movement, until the barometer needle is set to the current local pressure. Only adjust during settled weather.

For 'calibration', contact Chelsea Clock:
sales@chelseaclock.com
repairs@chelseaclock.com
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Old 17-03-2006, 19:34   #3
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Sean, I have the Chelsea, and it works great. To calibrate it you need to remove the needle and put it back at the correct setting if I recall. The directions came with mine. If you need them I will dig them out. I have the clock as well, ad it is the most accurate mechanical movement I have ever owned.
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Old 17-03-2006, 21:11   #4
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No we're not talking about the wind up mickey mouse clocks here either?

A REAL precission jeweled piece here that we're talking about.

Oi, oi, oi!!
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Old 17-03-2006, 22:07   #5
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Re: Barometers?

Quote:
ssullivan once whispered in the wind:
You know those barometer and clock sets every sailboat seems to have?

Anybody ever seen the barometer in one of those sets work???
No.

I had one of the Weems and Plath barometers as part of a clock/barometer/temp/humidity set. The barometer manual said it was calibrated at the factory, but when I called to ask about it, they said "Oh, you have to calibrate it".

I found that it would only hold a calibration for a few days. I even tried putting a wire through the slot in the calibration screw so it couldn't creep. It still went out of calibration very quickly.

I eventually tossed it and bought the least expensive recording barometer from Robert White Instruments. It was expensive, but it works great.
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Old 17-03-2006, 23:20   #6
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One characteristic of the Chelsea's is that you need to tap the center. They often will stick with minor changes. As I said, mine works great. I have had no issues with the calibration, of course the change is far mor important than the actual reading.
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Old 18-03-2006, 06:09   #7
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To Calibrate, take it to a local meterological office such as at a local major airport. Do the calibration based on what the station's master barometer is showing. Dont use 'radio' or TV listings of barometric pressure as these are usually many hours old, and there are cyclic compensations/corrections necessary due to 'local' changes of temperature AND relative humidity. The 'local' variations of temperature and humidity can significantly change the readings ... based on the 'local' average molecular weight/mass of the gas.

Simply - Take your barometer to the local weather station office, let it equilibrate to the ambient temperature at the station, then simply duplicate the pressure of the 'master' barometer, both barometers being at the exact SAME altitude. Otherwise you need to correct a 'remote' calibration for temp., relative humidity AND altitude.

Why relative humidity? dry air has a molecular weight of 29. Water has a molecular weight of 18. A mixture of air and water vapor wll be less than 29 .... and the 'pressure' of the gas will less than pure dry air. High humidity air will weigh less than pure dry air (ref. -40 degrees C or F) and the 'pressure' will vary due to the relative humidity. Therefore the barometer when calibrated will need to also be corrected for RH. If you take the barometer to a weather station .... just match what the master barometer is reading and inquire what corrections for T, RH, and altitude were made. Then correct for YOUR usage at 0 ft. (sea level) use. But noone I know of on a boat uses or needs a 'wet bulb' temperature gage to measure the RH.

Of course its not really the actual 'numbers' on a barometer that are important but the 'trend' over time that is really important (unless you are using it as an altimeter) ... thats why a recording barometer is of more value. Nevertheless, there are 'reading' output 'cycles' during the day for temperature that need to be corrected (added or subtracted from what you see on the gage) as the reported 'broadcast' pressures are already/usually corrected for these 'cycles'.
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Old 18-03-2006, 07:00   #8
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Hi Rich,

Always appreciate your electrical knowledge but, being a chemist, I have to correct some of your assumptions here. "Air" is a mixture of gasses and doesn't have a molecular weight. Nitrogen, the most common component has a MW of 28 and oxygen has a MW of 32. Water vapor is MW 18, like you note. For those of us in the northeast, ozone has a MW of 48, while for those in California, nitrous oxide has a MW of 44 !

Pressure doesn't care about the MW of the gas in a direct sense, but your intuition about the composition of the gas influencing pressure is correct.

I don't mean to be pedantic Rich, it's just that I get so few chances to actually add knowledge to this board!

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Old 18-03-2006, 09:00   #9
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Much easier to calibrate in UK as whenever conditions are good for this (high pressure area), the met office make a point about commenting on it on the TV weather forecast. Personally I disregard this as I have two better sources. The first is a weather station on two buoys close to my sailing area, which include pressure readings, and second, I have a proper barometer at home which reads from a true 30" of mercury - cant get anymore accurate than that, although I have to correct for pressure difference due to altitude.

The actual change in pressure is about one inch of pressure per thousand feet of elevation change.
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Old 18-03-2006, 10:14   #10
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Mark -
Actually the mixture of all the component gases that makes up 'air' at standard conditions is 28.9 mw. .... its an average thats pretty consistant.

You can take the standard conditions (14.7 @ 60 deg. F) and backcalculate the partial pressures (versus the standard) and arrive at the same --- 28.9mw. The chem process and 'gas' industries have been using 29mw as the average molecular (@ STP) weight (pumps, blowers, compressors, etc . etc. etc.) since Methusela was a pup. Damn important if you are mixing with other gaseous components, etc.

Actually I am (was) P-chem + MechEng ( but I dont remember how to spell them anymore so I abbreviate <g>).

regards
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Old 18-03-2006, 10:27   #11
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Barometer

Spend some time in NZ, you can taste and feel the weather patterns with out the assistance of a barometer. I have to adjust mine as the puddle here is 1108 feet above the ocean. Itried a couple of cheap models and set them alongside the good unit. The cheap ones hardly moved so I returned them.
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Old 18-03-2006, 18:21   #12
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Call your local airport.

Since you are at sea level, Sean, call the airport and ask for the current pressure corrected to sea level. They give it out to pilots regularly to calibrate their altimeters. If you are off by a little bit due to the variation in localle, it makes no difference.
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Old 18-03-2006, 22:30   #13
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I don't much see the point in haveing a Barometer down here.
The weather principles are simple.If it is a Sunny day, a cold front is on the way. If it is Raining, a fine day will follow in a day or so
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Old 19-03-2006, 08:24   #14
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YUP

That works for me, Wheels.
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Old 24-03-2006, 15:00   #15
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As a follow up, there was a small screw recessed WAY inside a hole in the back of the barometer. I turned the screw in various directions and gave the needle a little flick each time I adjusted it. Eventually, I found the right number of turns on the adjustment screw to get the needle to point to the correct reading.

The key when adjusting is to continually flick the needle so you can see how far the needle is deflected by your adjustments.

For a source... I just used NOAA's pressure for my area. Close enough.
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