Weight of a 21

[Brian B-P]

Originally Posted by Floating Cloud

To state what apparently is not the obvious, incorrect use of the scale previously leaving weight on it instead of following the directions to remove all weight after each weighing, may have affected it.

Perhaps the technical description is not clear to some. The 5 runs which Tom performed each include completely removing the force. So five times he ran it up to an indicated 300 pounds after completely removing the force; on average, the actual force at that time was 305 pounds (not bad) but the deviation (I don't know if this is the standard deviation of the distribution or the spread between the extremes of the five values) was 30 pounds - the reading is not reliably repeatable. Even if you ignore the rest of the data, those five readings were immediately following complete removal of the force. The consistency of the other results with the 300 pound results confirm for me that the test method is sound.

Again, in my opinion if this device is destroyed by a few seconds of leaving a load on it within its working range, it's not worth much. The basic principle of operation is perfectly sound; I suspect they need a better seal.

Originally Posted by Floating Cloud

We have the 1,000 and it works just fine.

How would you know? When you put the tongue down on it and is says - for instance - 500 pounds, how do you know it is not really 450 pounds or 550 pounds on the scale? Just for laughs, what would happen if you weighed your tongue weight five times in a row, faithfully following the instructions, with absolutely no change in the trailer between measurements. That would be a good test of repeatability, and I would appreciate seeing the results.

Tom does know what his is doing.

[TAfraser]

I spent the last 26 years working and running a calibration lab. We worked a wide variety of equipment, including Force, Dimensional, Electronic, and Production. Anything that had to do with the production, testing, and release of Boeing aircraft was certified in one of our labs. My field was in Electronics with a focus on Ultrasonics. I wrote the majority of our processes and procedures covering all fields that we were responsible for. In my career, I've calibrated and certified hundreds, if not thousands of vacuum and pressure gauges. I've also calibrated and certified vacuum and pressure systems in the field, some with pressures over 20,000 psi.

When I provided the first set of tests and measurements to Sherline, they didn't question their validity, (testing methods, or the outcomes) they just sent a new gauge. Brian clued me in to some finer points to testing on the trailer, thank you Brian. I'll send them this next set of tests, the setup, equipment used, and the outcome, and see what they have to say. I'd like to know if I overlooked something in my testing. I've been wrong in the past!

[cpaharley2008]

I've been wrong in the past!
I hope you were not on the Boeing plane I'm on……

[TAfraser]

I can't bring myself to rely on a gauge that I don't have confidence in, especially when it has to do with the safety of my family and others on the highways. With many of us using tow vehicles near their maximum intended towing capacities, accurate gauging, or measurements, is very important. I'll send Sherline this next set of tests, the setup, equipment used, and the outcome, and see what they have to say. Hopefully I've overlooked something again and this is actually a good gauge that we can all rely on. If not, we will all benefit from the knowledge. If you all would like me to stop this quest, I'll continue off line. Just let me know, in private please.

[gbaglo]

Please keep the thread online. Whatever the outcome, we will learn something.

[Brian B-P]

My personal opinion is that none of needs to support our posts with our qualifications or experience; the posted material can stand on its own. Tom's work with the Sherline gauge is obviously valuable; it would be regardless of who did it.

Everyone has their own priorities, preferences, and standards. Personally, I'm glad Tom's standards are so high and I appreciate that he is willing to do the work required to back them up. I have no issue with different standards, and I do understand that the accuracy (well, repeatability is really the issue) of the Sherline scale, and for most trailer owners' purposes it is a useful and valuable piece of equipment. It's even more useful if its limitations are understood.

Of course, now (or at least in the spring) I should really calibrate my home-made scale ... my guess is that it will have even more friction issues than the Sherline, although it has still proven useful.

