Sunday, June 17, 2018

Specific gravity

Now there's a term I've not had cause to use since ... probably 1970. It came to mind this morning when I was thinking about cream.

In Tesco I asked the boy on the till if he had scales and if he could weigh the pot of cream for me. 
'Because I want to know how much it weighs.'
He looked puzzled. 'Why?'
'Because my recipe is in grams and this is in mls.'
'Oh. No.'

I bought three pots just in case.

Back home I emptied one into the basin of the scales. It turned out that 300mls weighed just less than 300g. And that's when it struck me. Well, not exactly struck so much as there was a stirring of the brain cogs.

Something was sending my brain a message. Something from school days. Something about specific gravity. Possibly. Whatever was relaying the message wasn't sure. But the theory goes like this. The specific gravity of something is to do with water. Something like 1 litre of water weighs 1 kilogram. Or maybe that was how they defined measurements in the beginning. So it might have nothing to do with SG.

Cream is only a bit thicker/heavier than water so ... a very rough estimate would have 300ml weighing in at 300g. I'm sure Stu will tell me if I'm wrong. Or Husband. Whoever gets here first. 

All these years and I never knew that. Well, I did know it but hadn't applied my knowledge. I wonder if in a year or so I will realise the value of log tables. What on earth were they about? Do they have anything at all to do with life?

Incidentally when I came to pay for my shopping I realised I had left my purse at home. 


SmitoniusAndSonata said...

I can glibly recite that 1 litre weighs 1 kilo with the best of them … but what that actually means in terms of anything practical escapes me so I couldn't help. My cakes are fairly inedible, too, I'm afraid.

PipeTobacco said...

In the US we always use 1ml of water = 1g as the specific gravity in our Chemistry courses. The cream would be close to the same as it is mostly water, but the cream solutes change the density of the molecules to an extent the leads to the specific gravity (minor) difference you see. So as a general rule of thumb, if the solution is mostly water, ml will very nearly equal grams, but if it is less water based (say an oil or a higher fat mixture (like peanut butter)) the number could be significantly different.

So, in baking/cooking when ingredients are in grams, the higher the water content, the simpler the conversion from liquid volume to mass.