Meniscus-measuring (without parallax error!) скачать в хорошем качестве

Meniscus-measuring (without parallax error!)

When you measure liquids, edges of the liquid get pulled up walls of the container leading to a smile we call a meniscus & it’s important to measure from BOTTOM of meniscus (assuming you’re not measuring mercury or something where the meniscus is convex) ⚠️ BUT where bottom looks like it is depends on angle you’re looking from. To get a good eye-level, direct, glance, adopt a “titration stance” (that’s what my o-chem teacher called it) more on accuracy, precision, & error: https://bit.ly/accuracyprecisionerror Still, chances are, despite good form, each time you go to measure, you’re looking from a slightly different angle, so location will be slightly different - but different in different, random, ways (1 time your glance is more right-angled another time it’s more left-angled, etc.) This would be an example of RANDOM ERROR & I learned it has a cool name: PARALLAX ERROR. RANDOM ERRORS are “unbiased.” This is like having a shaky arm that leads to poor aim - your tails rarely directly hit their target in pin the tial on the donkey. BUT they’re not biased towards any one direction (e.g. you don’t have a tendency to pin left of target vs right) & they vary in how “off” they are. As a result these errors “cancel out” if you average enough of them, so RANDOM errors do NOT affect average, just the variability around the average. SYSTEMATIC ERRORS, on the other hand, are BIASED (error’s always in a certain direction) & they don’t “fix themselves.” This is like always pinning to the left of the target (or the right, or above, or below (but not all of them)) Since these SYSTEMATIC errors are DIRECTIONAL, the average measurement will reflect this error - e.g. if you always err to the left, so will your average If you know the amount & direction of systematic error, you might be able to correct for it after the fact. If I keep pinning the tail on the donkey’s ear, I can “cheat” after the fact & move the donkey so the tails are in the right spot. BUT I need to know how to move it… You can also get SYSTEMATIC ERRORS when measuring liquids. They can be “fixable” like if you measure volume & then realize you left stir bar in - you can subtract out volume of the stir-bar from the volume of the stir-bar + liquid  In that case you can see stir bar (or get an unwelcome surprise when you’re pouring the liquid & hear a thunk…). But the important thing is you found it! What’s even more dangerous is when you don’t know that a systematic error’s there. if you keep measuring & getting similar results, you might feel pretty confident - I mean, why doubt it? But what if marking lines on your graduated cylinder are off, so it’s like you always forgot the stir bar in there? Some systematic errors are harder to correct for even if you know they exist - like if you only learn that you’re supposed to be reading from BOTTOM of meniscus after you’ve already made measurements. If you’d been reading from top or edge of meniscus, your recorded values will ALL be too high, but how much “too high” is hard to know. more practical lab tips: https://bit.ly/lab_tricks_page     more pipetting tips: https://bit.ly/pipetting_problems &    • Pipetting viscous liquids - reverse pipett...   &    • Random tips for choosing & using micropipe...   &    • Tips for when liquid jumps back & gets stu...