What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Bathroom scales

scrollF4

scrollF4

Moderator, Asst. Line Boy
Ambassador
There are no such things as stupid questions, right? OK, here are mine...

I met a gent who is retiring as an A&P. He's selling off his stuff, including five 300-lb bathroom scales. He said he used them to weight small aircraft for weight and balance purposes. He specialized in EAB aircraft. For the mains, he said he'd set two scales beside each other, set a board across the scales, and set the main tire on the board, add up the two weights. These are definitely NOT certified scales.

My questions: Are bathroom scales commonly used for aircraft weight and balance? Mel, is this acceptable?
 
scrollF4 said:
There are no such things as stupid questions, right? OK, here are mine...
I met a gent who is retiring as an A&P. He's selling off his stuff, including five 300-lb bathroom scales. He said he used them to weight small aircraft for weight and balance purposes. He specialized in EAB aircraft. For the mains, he said he'd set two scales beside each other, set a board across the scales, and set the main tire on the board, add up the two weights. These are definitely NOT certified scales.
My questions: Are bathroom scales commonly used for aircraft weight and balance? Mel, is this acceptable?
Click to expand...


It depends on several things. If the scales are the "strain gauge" type and can be calibrated, then yes, it is perfectly acceptable.

Our EAA Chapter did this for years, very successfully I might add. It is a bit of a hassle, but it does work. We had a piece of railroad iron
that we had calibrated and used it to periodically check and calibrate the scales.
 
Last edited:
Close enough?

horseshoes-and-handgrenades.jpg


Using scales of unknown accuracy will yield the same results in your W&B numbers (when is the last time those scales were calibrated?).

It's not up to Mel to determine if the accuracy of the scales you used was acceptable, I think thats on you.
 
Calibration checks would determine if they are accurate and repeatable. Often bathroom scales are weak on repeatability, especially if not loaded evenly.
 
Well, good morning sunsine!

Walt said:
Using scales of unknown accuracy will yield the same results in your W&B numbers (when is the last time those scales were calibrated?).

It's not up to Mel to determine if the accuracy of the scales you used was acceptable, I think thats on you.
Click to expand...

I like Mel's response better! :p
 
Last edited:
If they are the mechanical spring type with the whirling dial - no.

If they are strain gauge type that can be calibrated, as others have said, you can use them with careful use and proper placement of the load.

Weighing yourself at the doctor's office, then going home and jumping on your scale is a poor calibration. Using bags or buckets of materials from Home Depot to calibrate is also a poor calibration. They vary.

An elevator company that I used to work for used large containers that could be filled reproducibly with water. If you weigh a suitable, water filled, container, on calibrated scales, you might be OK. Make sure that it can be filled reproducibly. It makes a suitable, easy to store, calibration weight if the temperature remains constant and there isn't anything dissolved in the water. You'll need a large container! Not saying that this is the easiest way, just an alternate idea to heavy, bulky items. It's easy to fill and drain the tank when needed.
 
vfrazier said:
If they are the mechanical spring type with the whirling dial - no.

If they are strain gauge type that can be calibrated, as others have said, you can use them with careful use and proper placement of the load.
Click to expand...

EXACTLY. Verifying the accuracy of scales is not very difficult. Get a weight of known quantity (no less than 10% of maximum capacity) and weigh it on a calibrated scale. Then use this known weight to verify the accuracy of your scale to within the percentage of maximum capacity.

Example for a 300lb .1% scale.

Make a 30lb weight and find the actual weight. I used Quickrete Concrete Mix, added water to harden it within the bucket it comes in. After about 1week of drying it came out to 32.5lbs on a calibrated scale. I then put it on the scale I want to verify and it should display 32.5lbs give or take .3lbs or between 32.2 and 32.8lbs. Once a year I weigh the bucket to verify the weight.

BTW: This is how scales are "calibrated" in a lab.

:cool:
 
Last edited:
GalinHdz said:
BTW: This is how scales are "calibrated" in a lab.

:cool:
Click to expand...

The calibration company that does my scales provides a sheet with 4 or 5 cal points from min to max, not sure how one point could confirm accuracy for the entire range.
Thats like saying an altimeter that reads correctly on the ground must be good at 20K feet too.
 
Last edited:
GalinHdz said:
EXACTLY. Verifying the accuracy of scales is not very difficult. Get a weight of known quantity (no less than 10% of maximum capacity) and weigh it on a calibrated scale. Then use this known weight to verify the accuracy of your scale to within the percentage of maximum capacity.

Example for a 300lb .1% scale.

Make a 30lb weight and find the actual weight. I used Quickrete Concrete Mix, added water to harden it within the bucket it comes in. After about 1week of drying it came out to 32.5lbs on a calibrated scale. I then put it on the scale I want to verify and it should display 32.5lbs give or take .3lbs or between 32.2 and 32.8lbs. Once a year I weigh the bucket to verify the weight.

BTW: This is how scales are "calibrated" in a lab.

:cool:
Click to expand...
Definitely want to check more than one point. The 10% Full Scale will work for the lower check, but you need to check at or near max value also as a minimum for cal. As Walt said, typically 4 points along the span are verified during cal checks to define the error curve. If you are trying to cut corners, at least do a point very near the weight you expect you will be weighing.
 
