Conveyor Scale, Checkweigher, Monorail, and Static Scale Accuracy

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Oct 22, 2013

So you have a checkweigher, conveyor scale, monorail, deck scale, or bench scale, but is your scale accurate statically?

Lets first define accurate, since we are talking about a scale here. We’ll use the definition that “NIST” gives us. (NIST is the National Institute for Standards and Technology). This is the Government agency that makes the rules for legal for trade scales).

conveyor scale

NIST defines accurate as: A piece of equipment is  “accurate” when its performance or value-that is, its indications, its deliveries, its recorded representations, or its capacity or actual value, etc., as determined by tests made with suitable standards-conforms to the standard within the acceptable tolerances and other performance requirements.Equipment that fails to conform is “inaccurate.”

To simplify this, a scale need only be within tolerance to be considered accurate,therefore your scale can be off a small amount and still be considered accurate.

in motion checkweigher

How do you determine the tolerance for your scale so you can determine if your scale is accurate?

Table 7a below describes the different classes of scales. Legal for Trade scales will have the class marked on a tag somewhere on the scale. This is important because there are different tolerances for each class of scale.

Table 7a

Typical Class or Type of Device for Weighing Operations

        Class

Weighing Applications or Scale Type

I

II

III

          IIIL

IIII

Precision laboratory weighing
Laboratory weighing, precious metals and gem weighing, grain test scales
All commercial weighing not otherwise specified, grain test scales, retail precious metals and semi-precious gem weighing, animal scales, postal scales, scales used to determine laundry charges, and vehicle onboard weighing systems
Vehicle, axle load, livestock, railway track scales, crane, hopper (other than grain hopper) scales, and vehicle onboard weighing systems
Wheel-load weighers and portable axle-load weighers used for used for highway weight enforcement
NOTE: A scale with a higher accuracy class than that specified as “typical” may be used.(Amended 1985, 1986, 1987, 1988, 1992,  and 1995)

Table 6 below describes the tolerances that a scale is allowed. We are going to look at a class III scale, the 1 , 2 , 3 , and 5 that you see are the number of divisions that a scale can be off and still be within tolerance or “Accurate”.

Looking across the chart for a class III scale we see that between 0 – 500 divisions a scale can be off by one division, and that between 501 – 2,001 divisions you are allowed two divisions.

Table 6

Maintenance Tolerance

(All values in this table are in scale divisions)

Tolerance in scale divisions

1

2

3

5

Class

Test Load

IIIIIIIIIIIIIL

0 – 50,000

0 – 5,000

0 – 500

0 – 50

50,001 – 200,000

5,001 – 20,000

501 – 2,000

51 – 200

200,001+20,001+2,001-        4,000201-               400 4,001+401+
Add 1d for each additional 500d or fraction thereof

What is division?

Let’s use a 5,000 pound deck scale for an example.

Most 5,000 lb deck scales will weigh in 1 lb increments, a division is essentially 1 increment.

In other words a division is the number that the scale counts by.

On a 5000 lb deck scale, from 0 to 500 divisions (0 lb to 500 lb) you can place a 250 lb test weight on the scale and have the scale read anywhere from 249 lbs to 251 lbs and still be accurate.

If you test with a 1000 lb weight, then you’re to the 501 – 2,000 column. Now you can be off by 2 divisions and still be accurate (998 lbs – 1002 lbs).

Now, use a bench scale for an example.

The bench scale has a capacity of 100 lbs and weighs in increments of .02 lbs.

The division size is now .02

To determine the weight ranges to use for the table:

.02 x 500 = 10 lbs  Between 0 lbs and 10 lbs the scale is allowed 1 division of error.

.02 x 2000 = 40 lbs Between 10.02 lbs and 40 lbs the scale is allowed 2 divisions of error.

There are other factors that need to be considered when determining if the scale is accurate:

  • Does it return to zero?
  • Does the scale repeat?
  • How do the corners test?

The purpose of this article, however, is to show that scales can have a small amount of error in them and still be accurate.

Table 8

Recommended Minimum Load

Class

Value of scale division

(d or e*)

Recommended minimum test load

(d or e*)

I

II

III

IIIL

IIII

Equal to or greater than 0.001 g

0.001 g to 0.05 g, inclusive

Equal to or greater than 0.1 g

All**

All

All

100

20

50

20

50

10

*For class I and II devices equipped with auxiliary reading means (i.e., a rider, a vernier, or a least significant decimal differentiated by size, shape or color), the value of the verification scale division ”e” is the value of the scale division immediately preceding the auxiliary means. For Class III and IIII devices the value of “e” is specified by the manufacturer as marked on the device; “e” must be less than or equal to “d”** A minimum load of 10 d is recommended for a weight classifier marked in accordance with a statement identifying its use for special applications.(Amended 1990)

A scale is suitable for its intended use if the minimum load which will be weighed on it is 20 measurement divisions or more.

A scale whose smallest division is 0.05 lb is suitable for minimum loads of 1 lb. or more.

conveyor-scales

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