Topic: Trouble with Fortin temperature correction - 4
posted January 23, 2021 10:33 AM
The Problem: I have Fortin and its uncorrected readings are within a mb of my station pressure as read from my aneroid Fisher and if I apply the standard temperature correction it sends the values 3+ mb to low. I think my correction calculation is correct (OK everyone roll-their-eye and say, ‘yeah they all say that at first’) and I think the scale correction is minimal and the vacuum is good. But in any case, maybe you can help find the kink.
Introduction: I purchased an eBay Fortin (F. D. & Co. LTD Watford) in good stead. I clean the interior of the tube with a dilute nitric acid solution to remove the mercury oxide, rinsed with distilled water and anhydrous methanol and dried under vacuum. Printed on the base of the cistern is: ‘Standard Conditions: g=980.665 cm/s^2; Temp = 0C. Refilled with clean triple distilled mercury; when tilted the mercury run to the top of the tube with only a sub-mm side bubble and when rocked I hear the distinctive ‘snap’, so I the vacuum looks good. I measured the distance from the bottom of the pointer to the 700mm mark on the scale with a meter long digital caliper and it is in within a quarter of a mm. I know this does not constituted a proper scale calibration but suggests it is very close.
As my standard barometer I am using my recently purchase aneroid Fisher which I have compared to six other local barometers, all within a few miles of my location in the Tucson area:
CWOP - C2274 (https://weather.gladstonefamily.net/site/C2274)
CWOP - E4883 (https://weather.gladstonefamily.net/site/E4883)
CWOP - AV579 (https://weather.gladstonefamily.net/site/AV579)
METAR – KTUS: TUCSON INTERNATIONAL AIRPORT (https://weather.gladstonefamily.net/site/KTUS)
METAR – KDMA: Davis-Monthan Air Force Base (https://weather.gladstonefamily.net/site/KDMA)
UA: Physics and Atmospheric Sciences Building, University of Arizona (http://www.atmo.arizona.edu/products/wxstn/)
The UA is bang-on with the Fisher and the others slightly higher but no more than 2mb. So, I am calling the Fisher my standard (Figure 1).
The Observations: When I compare the station pressure from the Fisher to the ‘raw’ uncorrected Fortin reading the Fortin is about a 1 mb or less (yes, I made the mm to mb conversations) higher than the Fisher (See Figure 2). My office temperature is currently fairly constant at 25C +-1C and so the temperature corrections are approximately -3.6 to -3.9mb and for my location, 32.25 degrees north and 2519 feet elevation, the gravity correction is about -1.15mb. Figure 3 the temperature, gravity and elevation (SL) corrections applied to the Fortin compared to the Fisher (SL). As you can see the corrections send the Fortin well below the Fisher and UA data.
I also tested the Fortin at lowers temperature (11 to 3.5 C) by hanging it in a refrigerator, these lower temperature data plotted slight off the trend of the normal room temperature (~25C) uncorrected station data (See Figure 2; lower temperature data indicated). However, when the temperature correction is applied the lower temperature data plot well away from other normal room temperature (~25C) corrected data (See Figure 3).
So: Is this a scale correction problem, is the temperature correction for ‘this Fortin’ just funky, something else or what am I doing wrong?