Tuesday 15 October 2019

Forensic plate pouring

At almost the last minute, I got to teach one lab section of Yr3 Food & Fermentation Microbiology. That was because a couple of last minute registrations pushed the numbers in the year just over the magic number of 36 (= 18 x 2;  18 being the health & safety limit to the number of bodies in the room. I think that, if surplus hadn't appeared, I would have had nothing to do on Thursday afternoons and had short hours for my last year teaching at The Institute; that would have been okay also!

I've had two weeks with these 12 students, about half of whom I had for QM [remedial math] 2 years ago. As in recent years, I'm going to appoint each pair in rotation to the rank of Autoclave Liaison Officer ALO. At least some of our students will be confident and comfortable with this potentially lethal scalding instrument when they move into their final year. They also need to be confident, comfortable and reliable when pouring Petri dishes if they want a career at the microbiology end of biology.  reliable here means: can you pour 20 plates from 500ml of agar so that they are all the same weight / depth? You only get to be able to do that by deliberate practice and it's important to get some feedback on how you're progressing towards the goal. Accordingly, last Friday while putting away the previous day's plates for use next week, I recorded the weight of each one.

Each pair of students had poured a wildly different number of plates: in general those who had poured light plates got more out them out of the bottle. That's not necessarily a good thing: the empty plates cost about €1/dozen so it's better to pour fewer plates so long as you have enough. Also thin plates dry out and blow away more readily, so don't last as long waiting to be used. Again, the shelf life needs to be enough - either one week or two weeks usually. Fat / thick plates otoh aren't getting as many functional plates from each run through the autoclave. In the table [L] I've recorded
1) the number of each type of Petri dish
2) the weight of just the plastic - count x 13.15g
3) the total weight of all the plates less the plastic
4) the average weight of each filled plate
5) the standard deviation of each batch
I've highlighted the lowest Standard Deviation [in the 3rd data column] because those boys were clearly in the zone when they were pouring - each plate the same as the last. But they still have things to learn. Expecting to pour 20 plates, they had labelled 20 empties before pouring because they poured a little mean, they only used 401g of their half litre of agar. That's enough to make 4 or 5 extra plates from the batch. But the lazy arses decided not to label the extra plates; preferring to throw away the surplus agar. If we have enough starch agar plates for the class next week, that was a good call. If not then someone will have to go sort in their investigations at the frontier of science. In the interests of nerdnik completeness here is the analysis of variance which shows clearly that there is a significant difference in the average weight of Petri dishes poured last Thursday - tsk on the quality control, lads.

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