Should you trust your digital scales?

[The-Engineer-That-Brews]

100ml of plain water weighs 100grams, right...? Hmmm... my old physics teacher would have grimaced and said "No, 100ml of water has a mass of 100g".

Stay with me here...

For various reasons today I decided to check the calibration of my hydrometer. It's a decent Stevenson Reed glass one, calibrated at 20ºc.

First I checked two 0.1g kitchen scales using a 200g calibration weight. The first read 200.0g and the second 200.5g.
Next I weighed my calibrated lab-glass 100ml measuring cylinder. One balance read 142.8g and the other 143.4g.
Then I very carefully measured 100ml of plain tap water at 20ºc into the cylinder (as a cross-check I did so using ten x 10ml syringe-fulls).

Then I checked the SG of the water with my hydrometer, which read 1.000 on the nose.
The weight of the water? Well, minus the weight of the cylinder, the first balance said 98.8g and the second 99.3g. Hmmm. (I then repeated the experiment and got 98.8g and 99.1g).

Not content with this I tried again using strong saline solution, measured by hydrometer at SG 1.100.
This time I got 108.0g and 108.7g; and on the rerun 108.4g and 108.8g.

What the heck?? Either the hydrometer, both volume measurements or both balances are out by about 1.5%. That doesn't sound like much, but it's the difference between measuring the SG as 1084 versus 1100 !!

That was when I thought back to my physics teacher... how much force would a 100g mass exert on the scales? Ummm well the 'schoolboy' answer is 1 Newton; but the more accurate answer is 9.81 Newtons.
Now... that 9.81 does look rather similar to what the scales gave when I measured the 100ml of SG 1.000 water... and it's also consistent with the measured weight of the SG 1.100 saline...

So what do you reckon is more likely? My SR hydrometer is telling fibs, or the designers of kitchen scales cut corners and assume the value of 'g' is 10 rather than 9.81 ??

I should add that the '200g' calibration weight was supplied along with the kitchen scales... so does it really weigh 200g?

 

[pilgrimhudd]

Hmmmm.....




I



Reckon


You


Have

Too much time on your hands!!!!

 

[moto748]

I'm sure scales manufacturers are well aware of 9.81. Isn't to more reasonable to say that your discrepancies lie within their margin of error? Are the scales warranted for any particular accuracy?

Hydrometer readings must vary a little with altitude? Probably not enough to be significant, though.

 

[johncrobinson]

Those digital scales are remarkable value for money.But they are not scientific instruments.

For example I needed a light meter for my work about 20yrs ago.
The local photographic shop could do me one for a tenner,But the radiometer I ended up buying cost £4000.+VAT.!!!

Thats the kind of difference precision costs.

 

[Buffers brewery]

100ml of plain water weighs 100grams, right...? Hmmm... my old physics teacher would have grimaced and said "No, 100ml of water has a mass of 100g".

Stay with me here...

For various reasons today I decided to check the calibration of my hydrometer. It's a decent Stevenson Reed glass one, calibrated at 20ºc.

First I checked two 0.1g kitchen scales using a 200g calibration weight. The first read 200.0g and the second 200.5g.
Next I weighed my calibrated lab-glass 100ml measuring cylinder. One balance read 142.8g and the other 143.4g.
Then I very carefully measured 100ml of plain tap water at 20ºc into the cylinder (as a cross-check I did so using ten x 10ml syringe-fulls).

Then I checked the SG of the water with my hydrometer, which read 1.000 on the nose.
The weight of the water? Well, minus the weight of the cylinder, the first balance said 98.8g and the second 99.3g. Hmmm. (I then repeated the experiment and got 98.8g and 99.1g).

Not content with this I tried again using strong saline solution, measured by hydrometer at SG 1.100.
This time I got 108.0g and 108.7g; and on the rerun 108.4g and 108.8g.

What the heck?? Either the hydrometer, both volume measurements or both balances are out by about 1.5%. That doesn't sound like much, but it's the difference between measuring the SG as 1084 versus 1100 !!

That was when I thought back to my physics teacher... how much force would a 100g mass exert on the scales? Ummm well the 'schoolboy' answer is 1 Newton; but the more accurate answer is 9.81 Newtons.
Now... that 9.81 does look rather similar to what the scales gave when I measured the 100ml of SG 1.000 water... and it's also consistent with the measured weight of the SG 1.100 saline...

So what do you reckon is more likely? My SR hydrometer is telling fibs, or the designers of kitchen scales cut corners and assume the value of 'g' is 10 rather than 9.81 ??

I should add that the '200g' calibration weight was supplied along with the kitchen scales... so does it really weigh 200g?