I picked up a 200 kg (440 lb) capacity electronic scale recently, which doesn't have the annoying behaviour of the typical bathroom scale, as well as the higher capacity, and convenient remote (corded) display. I have been hoping to get good information on the mass of individual trailer components, as well as a more convenient alternate tongue weight measurement tool. I tried it out a couple weeks ago, and found it had surprisingly poor repeatability on light loads. Life is full of disappointments, but maybe this scale will be fine in its mid-range.


This all really has little to do with the weight of a 21' Escape, although it goes a long way to explaining why we have difficulty accurately reporting the tongue weight of any trailer.

[Brian B-P]

Originally Posted by TAfraser

If you all would like me to stop this quest, I'll continue off line.

Stop - No! But you knew that...

Originally Posted by gbaglo

Please keep the thread online. Whatever the outcome, we will learn something.

Absolutely. I'm looking forward to hearing about Sherline's response: they do seem to genuinely interested in providing a good product.

Perhaps this whole tongue weight measurement issue should be split off to its own thread? I'm fine with either way. That would be from about post #27, but #33, #37, #38 and #40 are still on the original topic.

[Dave Walter]

I work in a field of engineering where we do quite a lot of fairly sophisticated soils and rock testing using compression testing machines with load cells for accurately measuring loads put on samples during testing. One of the effects that impacts our testing and which is likely a significant factor in the repeat ability of the Sherline gage, is friction and alignment. In soils and rock testing, we try to minimize friction between the sample and the loading device (the plattens). For the Sherline gage, it seems to me that small changes in alignment between the trailer coupler and your gage could have significant impacts relative to how well the load is transferred from the coupler to the gage. Also, reducing friction between the coupler and the gage may help to improve the transfer of loads between these devices. Alignment and friction could likely be controlled in a testing lab to a much greater extent than would be the case in actual practice, so my expectation is that field repeat ability of readings would be poorer in practice than in the lab.

[float5]

ice-breaker, all of your points are excellent and make good sense. Part of what you are saying is that the piston needs to be perpendicular. I imagine with the shape of the coupler that there is often a mistake made with regard to that and there could be such a mistake even if the coupler were flat.

Taking the weight off between weighings seems also applicable to your points on friction. Yes, the lab is different and in real life, many mistakes can be made that would not apply to the lab.

Usually a company knows its product much better than the end user. There is no indication yet that these tongue weights have been done following directions.

People who have Sherlines no doubt usually want to know first, their total loaded weight, so that they know what tongue weight to expect. That is one way they check that the Sherline is working, by matching weights with truck scales. Our weights match pretty closely. I also can't imagine any Sherline owner not doing repeat weighings. Nobody spends the money on the scale and then does one tongue weight.

[Brian B-P]

Originally Posted by Floating Cloud

Yes, the lab is different and in real life, many mistakes can be made that would not apply to the lab.

Yes, I agree, the results will be even more inconsistent in real life, which is what Tom was seeing.

Originally Posted by Floating Cloud

Usually a company knows its product much better than the end user.

A reasonable assumption, but this is a pretty simple device, built by a company that realized that its precision machining capabilities could produce another useful product; they're not into pressure or force measurement in general. They know it's not a precision instrument, and I doubt they worry much about measurement technique, judging from what they publish; they just don't want the seal to fail and leak.

Originally Posted by Floating Cloud

People who have Sherlines no doubt usually want to know first, their total loaded weight, so that they know what tongue weight to expect. That is one way they check that the Sherline is working, by matching weights with truck scales. Our weights match pretty closely.

That's quite an assumption. Many rigs are limited by tongue weight and not overall weight, so an owner might only care about measuring the tongue weight. But I agree, most Sherline owners seem likely to visit scales and weigh the whole rig... like everyone in this disucssion.

