Several years ago, at the other place (FTL!) I reported the results of an experiment I’d conducted whose results surprised me.

After eating a big meal I weighed myself and carefully noted the reported mass. I repeated the weighing several times to verify repeatability, then put lead ingots into a suitcase until the mass was close to my weight. I also carefully noted the mass of the suitcase of lead. After sitting at my desk for two hours (doing maths), having eaten and drunk nothing, and having excreted nothing, in that time I weighed myself again to find a loss of about 7%. I weighed the suitcase of lead again and verified that its reported mass hadn’t changed. I’ve repeated this experiment several times with similar results. I still don’t know what happens to the missing mass.

I recently repeated this experiment, but in a different form, again with surprising results. After micturation I weighed myself, carefully recorded the mass, and put bricks into a suitcase (I no longer have access to lead ingots) to an approximation of the mass. Again, I repeated the weighing of each several times to verify repeatability. After two more hours of maths, (again seated at my desk), I weighed myself again. This time my mass had increased by about 2%. Again, the reference weight hadn’t changed. Again, I ate, drank, and excreted nothing in that two hours.

I have absolutely no idea where the mass comes from or goes to.
.

Guess: respiration.

Your brain uses a lot of energy as well.

Michael V said:

Guess: respiration.

Well, that’s a reasonable place to start. Let’s consider, though. My RMV has been measured at 30L/min, so after two hours I’ve breathed 30*120 = 3600L. The density of pure water vapour at 25^o is 0.804g/L, so, assuming I breathed out pure water vapour, that’s about 2.894kg. Exhaled air only contains about 3% water vapour though, so only about 87g of water is accounted for – assuming the air I breathed in was completely dry. Assuming a difference of 6kg, that’s still a lot of mass to account for.

Divine Angel said:

Your brain uses a lot of energy as well.

Entirely true. Even sitting in a chair uses energy. Let’s consider that, too. E=mc², and with m=6kg and c=299692458, in two hours I’ve used about 539 PJ (5.39 × 10¹⁷ joules.)

This difference may, in some small way, account for the loss of mass in the first experiment; what I find really intriguing, though, is where the additional mass came from in the second experiment.

After sitting at my desk for two hours (doing maths), having eaten and drunk nothing, and having excreted nothing, in that time I weighed myself again to find a loss of about 7%

7% loss of body weight in two hours??? hmmmm

Dropbear said:

After sitting at my desk for two hours (doing maths), having eaten and drunk nothing, and having excreted nothing, in that time I weighed myself again to find a loss of about 7%

7% loss of body weight in two hours??? hmmmm

I’d be losing about 8 or 9 kg.

:)

I was researching the amount of oxygen we use, recently, and found out that humans use about 0.84 kg of oxygen per day at rest. That gets converted into carbon dioxide and water. Even if it was all carbon dioxide, then the maximum weight loss due to metabolism alone would be around 300g per day.

After respiration, my next guess would be evaporation. If I drank say 1 liter of water and that evaporated from the skin, through the clothing, in two hours on a hot day then that would account for a weight difference of 1 kg.

A 7% mass loss is huge. For example, a 7% mass loss over 2 hours is a 100% mass loss in just over a day. Are you really losing 6 kg in only two hours? I don’t think so.

mollwollfumble said:

After respiration, my next guess would be evaporation. If I drank say 1 liter of water and that evaporated from the skin, through the clothing, in two hours on a hot day then that would account for a weight difference of 1 kg.

A 7% mass loss is huge. For example, a 7% mass loss over 2 hours is a 100% mass loss in just over a day. Are you really losing 6 kg in only two hours? I don’t think so.

Then please tell me the flaw in my experimental method. I’ve described it exactly as conducted.

Wocky said:

mollwollfumble said:

After respiration, my next guess would be evaporation. If I drank say 1 liter of water and that evaporated from the skin, through the clothing, in two hours on a hot day then that would account for a weight difference of 1 kg.

A 7% mass loss is huge. For example, a 7% mass loss over 2 hours is a 100% mass loss in just over a day. Are you really losing 6 kg in only two hours? I don’t think so.

Then please tell me the flaw in my experimental method. I’ve described it exactly as conducted.

Your measurents suck… That’s the most likely explanation..

How is the measuring instrument supported?

Dropbear said:

Your measurents suck… That’s the most likely explanation..

Yes. That’s why I took such care to record the masses. That’s why I had a reference mass, whose mass didn’t change over the two hours. Occam’s razor says my measurements being wrong is the most likely answer. But I’ve done all I can to account for this.

You sure you didn’t do a really big fart?

Wocky said:

Dropbear said:

Your measurents suck… That’s the most likely explanation..

Yes. That’s why I took such care to record the masses. That’s why I had a reference mass, whose mass didn’t change over the two hours. Occam’s razor says my measurements being wrong is the most likely answer. But I’ve done all I can to account for this.

In that case,the scientific method would say to repeat your experiment several times, at different times of the day and see if the pattern stays

Duplicated experiment here, though over 1 hr and without checking scales for accuracy.

