Sherline Tongue Weight Scale question

[dnhx]

I weighed my TV and TT at the CAT scale and the tongue weight came out to 620 lbs. I have a Sherline tongue weight scale, and weighing right at the trailer ball coupler, it only showed 560 lbs. I would think the scale should be more accurate, so wondering what I did wrong.
The trailer was level and we did not move anything around in the trailer between the time it was weighed on the CAT scale and the time I weighed with the tongue weight scale. All the tanks were empty both times.
All I can think is maybe I read the scale too soon after raising the tongue jack.

thanks
Don

[ktmrfs]

atreis wrote:

RCMAN46 wrote:

atreis wrote:
(It's accurate, but not necessarily precise.)

???????? What a contradictory statement.

Accuracy and precision are not the same thing. Accuracy is how close the reading is to the "true" value, as defined by the central bodies that define and standardize those true values. Precision is how many decimal points the measure can be made at WHILE STILL BEING ACCURATE. (Overly precise is when more digits of precision are provided than can actually be measured accurately.)

As an example, the Cat Scale is certified to be accurate, and the actual value printed out on the sheet will lead you to believe that it's also precise, but it isn't. It might have given you a reading of, say, 4,578 lbs, when it is really only accurate to, say, 4,600 lbs. The rest is the computer taking the reading from the sensors and reporting it, but those sensors can't read accurately at that precision. (They ought to be rounding at whatever point their precision ends.)

add to that repeatability. If I measure something 10 times, what variation in measurement do I get?

[ktmrfs]

atreis wrote:

RCMAN46 wrote:

atreis wrote:
(It's accurate, but not necessarily precise.)

???????? What a contradictory statement.

Accuracy and precision are not the same thing. Accuracy is how close the reading is to the "true" value, as defined by the central bodies that define and standardize those true values. Precision is how many decimal points the measure can be made at WHILE STILL BEING ACCURATE. (Overly precise is when more digits of precision are provided than can actually be measured accurately.)

As an example, the Cat Scale is certified to be accurate, and the actual value printed out on the sheet will lead you to believe that it's also precise, but it isn't. It might have given you a reading of, say, 4,578 lbs, when it is really only accurate to, say, 4,600 lbs. The rest is the computer taking the reading from the sensors and reporting it, but those sensors can't read accurately at that precision. (They ought to be rounding at whatever point their precision ends.)

The sherline scale basically measures hydraulic pressure 1psi/lb load, and the scales are std pressure gauges with a dial showing weight rather than pressure readout. They just replace PSI with Lbs.

they say the gauge they use has a typical accuracy of about 2% at the center of the scale, and degrades at either end.

Add to that the limited resolution of the small dial,
and then add to the error from the tolerance of the cylinder bore, and any side sticktion.

So, pick a number. 5%??? or so.

I at one time weighed my tongue using the sherline scale then took my trailer to the local highway weigh station. In oregon they leave them open for use when closed. Unhooked the trailer tongue over the scale. Weigh station weight readout is in 50 lb increments. My tongue weight is around 1400lbs. depending on how much water and other supplies are in the trailer. Sherline and weigh station were within 75lbs of each other.

To me that says the sherline is good enough for my intended purpose. Give me a good idea of what my tongue weight is.


Sherline quotes

" The supplied gauge was chosen as the most cost effective solution to providing a result that is sufficiently accurate for trailer weighing purposes. (The Sherline scale is not intended to be used for trade; i.e., items priced by weight.)"

[RCMAN46]

atreis wrote:

RCMAN46 wrote:

atreis wrote:
(It's accurate, but not necessarily precise.)

???????? What a contradictory statement.

(They ought to be rounding at whatever point their precision ends.)

If you have ever looked at a CAT scale weight sheet you will discover that is exactly what they do.

Here is the specs on a CAT scale:
Division: 20 pounds Accuracy: HB 44, Class III L

I would like to see documentation of the accuracy and or precision of a Sherline scale.

[atreis]

RCMAN46 wrote:

atreis wrote:
(It's accurate, but not necessarily precise.)

???????? What a contradictory statement.

Accuracy and precision are not the same thing. Accuracy is how close the reading is to the "true" value, as defined by the central bodies that define and standardize those true values. Precision is how many decimal points the measure can be made at WHILE STILL BEING ACCURATE. (Overly precise is when more digits of precision are provided than can actually be measured accurately.)

As an example, the Cat Scale is certified to be accurate, and the actual value printed out on the sheet will lead you to believe that it's also precise, but it isn't. It might have given you a reading of, say, 4,578 lbs, when it is really only accurate to, say, 4,600 lbs. The rest is the computer taking the reading from the sensors and reporting it, but those sensors can't read accurately at that precision. (They ought to be rounding at whatever point their precision ends.)

[Ron_Gratz]

dnhx wrote:
The 620 came from subtracting the weight of truck with weight with the truck hooked to the trailer.

When you weighed the truck by itself, did you have the WDH mounted in the receiver with the WD bars in the WDH (or in the back of the truck)?

If not, the calculated "tongue weight" will be too high by an amount approximately equal to the weight of the WDH and bars.

Ron

[RCMAN46]

atreis wrote:
(It's accurate, but not necessarily precise.)

???????? What a contradictory statement.

The issue with the Sherline was getting different readings with several tries.

I will bet you can weight on the CAT scale several times and the weights will not be more than 40 lbs different.

[atreis]

It's hard to say - CAT scale is certified, but is not accurate to such small weight differences. It's designed and certified to weigh things that weigh tens of thousands of pounds. (It's accurate, but not necessarily precise.)

Sherline scale is designed to weigh things at about the right range of weight for a tongue, but isn't certified. (It's precise, but not necessarily accurate.)

I'd be inclined to go with the Sherline weight, but you might want to get a third reading. (Another CAT Scale, another Sherline...)

[J-mans_Dad]

Were you sittin in the TV when you weighted at the CAT scale?

[dnhx]

The 620 came from subtracting the weight of truck with weight with the truck hooked to the trailer.
I'll retry doing several readings and let the trailer settle on the scale more. I asked Sherline and they suggest I "nudge" the scale a bit, because as others of said, how the weight is sitting on the scale has a lot to do with the accuracy. Ill also make sure I have the blocks the scale is sitting on perfectly level and stable and see what I get.

[ktmrfs]

lbrjet wrote:
The Sherline is finicky as Barney suggested. Do several readings. The slightest bit of angle can throw off the reading.

yup a good tool but not a certified scale. gives you good enough data.

[lbrjet]

The Sherline is finicky as Barney suggested. Do several readings. The slightest bit of angle can throw off the reading.

[BarneyS]

Often stiction because of slight sideways pressure via placement of the tongue on the Sherline will cause erroneous readings. I would take several readings, moving the scale ever so slightly between them, and then average the readings. The height of the tongue will make the reading vary also. Make sure you measure at the same height each time - preferably when the trailer is as level as you can get it.
Barney

[fj12ryder]

How did you determine the tongue weight on the CAT scale? Direct weighing or subtracting weight of truck with weight with the truck hooked to the trailer?

I would rely on the Shurline scale over the CAT scale for that small of a reading.