Weight of Water (Steam)

Preparing my lunch for work today, I bang the lunchbox on my kitchen scales, add the pork loin, then add some freshly boiled broccolli. The weight held steady after the pork was added, but when the broccolli was added it was steaming profusely, and the weight was visibly dropping a gram a second. it’s currently down from 479 to 443. Does steam really weigh that much, or is somethign else going on here?

That sounds like something other then steam. Water is fairly heavy, but 36 grams of water would be just over two cubic inches, or 36 cc. That’s quite a lot of volume, unless you had a LOT of freshly steamed broccoli.

a full head - 250g according to othe label

What kind of scale? I’m wondering if the reading is affected by temperature.

Small square base, metal circular section where you put the thing to be weighed. digital read out. Just a standard kitchen scale i got from amazon for £12 or so

How good is the scale? It didn’t go up adding the pork, and then went down sharply after the broccoli? Perhaps it needs cleaned.

I turned it on with the box in place, so the weight of the box would be calibrated out (it turns on at 0 whatever is on the scale)

Then I added the pork - went up to 157

Then the broccolli - went up to 490 and dropped a gram or 2 at a time down to low 400’s

I’ll wager something is wrong with your scale. Even if the broccoli is fresh out of the boiling water, and even given as much surface area as a head of broccoli has, it seems unlikely that you would see two cubic inches of liquid water evaporate in thirty seconds’ time.

I don’t find that implausible. If there have been no other apparent problems with the scale before or since, I’d say evaporation is the most likely explanation.

Nah, got to be something. Convection currents, a downward reaction to the upward thrust of the steam.

I’d say cook some more broccoli and repeat this, but this time use a container with a tight cover.

Not for the quantity involved here. Suppose the OP’s lunch container is 4 inches wide, 4 inches long; we’re talking about filling it 1/8" deep with water, and then having all of that evaporate in the space of 30 seconds.

Broccoli like any vegetable are mostly made of water, say 80%. So you would have a good 200 g of water, about 1/6 of which evaporates. And because of their fractal shape, they have a LOT of surface in contact with the air. This increases the plausibility of 36g of water evaporating.

Actively boiling water, you might be able to lose that much that quickly, if you have a really hot stove. Some vegetables you’ve already taken out of the cooking pot, though, not a chance.

ill try this tomorrow am

Repeating the measure with a balance scale would also be a good way to confirm that is it not a scale dysfunction issue.

Tried again today. Box on scales when turned on so weight calibrated out. Added broccolli straight from Pan and whacked lid on box. - weight = 367g and was not changing I then removed the lid and the weight has started dropping from a starting point of 320g.

The rate of dropping is nothing like as fast as yesterday though. Just gone for a look and its down to 306, and it’s not steaming all that fiercely now so I can’t see it dropping much more (if steam is indeed the mechanism at work here) Steamin stopped and weight holding steady at 298g

Freshly boiled broccolli, isn’t boiling. Out of the pan it has a maximum temperature of 100C, and there is no further source of energy input. The only energy available is what is already in the broccoli. Assume it is 100% water, and room temperature is 20C, and 250grams of broccolli. Plus 70 grams of water either absorbed or wetting the bunch. To a total weight of 320 grams. Specific heat of water is 4.186 J/g/K so 80 x 320 x 4.186 = 107kJ.

Latent heat of vapourisation of water is 2260 J/g, so there is, at the limit, enough energy to vapourise about 47 grams of water before you get back to room temperature. In reality you would probably halve that, since the food will lose energy from other means, and the rate of evaporation drops precipitously as it cools. But as a sanity check, of the order of 20 odd grams is reasonable.