Walt said:
The calibration company that does my scales provides a sheet with 4 or 5 cal points from min to max, not sure how one point could confirm accuracy for the entire range.
Click to expand...

Good to see they do that but you only need to verify accuracy to within parameters and accuracy parameters are at max scale. Anything else is optional unless your scales parameters say X% accurate at "X" reading(s). At .1% full scale a 1,500lb scale is accurate to only 1.5lbs. So a 30lb weight can read from 28.5 - 31.5lbs and still be "accurate" while a 300lb scale will read from 29.7 - 30.3lbs.

Walt said:
Thats like saying an altimeter that reads correctly on the ground must be good at 20K feet too.
Click to expand...

Altimeters have accuracy parameters at different altitudes (X% accurate at "X" readings) that must be verified. Apples and Oranges.
:cool:
 
Last edited:
Ok, to wrap up all the above in a nutshell: there's enough higher math energy going into ensuring the scales' accuracy that I don't trust the simple spring-driven Walmart scale approach. Thanks for the wisdom, Gang.
 
I gave away my stack of bathroom scales when I moved out of Houston - I quit using them long before that, since I had a neighbor there with a nice set of car scales from Summit Racing. I just looked them up, and now you can get a four-pad set of WIRELESS (ooh, shiny new stuff....) scales for $730 (http://www.summitracing.com/parts/pro-67650). All it takes is for a couple of guys on the field to chip in together, or the local EAA chapter - or go see if Randy has a set.

Yup - the days of bathroom scales is long over for me....
 
Cost to weigh your plane?

So, if you don't want to spring (pun?) for a set of racing scales, or buy at least five bathroom scales, and your EAA chapter is unwilling to go co-op, what should one expect to pay to have an FBO weigh your plane on calibrated scales?
 
There is another problem with electronic bathroom scales. Some seem to have a memory built in to them to look more repeatable and take away variations/oscillations in the last 1/10 of a pound.

Try this test at home. Weigh yourself, note the number, and then weigh yourself again holding a one pound weight or so.

A lot of scales will simply give you the same number as the first weighing...:)

Don't know how long this "memory" function lasts, but it will screw up your use for aircraft weighing attempts.
 
Ironflight said:
I gave away my stack of bathroom scales when I moved out of Houston - I quit using them long before that, since I had a neighbor there with a nice set of car scales from Summit Racing. I just looked them up, and now you can get a four-pad set of WIRELESS (ooh, shiny new stuff....) scales for $730 (http://www.summitracing.com/parts/pro-67650). All it takes is for a couple of guys on the field to chip in together, or the local EAA chapter - or go see if Randy has a set.

Yup - the days of bathroom scales is long over for me....
Click to expand...

I bought a set of these several years ago, and I am very glad I did. I have weighed my plane 3-4 times (after W&B changes) since I purchased them. They have paid for themselves in these 3-4 uses because they are reliable and (most importantly) available when I need them. It provides a great peace of mind for me.
 
Just as a reference.......

koda2 said:
So, if you don't want to spring (pun?) for a set of racing scales, or buy at least five bathroom scales, and your EAA chapter is unwilling to go co-op, what should one expect to pay to have an FBO weigh your plane on calibrated scales?
Click to expand...

We have a guy on the field at McKinney airport that will travel a reasonable distance and weigh aircraft for EAA members for $150. (Certified scales)
 
koda2 said:
So, if you don't want to spring (pun?) for a set of racing scales, or buy at least five bathroom scales, and your EAA chapter is unwilling to go co-op, what should one expect to pay to have an FBO weigh your plane on calibrated scales?
Click to expand...

I charge $250 to do it, I give a discount if you are there to help and drain all the fuel yourself.
 
Last edited:
Ironflight said:
I gave away my stack of bathroom scales when I moved out of Houston - I quit using them long before that, since I had a neighbor there with a nice set of car scales from Summit Racing. I just looked them up, and now you can get a four-pad set of WIRELESS (ooh, shiny new stuff....) scales for $730 (http://www.summitracing.com/parts/pro-67650). All it takes is for a couple of guys on the field to chip in together, or the local EAA chapter - or go see if Randy has a set.

Yup - the days of bathroom scales is long over for me....
Click to expand...

After some investigation into the racing scales I opted not to go with them. The reason is they can typically only be calibrated at a single point and all other points are based on the linearity of the load cells. The claim is that they "hand pick" the cells based on their linearity so it should never be an issue. When I quizzed the pros at a local scale shop (that is basically all they do) their recomendation was to avoid electronic scales that cannot be calibrated at muliple points because when they go out of tolerance you throw them away. I went with Intercomp aircraft scales.
 
Last edited by a moderator:
we have a certified weight guy

Mel said:
We have a guy on the field at McKinney airport that will travel a reasonable distance and weigh aircraft for EAA members for $150. (Certified scales)
Click to expand...

Our guy has been weighted and certified to 250#. He can travel to your location and stand on the scale to certify the weight as long as doesn't eat too many twinkies that day.
 
You must log in or register to reply here.
Back
Top