17:45 – weight 106.2 kg.
18:45 – weight 106.4 kg.

Dropbear said:

Wocky said:

Dropbear said:

Your measurents suck… That’s the most likely explanation..

Yes. That’s why I took such care to record the masses. That’s why I had a reference mass, whose mass didn’t change over the two hours. Occam’s razor says my measurements being wrong is the most likely answer. But I’ve done all I can to account for this.

In that case,the scientific method would say to repeat your experiment several times, at different times of the day and see if the pattern stays

I’ve done the first one several times at different times of the day and different times of the year, always with similar (but not identical) results. I’ve only done the second one once. But I will do it again.

I’ve read of experiments wherein subjects’ intake (food and drink) was monitored, and their output (solid and liquid, though not gaseous) monitored; the subjects’ mass was also monitored. Although the mass didn’t change, the intake was greater than the output (but I don’t know by how much.)

How did you make absolutely sure of no guage error from varied distribution across the scales and other things that may contribute to errors of the scales, or are you relying on doing a number of measurements to reduce this.

?

Michael V said:

Guess: respiration.

99% perspiration
1% inspiration

:)

Can you supply the raw data? I am betting the 7% is a maths error, not a measurement error.

>Can you supply the raw data? I am betting the 7% is a maths error, not a measurement error.

gravitational anomaly

Approximately a fiftieth (that is therefore 20%!) of your body weight exists from proteins.
———————
This line fits in with this OP…

Stealth said:

Can you supply the raw data? I am betting the 7% is a maths error, not a measurement error.

I second the above. I’d guess it was really more like 0.7%.

I’ve tried out other possible sources for the anomaly on my home scales. I can now reject the following hypotheses:
1) Respiration
2) Sweating
3) Flat battery in scales
4) Improper zeroing by moving the scales
5) Putting the scales on an uneven surface
6) Putting the scales on carpet
7) Moving the body while the scales are taking their measurement

None of those can account for more than about 0.4 kg difference on my home scales.
8) Clothing
May account for 1.5 to 3 kg or so, but not 6 kg.
9) Eating
May account for ~1.5 kg.

So a math error looks like by far the most likely explanation.

Fluctuation in my body weight over a day, scales only measure to nearest 0.2 kg:
Time , Weight(kg) , event
8:20 , 105.8 , after breakfast before shower
8:30 , 106.0 , after shower
8:40 , 107.4 , after dressing – clothes weight 1.4 kg
9:40 , 107.2 , after model-making – mostly sitting
9:45 , 106.8 , after defecation
10:50 , 107.2 , after model-making & after replacing flat battery in scales
10:50 , 107.4 , after a drink of water
12:20 , 107.4 , after doing nothing – sitting
13:35 , 107.6 , after lunch
13:40 , 107.0 , after urination
13:50 , 107.8 , after coffee
15:45 , 107.4 , after model-making
15:50 , 107.4 , after a drink of water
18:40 , 107.2 , after doing nothing
19:25 , 108.6 , after dinner – dinner weight 1.4 kg
19:25 , 108.4 , re-check previous weight
20:30 , 108.4 , after doing nothing
20:35 , 108.0 , after urination
22:12 , 106.6 , after undressing
10:40 next day , 105.8 , after breakfast before shower
13:10 , 107.0 , after dressing and going shopping

Summary, the only large weight changes during the day were dressing (1.4 kg) and dinner (1.4 kg). 0.8 kg was lost overnight.

What about changes in measurement with changes in ambient temp? Nothing corresponds with that? Heating of the scales.

Oh but you have a reference I see, so can’t be that.

transition said:

What about changes in measurement with changes in ambient temp? Nothing corresponds with that? Heating of the scales.

Didn’t check that. I did consider the effect of ambient temperature on sweating, and the effect of fear on sweating.

0.8 kg was lost overnight.
——————————-

You Tiger you.

Stealth said:

Can you supply the raw data? I am betting the 7% is a maths error, not a measurement error.

When I first read this, I must confess I took offence at the idea that my mathematical skills might be somewhat deficient. On subsequent consideration, though, it’s actually a reasonable request. So here are the raw data, for experiment one, only. I’ve only conducted experiment two once, a few weeks ago, and only have one measurement for it. I’ll do that again, several times. Now, hopefully this’ll work:

Date	Time	Body mass (kg)	Reference mass (kg)	accuracy	Notes
1988-08-13	1432	91.8</center?<>	93.3	2%</center?<>	1
	1640	85.9	93.3	2%	1
1994-12-27	1946	94.7	95.0	2%
	2153	89.7	94.9	2%
1998-03-23	1328	96.8	95.0	2%
	1546	93.2	95.0	2%
2001-03-29	1442	106.2	105.0	1%
	1701	100.9	104.9	1%
2006-09-21	2002	122.2	123.0	1%
	2215	115.8	123.1	1%
2011-07-19	1312	132.6	130.2	1%
	1516	129.4	130.2	1%

Note 1: This measurement (body mass) was obtained by weighing myself 7 times. The same reading was obtained each time.