Truck scales typically have 20-pound or 10-kilogram reading increments, so any number from them is plus or minus ten pounds in the ideal case. They are not perfect mechanisms, so they have repeatability issues from both their internal imperfections and the way the vehicle sits on the scale; it's not at all unusual to go back over the scale and get a different number (I've done it with a van pulling a couple different trailers, a motorhome, and two or three cars). To get a tongue weight, if you don't unhitch on the scale (with the trailer axles off the scale) and put a stand directly under the coupler, you need to combine two or more readings to get the tongue weight - combining readings adds their errors. The result is that the tongue weight value from a truck scale is quite approximate, especially for those on the lighter end.

Matching pretty closely to a number which can easily be out by 20 pounds gives only a rough indication that the tongue weight scale is working, not its accuracy or its repeatability.

Originally Posted by Floating Cloud

I also can't imagine any Sherline owner not doing repeat weighings. Nobody spends the money on the scale and then does one tongue weight.

I can imagine that. In fact, in years of participating in these forums, I've never noticed anyone reporting a set of measurements - they announce a single number. No one seems to say "I weighed the tongue five times, and got reading from 300 to 320 pounds, so I'm going with 310 plus or minus 10". This is what I was saying about different people having different standards and expectations - most will get one number, some will do repeat checks... some fire up a metrology lab.

Of course few are likely to buy a device like this to use it once and then put it away forever. To be useful to understand the scale performance, repeat weighings means of the same load, not just weighing the trailer before each trip.

So, Floating Cloud, you do make repeat measurements without changing anything. How much variation have you found?

[float5]

Truck scales being off 20 lbs. or getting somewhat different weights is no problem. A tongue weight of 10-15% is needed. On a load of 3500 lbs, for instance, that leaves plenty of leeway. No problem whatsoever in seeing that you have proper tongue weight.

Why don't we give repeat weighings? Because we get exactly the same thing over and over. The scale works great. If we ever get a difference, it's a very large one where we have obviously done something wrong, which is the case here as far as I see. We then look at it and figure out the mistake and do it again a few times. At this point though, we expect to get it right the first time.

[stoney]

Morning all on the way home from Chilliwack we stopped on a scale to check our weights and found the following
-truck alone 4960lb
-truck hooked up 5290lb
-trailer hooked up 3170lb
-hitch weight would then be 330lb
These measurements are with a pretty empty trailer a porta potti , suitcase , n some snacks and a flat of water , we use a tacoma double cab 4x4 with timbren overloaders and no wdh hitch hope this helps , cheers mike

[cpaharley2008]

Those numbers are real close to the factory spec's, Stoney. Thanks.

[paulw]

Originally Posted by stoney

Morning all on the way home from Chilliwack we stopped on a scale to check our weights and found the following
-truck alone 4960lb
-truck hooked up 5290lb
-trailer hooked up 3170lb
-hitch weight would then be 330lb
These measurements are with a pretty empty trailer a porta potti , suitcase , n some snacks and a flat of water , we use a tacoma double cab 4x4 with timbren overloaders and no wdh hitch hope this helps , cheers mike

It does! Thanks Mike...

[Brian B-P]

Originally Posted by Floating Cloud

Truck scales being off 20 lbs. or getting somewhat different weights is no problem. A tongue weight of 10-15% is needed. On a load of 3500 lbs, for instance, that leaves plenty of leeway. No problem whatsoever in seeing that you have proper tongue weight.

I agree that truck scales are close enough if you only want to ensure about 10-15% tongue weight.

My comments about truck scales were unrelated to that: my point was that they are not precise or accurate enough to be of any value in assessing the performance of a tongue weight scale such as the Sherline; maybe I wasn't clear when I said this "Matching pretty closely to a number which can easily be out by 20 pounds gives only a rough indication that the tongue weight scale is working, not its accuracy or its repeatability."

Originally Posted by Floating Cloud

Why don't we give repeat weighings? Because we get exactly the same thing over and over. The scale works great. If we ever get a difference, it's a very large one where we have obviously done something wrong...

Excellent - reading the same to the pound every time; that's exceptional for any scale. I don't think anyone else experiences that performance with any scale, but I'm glad this one is perfect.

