KEY POINT

GAMMA

ELECTRO-MECHANICAL

1.

Installation

Bolt onto conveyor frame between convenient idlers.

Requires special provision for mounting, special idlers, leveling and straight conveyors.

2.

Belt length

Unimportant

Very short or very long belts may cause calibration problems, due to tension effects.

3.

High rise, vertically or horizontally curved conveyors.

Unimportant, but belt should not physically lift off idlers at point of installation.

Causes calibration problems. Scale should be on flat conveyor with no curves.

4.

Mis-alignment idlers and conveyor stability

Unimportant

Important; essential idlers be aligned, and frame be rigid on a sound foundation.

5.

Belt tension

Unimportant. Even belts with changing direction are suitable.

Important. Changing tension cause calibration problems with short weigh frames.

6.

Cleanliness, corrosive atmosphere or degree of spillage.

Important. Must be cleaned off with air blast or water.

Affects calibration. Spillage causes wear or binding.

7.

Overloading

Physically unimportant to scale components, but causes large errors for duration.

Can destroy scale calibration on short frame models or break the loadcell.

8.

Belt shape - or use of skirt boards.

Can be any shape or idler angle. Belt shape or skirt boards do not affect operation unless coupled with overloads.

Flat bottomed belt with maximum of 35 degree idler trough necessary for short weigh frames. 45 degree maximum for six idler systems. Skirt boards cannot be used near weigh frame.

9.

Required space

About 30cm of belt length between any convenient idlers.

Requires several meters of belt length and some degree of straight run prior to and after the scale.

10.

Windy conditions

Unimportant

Can contribute errors of significance if not protected.

11.

Belt speed

Very important. Speed compensation is necessary if speed varies.

Very important. Speed compensation is necessary if speed varies.

12.

Accuracy

Approximately 1 to 2% for approved installations.

Approximately 0,2% to 1% for approved installations.

13.

Initial calibration

Calibrated against product initially. Typical static loading of 1 meter belt section is adequate for many installations.

Same technique used but requires more products on longer belt section. Also use auxiliary devices such as calibration chains, assized weights and static loads.

14.

Subsequent calibration

Each month, zero checked with belt empty. Calibration also checked by built-in absorber. Time: 10 to 15 minutes.

Monthly zero check required. Place weights on scale, or use calibration chain on belt itself. Takes approximately 10 to 30 minutes.

15.

Calibration maintenance

Maintenance has serious effect on accuracy. Complete rechecks can be done in a short time.

Maintenance has serious effect. Idler alignment, dirt, belt drift and tension affects calibration of short weigh frames. Long weigh frames require less maintenance.

16.

Ease of repair

Gauge is non-contacting. No moving parts to wear out. Electronic parts etc. located on replaceable circuit boards, good for long life.

Mechanical parts require shut down of belt. Recalibration probably also required. Electronic parts etc. located on replaceable circuit boards, good for long life.

17.

Slump angle changes

Sharp slump angle changes due to moisture change or different particle size will introduce errors from 0,5 % to 14 %.

Not an important consideration.

18.

Particle size effects

Can be calibrated for one material only. Nature of product to remain constant. Changes will cause errors.

Large particles can cause errors in short weigh scales. Has no effect on long weigh frames.

19.

Moisture changes

If slump angle does not change with added or lessened moisture, there will be no serious effect on results.

Not an important consideration unless flooding causes spillage.

20.

Chemical composition changes

Can be a cause of errors. Material changes should not happen.

Not an important consideration. Multiple materials can be accommodated.

21.

Extra light loads

Low belt loading should be avoided.

Recommended that low belt loading be avoided.

22.

Maintenance costs

No moving parts to replace. Maintenance probably less than 30 minutes per month. Leak testing, once in two years, requires 10 minutes.

Maintenance cost is high for short frame scales and low for long weigh frame scales.

23.

Personnel requirements

Plant safety man serves as radiation safety man. Radiation safety training is usually provided by the supplier. Source register to be kept and Authority to Hold and Use is required.

Not a consideration.

24.

Summary

Instrument price is usually greater. Simpler installation, operation and maintenance will largely offset this. Perfectly safe and trouble free on approved applications. Lower accuracy device, which can be affected by material changes. Not certifiable.

Instrument price is less for short weigh frame scales, higher for long weigh frame units. Maintenance costs are higher. Much higher accuracy's achievable. Long weigh frame scales are certifiable.