You didn't measure the weight of the hydrometer or the volume of water it displaces

For completeness you understand

 

[dad_of_jon]

100ml of plain water weighs 100grams, right...? Hmmm... my old physics teacher would have grimaced and said "No, 100ml of water has a mass of 100g".

Stay with me here...

For various reasons today I decided to check the calibration of my hydrometer. It's a decent Stevenson Reed glass one, calibrated at 20ºc.

First I checked two 0.1g kitchen scales using a 200g calibration weight. The first read 200.0g and the second 200.5g.
Next I weighed my calibrated lab-glass 100ml measuring cylinder. One balance read 142.8g and the other 143.4g.
Then I very carefully measured 100ml of plain tap water at 20ºc into the cylinder (as a cross-check I did so using ten x 10ml syringe-fulls).

Then I checked the SG of the water with my hydrometer, which read 1.000 on the nose.
The weight of the water? Well, minus the weight of the cylinder, the first balance said 98.8g and the second 99.3g. Hmmm. (I then repeated the experiment and got 98.8g and 99.1g).

Not content with this I tried again using strong saline solution, measured by hydrometer at SG 1.100.
This time I got 108.0g and 108.7g; and on the rerun 108.4g and 108.8g.

What the heck?? Either the hydrometer, both volume measurements or both balances are out by about 1.5%. That doesn't sound like much, but it's the difference between measuring the SG as 1084 versus 1100 !!

That was when I thought back to my physics teacher... how much force would a 100g mass exert on the scales? Ummm well the 'schoolboy' answer is 1 Newton; but the more accurate answer is 9.81 Newtons.
Now... that 9.81 does look rather similar to what the scales gave when I measured the 100ml of SG 1.000 water... and it's also consistent with the measured weight of the SG 1.100 saline...

So what do you reckon is more likely? My SR hydrometer is telling fibs, or the designers of kitchen scales cut corners and assume the value of 'g' is 10 rather than 9.81 ??

I should add that the '200g' calibration weight was supplied along with the kitchen scales... so does it really weigh 200g?

hmm my hydrometer with tap water is .002 out but for working out abv it doesn't matter that much its the drop in points from og to fg that gets you abv and .002 wont make that much diff in checking if a beer has finished fermenting. should tap water come out at 1.000 if it has chlorine or chlorimine added? - I don't know. every bit of measuring equipment is prone to some margin of error unless you're measuring time with an atomic clock which is more accurante than almost every other measuring device we could be using?

 

[stripeyjoe]

I give a range for my ABVs, +/- 0.5% on what I think it might be based on the OG and FG difference measured on my iSpindel. I do use a hydrometer for pre and post boil for my efficiency but then just go by the iSpindel once it is in the FV, I pressure ferment so it’s not that easy to take an FG with the hydrometer as the beer is carbonated by then.
The iSpindel does seem to show a slightly higher OG than the post boil and I think it shows lower for FG, though that might be due to the final temp and pressure.

 

[Irishwizard]

So.. the answer seems to be......CALIBRATE THEM
Thats too simple - chuck em out and write some waffle

 

[chopps]

100ml of plain water weighs 100grams, right...? Hmmm... my old physics teacher would have grimaced and said "No, 100ml of water has a mass of 100g

Your science teacher was boring.

My science teacher came out of the store room one day with a beaker of p*ss, a battery an Avo meter and a light bulb and proceeded to show us why you shouldn’t take a leak on an electric fence. Now that’s real sciencing.

I have other stories including how to make official WW1 mustard gas during lunchbreak and get an early school day finish for everyone, how to launch a half scale moon rocket from the third floor fire escape, how to drink water while hanging by your feet in the lab like a bat and a demonstation of solid, liquid, gas states of matter using baked beans (in the can) a cricket pitch and mucho propane.

 

[itry]

You should read the book called water or watch the game of thrones

 

[peebee]

Do you want me to pap on some more about this?

I can't hear you! So I'll take it as "yes".

Shame on you. 100ml of water does not weigh (or have a mass) of 100g, especially at 20°C. Pretty damn close at 4°C though.

A hydrometer does not measure density. It measures relative density (so called "specific gravity"). Brewing hydrometers in the UK (and most of the rest of the world) are calibrated to measure 1.000 in water at 20°C compared to a reference of water at 20°C, which at that temperature has a density of 0.9982g per ml. Lab rats can use a hydrometer calibrated at 20°C but a water reference of 4°C; now that will measure 0.998 in water. But most lab rats cling onto their 60°F/60°F hydrometers, like the Americans do.

Now go and do your sums again. Write 1000 lines "I will not confuse density with SG", and then stand in the corner until the bell rings to say you can go home.