[TAfraser]

I’ve been testing the Sherline in a lab atmosphere, (controlled environment with known uncertainties) with some interesting results. I shared the results with sherline and here is their first answer:

Tom,
I forwarded your e-mail to our General Manger and she sent it on to Ashcroft, our gauge supplier, for their response. Ashcroft specifications for the gauge are 2% at mid-range and 3% at the extremes. This means it could be as much as 30 pounds off at 100 and 900 pounds and be within the acceptable range. They do make a 1% gauge (2% at extremes) but that one is quite a bit more expensive and we never considered the small difference significant when it comes to determining safe trailer loading.

Generally, a scale like this is used to determine gross misloading or exceeding a certain maximum, so a difference of 20 or even 60 or 80 pounds is not usually critical to overall trailer performance or safety, although it is certainly desireable to be as accurate as possible.

I will let you know what the gauge people say, as that is really the only "calibrateable" part of the system. The piston surface is 1 square inch exactly. The only variables other than that are piston alignment, friction with the O-ring and the cylinder face and potential for air in the system.
In any case, if the scale will not be accurate enough for your purpose, we will be glad to refund your purchase price.
Sincerely Yours,
Craig Libuse

As you can see, they’re concerned about their product, although I don’t understand being 60 or 80 lb off not being a problem, and are willing to back it up with a refund.

I then sent them the following set of data: (had to abbreviate because it didn't post well)
All deviations from the nominal were positive, all were at the edge of, but in tolerance, and the deviation from min to max of the 5 readings were in tolerance.

The offsets I'll be applying to this gauge are as follows: 300lb -20, 400lb -20, 500lb -20, 600lb -25, 700lb -25, 800lb -25, and 900lb -30.

From Sherline:
Tom,
Very thorough testing. The Ashcroft rep did call back and noted as you did that the gauge did register within tollerance limits. He did offer the 1% gauge for our use, but it would cost us 4x as much, and we don't feel these differences are significant as far as whether the readings are safe for towing. An estimating scale's job is to descriminate loads that are dangerous vs. ones that are safe. We've tried to combine a sturdy scale with a gauge that gives you enough information to be safe out on the highway. I see that Northern now offers an 1100 lb electronic scale for $99 that reads in tenths of a pound. The advantage of using the hydraulic method is that we can go up to 5000 lb readings by just changing gauges.
Stay safe.
Craig

I think I have enough data on this particular scale to comfortably apply known offsets (noted above). The measurement environment in the field is sure to contribute more to inaccuracies then the scale itself. So that’s all I have on this subject. Hope this helps in some way?

Tom

[gbaglo]

Good info. So I looked up the Northern scale. Only one I could find is a "hanging" scale, which wouldn't be very convenient.
I did change the gauge on my Sherline from 2,000 lb to 1,000 lb capacity to keep my target weight ( 320 lbs. ) closer to the middle.

[dbailey]

So I hear that as: not as accurate as we'd like, but maybe good enough...

Good enough for me, I think.

Tom: thanks for the thorough work.

[gbaglo]

"The offsets I'll be applying to this gauge are as follows: 300lb -20, 400lb -20, 500lb -20, 600lb -25, 700lb -25, 800lb -25, and 900lb -30."

Tom,
Can you clarify a couple things? I think much earlier in this thread you said you had a 1,000 lb. gauge. Is that correct?
You say 300lb - 20. Does that mean the actual weight is 280? or 320?

baglo

[jamman]

Originally Posted by gbaglo

Good info. So I looked up the Northern scale. Only one I could find is a "hanging" scale, which wouldn't be very convenient.
I did change the gauge on my Sherline from 2,000 lb to 1,000 lb capacity to keep my target weight ( 320 lbs. ) closer to the middle.

Agreed - I don't have a sky hook handy. But here is a link.
Roughneck Digital Hanging Scale — 1,100-Lb. Capacity | Suspended Scales| Northern Tool + Equipment