NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant frames
From: Brad Morris
Date: 2009 Apr 16, 10:51 -0400
From: Brad Morris
Date: 2009 Apr 16, 10:51 -0400
Hi Jeremy As indicated in the Sextant Handbook, you should allow the sextant to reach stable temperature before use. The reason is exactly that, the thermal coefficient of expansion. The sextant should not be warmed in the sun, as one side will be warmer than the other. This refers to the linear expansion of the material, typically in parts per million per degree Celsius (PPM/C). So if we had a one meter long bar of aluminum and changed the temperature by 1 degree C, it would change in length by about 23 microns (~1/40th of a millimeter). That's .000023 PPM/C The first issue that you encounter is that the parts of the sextant will not change uniformly during the transition to stable temperature. Thicker parts will remain cool while other thinner parts will quickly warm. This may distort your reading. The effect is exaggerated in plastics, which have a large coefficient of expansion relative to aluminium. When comparing aluminium to bronze, we encounter a subsequent issue. Since bronze is an alloy, the thermal coefficient of expansion is a function of the precise materials which form the alloy. You will be hard pressed to have manufacturers tell you the composition of that alloy and hence, we can only guess at the coefficient. In general, however, bronze has a lower thermal coefficient of expansion than aluminium. As such, the effects of temperature are reduced. Based upon the values, I would indicate that this issue is dramatically overblown. From my perspective, the effect is minor. If you were to change the temperature by 100 degrees C, you might have issues, until the sextant temperature stabilizes . But that is freezing to boiling of water. I don't think you keep the bridge that cold. I doubt the bridge wing is that hot. Best Regards Brad -----Original Message----- From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Anabasis Sent: Thursday, April 16, 2009 9:45 AM To: NavList Subject: [NavList 7968] Re: Sextant frames That certainly is interesting reading (and should be copied to the sextant group on yahoo). I have used sextants of all natures at sea (excluding plastic which I've only played with on land). I haven't noticed much difference in the accuracy of my sights, but there certainly is a cosmetic and weight difference. I found that my selection of scopes increased my accuracy, but the material of the sextant seemed irrelevant. One question however. Do the differing co-efficients of expansions make any difference in the consitancy of the sight? In other words, when I bring a sextant out from the AC of the bridge to the hot bridge wing, will a bronze or aluminum sextant tend to warp at all, and if so, to what degree? I would hope that that frame shape would take care of this issue and by the time the fog cleared on the mirrors, I could be confident of the sight. When I bought my sextant last year, I went with the bronze Cassens & Plath, not because it was bronze, but because it had the highest claimed accuracy of sextants available new, and because of the polarized shade option. The C. Plath Navistar is not bronze, but some sort of aluminum looking alloy. It is quite light, and has a very fine claimed accuracy. My issues with that sextant extend beyond the material and more to the ergonomics and usability (very poor shading). Jeremy "Confidentiality and Privilege Notice The information transmitted by this electronic mail (and any attachments) is being sent by or on behalf of Tactronics; it is intended for the exclusive use of the addressee named above and may constitute information that is privileged or confidential or otherwise legally exempt from disclosure. If you are not the addressee or an employee or agent responsible for delivering this message to same, you are not authorized to retain, read, copy or disseminate this electronic mail (or any attachments) or any part thereof. If you have received this electronic mail (and any attachments) in error, please call us immediately and send written confirmation that same has been deleted from your system. Thank you." --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---