 

[The Baron]

Unless you are doing it for the HM does it matter that at £5 for a hydrometer and £15 for some scales they are slightly out we are homebrewers.
As long as you calibrate them to make sure it is close your reading will be relative to the hobby

 

[Cwrw666]

You measured your 100ml of water with 10 squirts from a 10ml syringe? What on earth makes you think the 10ml syringe is remotely accurate?

 

[Davegase]

100ml of plain water weighs 100grams, right...? Hmmm... my old physics teacher would have grimaced and said "No, 100ml of water has a mass of 100g".

Stay with me here...

For various reasons today I decided to check the calibration of my hydrometer. It's a decent Stevenson Reed glass one, calibrated at 20ºc.

First I checked two 0.1g kitchen scales using a 200g calibration weight. The first read 200.0g and the second 200.5g.
Next I weighed my calibrated lab-glass 100ml measuring cylinder. One balance read 142.8g and the other 143.4g.
Then I very carefully measured 100ml of plain tap water at 20ºc into the cylinder (as a cross-check I did so using ten x 10ml syringe-fulls).

Then I checked the SG of the water with my hydrometer, which read 1.000 on the nose.
The weight of the water? Well, minus the weight of the cylinder, the first balance said 98.8g and the second 99.3g. Hmmm. (I then repeated the experiment and got 98.8g and 99.1g).

Not content with this I tried again using strong saline solution, measured by hydrometer at SG 1.100.
This time I got 108.0g and 108.7g; and on the rerun 108.4g and 108.8g.

What the heck?? Either the hydrometer, both volume measurements or both balances are out by about 1.5%. That doesn't sound like much, but it's the difference between measuring the SG as 1084 versus 1100 !!

That was when I thought back to my physics teacher... how much force would a 100g mass exert on the scales? Ummm well the 'schoolboy' answer is 1 Newton; but the more accurate answer is 9.81 Newtons.
Now... that 9.81 does look rather similar to what the scales gave when I measured the 100ml of SG 1.000 water... and it's also consistent with the measured weight of the SG 1.100 saline...

So what do you reckon is more likely? My SR hydrometer is telling fibs, or the designers of kitchen scales cut corners and assume the value of 'g' is 10 rather than 9.81 ??

I should add that the '200g' calibration weight was supplied along with the kitchen scales... so does it really weigh 200g?

Have you had your syringe calibrated ?

 

[Clint]

My science teacher had huge eye brows! One lesson he was demonstrating the attributes of concentrated sulphuric acid by dropping various items into a beaker of the stuff. At the end he absent mindedly tipped it down the sink....melting the plumbing out from our level down to the next!

 

[peebee]

Do you want me to pap on some more about this? ...

Actually; credit where it's due:

Not so long ago I used to make the same error, until I was made to stand in the corner by @Argentum (aka Silver_Is_Money; author of "Mash Made Easy" the - free - brewing water calculator).

 

[The-Engineer-That-Brews]

Thanks all - and an especially big round of applause to @peebee for not mentioning pycnometers (yet)
Well as you might imagine, the reasons for all this shenanigans was to calibrate my digital hydrometer based on the buoyancy of a submerged float (I'm going to call it the 'Dunk').
Anyhow, I found a much easier way to calibrate it just based on taking readings at a couple of different SGs measured using the normal glass hydrometer.

... and the sneak preview is that the Dunk appears to work very well indeed. I'm currently working on the bubble-dislodging mechanism.

 

[Buffers brewery]

the bubble-dislodging mechanism.

You know what I’m going to say.......

 

[peebee]

... especially big round of applause to @peebee for not mentioning pycnometers (yet) ...

Did I? ... Oh aye; well I'll put that right:

Knowing all this stuff (even the density of water at 20°C is off top of me head) is what I had to learn to make sense of a Pyknometer ( ). They might be accurate, but for now they do come with a bit of a disadvantage (i.e. space in head that could be containing something more useful).

 

[Applesnmore]

You measured your 100ml of water with 10 squirts from a 10ml syringe? What on earth makes you think the 10ml syringe is remotely accurate?

And repeated accurate, without splashback. In my student days we were taught to put solution into a burette and read from that point rather than fill to 100ml or what ever.

Secondly scales need to be level. The load cells are either one centrally or in an X formation and perform a calculation to determine the mass. These should be level, commercial balances having a little spirit level incorporated into them. Equally some balances have a range of accuracies depending on band being weighed. Then there is vibration. Our lab balances were on a concrete plinth, with anti vibration material cemented into it. Certainty enters a rabbit hole with this.

My point being that really for most of us it is about tolerable error (error we can live with) and repeatability.