The basic belt weigher formula in metric units is flow rate (t/h) = material loading (kg/m) × belt speed (m/s) × 3.6. It applies to weightometers and conveyor belt scales measuring material moving with the belt. The loading must represent net material, after the empty-belt reference and the installation’s weighing geometry have been accounted for.
Why is the conversion factor 3.6?
Kilograms per metre multiplied by metres per second gives kilograms per second. There are 3,600 seconds in an hour and 1,000 kilograms in a metric tonne, so 3,600 ÷ 1,000 = 3.6. This conversion is for kg/m, m/s and metric t/h; other unit combinations need different factors.
Worked example: calculate tonnes per hour
- Assumed net material loading: 45 kg/m.
- Assumed belt speed: 3.0 m/s.
- 45 × 3.0 = 135 kg/s.
- 135 × 3.6 = 486 t/h.
At a constant 486 t/h for 30 minutes, the total would be 486 × 30 ÷ 60 = 243 tonnes. If the rate changes, add the mass contributed over each time interval or use the totaliser readings. An instantaneous rate multiplied by the whole shift duration is not reliable when feed varies.
How do you calculate kg/m from tonnes per hour?
Material loading (kg/m) = flow rate (t/h) ÷ [3.6 × belt speed (m/s)]. The belt speed must be greater than zero. For example, 486 ÷ (3.6 × 3.0) = 45 kg/m.
The integrator derives its loading value from the load cell signal and the installed configuration. It is not simply gross weight on the frame divided by its physical length. See how a belt weigher works.
What is the effective weigh length?
Effective weigh length describes how a weighing arrangement relates the sensed load to belt loading per metre. It depends on idler spacing, the number of weigh idlers and the frame’s load path. Use the value defined for the installation. Do not substitute overall frame dimensions or a single idler spacing without checking the design.
For an assumed direct-loading arrangement, 54 kg of net equivalent material load over a 1.2 m effective weigh length gives 54 ÷ 1.2 = 45 kg/m. This is an explanatory example, not a universal formula for converting raw load cell readings on levered or inclined installations.
How do speed pulses become metres per second?
Belt speed (m/s) = pulse frequency (Hz) ÷ pulses per metre. For a wheel directly following the belt without slip, pulses per metre can be calculated as pulses per revolution ÷ wheel circumference in metres. Gearing and effective rolling diameter must be included where relevant.
Example only: a 300 mm effective-diameter wheel with 100 pulses per revolution has a circumference of π × 0.300 = approximately 0.94248 m. Its pulse constant is about 106.10 pulses/m. At 320 Hz, the calculated speed is approximately 3.016 m/s.
Actual arrangements may use a pulley sensor, encoder, measured travel calibration or configured fixed speed. Check the fitted system rather than entering the example values. See the belt weigher speed sensor guide.
How is a test-load target calculated?
Where the approved test method provides a known equivalent loading in kg/m, the expected indicated test mass is equivalent loading (kg/m) × test travel (m) ÷ 1,000. Test travel may be belt length multiplied by complete revolutions, where that is the instrument’s specified method.
Assume an approved arrangement gives 45 kg/m equivalent test loading and the belt travels 360 m during the test. The target is 45 × 360 ÷ 1,000 = 16.2 tonnes. That is an indicated test quantity; it does not mean 16.2 tonnes of material were physically conveyed during a static-weight test.
Do not assume a 90 kg weight on a 2 m frame always represents 45 kg/m. Loading points, leverage, inclination and any established reference correction must be handled by the applicable procedure. Matching a routine reference does not prove performance under every material condition. See belt weigher calibration methods.
How do input errors affect the result?
With the loading input otherwise correct, a speed reading 1% low makes the calculated rate 1% low: 486 × 0.99 = 481.14 t/h. If both inputs have errors, they combine multiplicatively. Two positive 1% errors give 1.01 × 1.01 − 1 = 2.01%, not exactly 2%.
An added 1 kg/m loading offset at 3 m/s contributes 1 × 3 × 3.6 = 10.8 t/h. Its relative effect is greater when actual loading is low. Filtering, cut-offs and run logic can also affect the displayed or accumulated result, so a hand calculation is a check of the inputs and units, not a complete calibration.
Frequently asked questions
Can I calculate tonnes per hour without a belt weigher?
You need a credible material loading and belt speed. Multiplying estimates produces an estimated rate; it does not establish measurement accuracy.
Does the formula use tonnes or tons?
The factor 3.6 above gives metric tonnes per hour. One tonne is 1,000 kg, while one US short ton is approximately 907.185 kg — about 92.8 kg (9.28%) less. For the same material flow, 1,000 metric tonnes per hour is approximately 1,102.31 US short tons per hour. The higher number reflects different units, not extra material. Check the integrator and reporting units before comparing figures. See NIST’s unit conversion tables.
Can this formula tell me which calibration setting to change?
No. It helps check relationships and units. Diagnose the cause and follow the model-specific procedure before changing calibration.
Need a manual for your equipment?
Talk to us about manuals for your equipment. Send the make, model, serial number and software version (a photo of the label is fine) and we’ll help you find the right document.
For unexpected readings or a calibration enquiry, contact Accurate Industries or see weightometer calibration and belt weigher servicing.