Checking bathroom scales

[Harry Bloomfield]

I/we've had this pair of digital scales for something like 15 years. When the opportunity has presented itself, I have checked them against surgery scales, whilst wearing the same clothes, on returning home, and they have always agreed, until recently. During my most recent check, with hospital scales, and a pharmacies scales, despite them claiming to be calibrated, they disagreed to 2kg between them, whilst wearing the very same, and up to 3kg out by my scales at home. It leaves me wondering which are correct, and just how to check mine?

The ones at home, are electronic, digital display, showing to 0.1Kg. You give them a kick, to wake them, they show 'HELLO', then self zero to 0.0Kg. I assume using strain gauges. I cannot see anyway to calibrate them. Others I have checked them against, have just been mechanical analogue.

 

[Ivor Windybottom]

Why worry? 2kg isn't a huge percentage. You can put that on by having a hefty meal with a pint or two.

 

[Ivor Windybottom]

Did you eat or use the toilet between the two?

 

[Harry Bloomfield]

Why worry?

I check my weight, once per week, the hospital ask for it each month, for my CKD appointment. So I like it to be accurate.

Did you eat or use the toilet between the two?

No.

 

[charliegolf]

I/we've had this pair of digital scales for something like 15 years. When the opportunity has presented itself, I have checked them against surgery scales, whilst wearing the same clothes, on returning home, and they have always agreed, until recently. During my most recent check, with hospital scales, and a pharmacies scales, despite them claiming to be calibrated, they disagreed to 2kg between them, whilst wearing the very same, and up to 3kg out by my scales at home. It leaves me wondering which are correct, and just how to check mine?

The ones at home, are electronic, digital display, showing to 0.1Kg. You give them a kick, to wake them, they show 'HELLO', then self zero to 0.0Kg. I assume using strain gauges. I cannot see anyway to calibrate them. Others I have checked them against, have just been mechanical analogue.

Check it with a 10 or 20 kg weight. Extrapolate from there.

 

[Ivor Windybottom]

Ours can give different readings depending on the slight uneveness of the floor tiles it happens to be on. Try moving it around and weighing yourself 3 times. You'll probably find out it varies. Similarly if you subtly change how you're standing, moving your weight from the front to back of your feet. They're all just a rough guideline, nobody cares about 2kg.

 

[Rodders53]

Bathroom scales: Salter give no useful specs that I can find, even for their "Medical" mechanical scales.

BHF a set by Omron https://shop.bhf.org.uk/omron-body-composition-digital-scales quotes 40.0 kg to 150.0 kg: ± 1% so for a 75kg person +/-0.75 kg (75 kg is roughly 12 stones). Two machines side by side at extremes of accuracy can therefore be 1.5kg different with the same nominal weight on them (albeit unlikely in practice).

1% accuracy is probably as good as one gets? {Although another scale on BHF claims +/- 0.5 kg}.
Then you have repeatability: each time you weigh, it may be slightly different.

A litre of water is a pint and three-quarters. One litre of water weighs 1 kg.

Using the same scales should ensure the number is as repeatable as possible. The hospital scales will be regularly checked with a test weight or two - stage weights or similar - and as long as you are weighed on the same scales every time then the numbers would be 'sufficiently accurate' to spot a trend.

I'm sure the various kidney charities can offer advice / reassurance that the precise weight differences are nothing to worry about if you ask them.

 

[Ivor Windybottom]

I once sold two digital thermometers to a customer. They were all of about £8 each. He returned them, stating that when placed side-by-side they were reading 0.1 degrees C different. Pretty unreasonable expectations! Besides which, the air really could be 0.1 degrees different 6 inches away, it's a negligible difference.

Anything that measures anything is only estimating. 2% isn't too bad when you're dealing with weight, when placed on a floor that is very likely to be uneven (e.g. tiles) and/or flexible (e.g. carpet or vinyl).

 

[Harry Bloomfield]

A litre of water is a pint and three-quarters. One litre of water weighs 1 kg.

That sounds like a good method to check them. I have a suitable barrel, just a matter of measuring water into it.

 

[fillyboy]

My wife once went on one of those 'speak your weight machines' and the machine said 'one at a time please'.

 

[pete01]

I weigh 16 stone with my glasses on.

I have no idea how much I weigh with my glasses off.

 

[Wayners]

Just stand on scales then look at weight.
Now hold a bag of sugar and weigh again.
Difference should be the weight of sugar

 

[Brigadier]

I once sold two digital thermometers to a customer. They were all of about £8 each. He returned them, stating that when placed side-by-side they were reading 0.1 degrees C different. Pretty unreasonable expectations! Besides which, the air really could be 0.1 degrees different 6 inches away, it's a negligible difference.

Anything that measures anything is only estimating. 2% isn't too bad when you're dealing with weight, when placed on a floor that is very likely to be uneven (e.g. tiles) and/or flexible (e.g. carpet or vinyl).

A bloke who worked in our company did a calibration of a rotameter, for which he had to record the temperature at the calibration site.

He documented the temperature at the calibration site (his office) as 5 degrees C.


When I asked him if he'd had the heating off, he seemed befuddled.

What did you use, to measure the temperature?, I asked him.






"The BBC Weather app!"




And he had a masters degree in a science(ish) subject........

 

[Brigadier]

I weigh 16 stone with my glasses on.

I have no idea how much I weigh with my glasses off.

On the bright side, you're not overweight.

Just under tall

 

[JohnD]

If you are "watching your weight" to see if its going up or down, your scales do not need to be very accurste, as long as they are fairly consistent.


If, in January, your daily weight averaged 20 stone, and in April, it averaged 18 stone, you have lost a useful amount. It does not matter if the numbers on your scales said 20 stone 2, and 18 stone 2.