(A true story, with some added sarcasm, to illustrate that using equations is safer in the long run than trying to avoid using equations)<\/p>\n
<\/p>\n
My lab has a new centrifuge that I recently needed to use for the first time. \u00a0Like most centrifuges, you can set the rotations per minute (RPM) and the number of minutes; my protocol said ‘spin at 125 g for 6 minutes.’ \u00a0Having used many centrifuges, and having written (in Lab Math) about the indefensible* conversion from RPM to g, I expected this.\u00a0 However, when I went to look at the conversion chart that I expected to find taped to the lid of the centrifuge, it wasn\u2019t there.\u00a0 No one had made a spreadsheet to calculate the conversion from g\u2019s to RPMs for frequently used values.\u00a0 No one had measured or looked up the radius of the rotor, or indicated whether it represented the distance to the middle or the tip of the holders.\u00a0 At least, no one had thoughtfully posted it in an obvious place for those who were to follow.\u00a0 So I went in search of someone who I knew had used this centrifuge to find out if this information was kept somewhere that I didn\u2019t know about.\u00a0 The person I found to ask was trained in a lab at Yale.\u00a0 He was told, during his training, that 1.0 RPM (the numbers have been changed to protect the innocent) would give him the correct RCF, and since our centrifuge is about the same size as the one in the Yale lab, he just uses 1.0 RPM. \u00a0\u00a0Always.<\/p>\n
SERIOUSLY?\u00a0 The equation is:\u00a0\u00a0 RCF = RPM2<\/sup> [min-2<\/sup>] x Radius [mm] x 10-6<\/sup><\/p>\n It is multiplication!\u00a0 Granted, getting out a ruler and measuring from the center of the rotor to the tip of the holder can be physically exhausting and is best left to the young athletic types in your lab.\u00a0 No argument there.\u00a0 But risk your experiments rather than do multiplication?<\/i>\u00a0 \u00a0And he learned this at Yale?\u00a0 This is a very smart person, a very good scientist, yet the thought of doing multiplication is so distasteful, that he relies on a number he once heard from someone he considered trustworthy.<\/p>\n What is this \u2018culture of equation avoidance\u2019 doing to our scientists? He may someday find himself with a new centrifuge, of a different size, and his 1.0 RPM could lead him to bad data. \u00a0Troubleshooting will be close to impossible, and he will abandon his beautiful experiment and not get a grant.<\/p>\n Please help him, and scientists like him.\u00a0 Change the culture.\u00a0 Use equations until it hurts.<\/p>\n <\/p>\n * I object to the use of \u201cg\u201d as unit for this purpose, although I appreciate that thinking in terms of our constant companion gravity is a comfort. The correct name for the parameter in question is Relative Centrifugal Field or RCF. \u00a0Without regard for reality, however, RCF is traditionally reported in units of g. \u00a0RCF has\u00a0dimensions of length over time squared (L T-2<\/sup>), which is mm\/minutes squared in the above equation (rotation is dimensionless). RCF is determined entirely by the rotations per minute and the radius of the rotor. On the other hand, gravitational force has units of, surprise, force, i.e. Newtons, meaning its dimensions are mass x length \/ time squared (M L T-2<\/sup>).\u00a0\u00a0 What happens to the mass when you convert to RCF?\u00a0 Traditionally, they\u2019re not telling.\u00a0 So, writing \u201can RCF of 125 g\u201d is an abomination. \u00a0However, I have gotten over this and moved on.\u00a0 Really.<\/p>\n","protected":false},"excerpt":{"rendered":" (A true story, with some added sarcasm, to illustrate that using equations is safer in the long run than trying to avoid using equations) My lab has a new centrifuge that I recently needed to use for the first time. \u00a0Like most centrifuges, you can set the rotations per minute (RPM) and the number […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[3,1],"tags":[],"_links":{"self":[{"href":"http:\/\/labmath.org\/index.php?rest_route=\/wp\/v2\/posts\/467"}],"collection":[{"href":"http:\/\/labmath.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/labmath.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/labmath.org\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/labmath.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=467"}],"version-history":[{"count":9,"href":"http:\/\/labmath.org\/index.php?rest_route=\/wp\/v2\/posts\/467\/revisions"}],"predecessor-version":[{"id":474,"href":"http:\/\/labmath.org\/index.php?rest_route=\/wp\/v2\/posts\/467\/revisions\/474"}],"wp:attachment":[{"href":"http:\/\/labmath.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=467"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/labmath.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=467"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/labmath.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=467"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}