Hey Ron that is correct I am located in North Dakota and my current elevation here is just over 1500 feet. Can you call me I did send you a private message and we can maybe figure this out on a five minute call
---------- Post added July 5th, 2014 at 12:58 PM ----------
Unless I'm doing something wrong I use that calculator and it still is incorrect
---------- Post added July 5th, 2014 at 01:03 PM ----------
Ron I believe I have it figured out thank you so much you're a lifesaver and I want to say thanks for taking your Saturday to make my weekend. the cobalt is reading correct I just wish it would've made it more clear when I got my instructions the first time but no worries
my local elevation is 596 meters (1660ft) and 29.85 inHg
i did the calculation and I believe it came out to 951 which is only 1 off from what my cobalt reads
You did it right- 1011 "corrected" at 1660 feet would equate to an actual reading of 951.6, just about exactly what your Cobalt says. For perspective, 30 mbar is about equal to one foot of sea water.
We do need to make this more clear re. checking the Cobalt. It seems most local weather services "correct" atmospheric pressure readings to what they would be at sea level- which I guess makes a kind of sense for weather prediction, but is confusing for checking the accuracy of a gauge. We have a local university weather station here that does give the real values, and we are only slightly above sea level, I was not aware until recently that most online services give these "sea level corrected" numbers.
Glad you are able to dive- enjoy!
Ron
---------- Post added July 5th, 2014 at 03:27 PM ----------
To follow up on the previous post:
You can check the Cobalt's depth sensor accuracy by using the atmospheric pressure reading on the System Info screen.
However, if you are above sea level you will need to convert local weather barometric pressure data to remove the altitude correction that is usually added. You need to know your actual altitude to do this. The Cobalt displays actual pressure data- the surface ambient pressiure is what is needed for decompression calculations. Most weather services provide a "sea level corrected" number. Unless the value is specifically listed as "station pressure, this correction must be removed to determine the physical atmospheric pressure. Online calculators such as
Above Sea Level Barometer Reading Correction Calculator can do this conversion, and will also convert units to mbar if the data is in inches or mercury or other units.
As a rule of thumb, the correction is about -35 mbar per 1000 feet of elevation, or -50 mbar per 400 meters. This is about what is applied for the first few thousand feet. But the calculator will be more accurate.
We have had some mixups recently where users seemed to have an inaccurate Cobalt, in fact the local pressure data they were receiving was the issue.
Ron
