Truck Scale / Weighbridge Project Guide - Edition 3

Truck scales or weighbridges are industrial scales that are capable of weighing trucks of all sizes. The most common use of a truck scale is to determine the weight of bulk goods being bought and sold in truckload-sized quantities. In those cases, information from the scale is a crucial part of the business transaction. The scale functions much like a cash register. Enforcement agencies use truck scales to check for a truck's compliance with road-going vehicle weight limits. They are also used to monitor intake and output volumes at facilities, such as solid-waste processing and recycling centers, construction sites and more.

In applications in which you need to weigh the entire truck, your truck scale must be long enough to hold all of the wheels of the longest truck you plan to weigh. That usually means 18 to 24 meters or 60 to 80 feet long for tractor-semitrailers, and up to 30 meters or 100 feet for double trailers. The overall maximum length of over-the-road tractor trailers is typically regulated by regional/state/provincial authorities.

The sensory devices used to measure the weight on the scale. Load cells are the heart of any truck scale. They are the sensors that measure the weight of objects on the scale deck. Most truck scales require 6 to 12 load cells. They must work together flawlessly to provide accurate weight readings.

Five main components must work together for a truck scale to function properly. 1. Foundation: permanent installations use a concrete foundation. 2. Weighbridge, also known as the scale deck: this is the structure that creates the driving surface for the trucks. The weighbridge is typically composed of modular sections that are placed together to span the desired length. 3. Load cells: these are the sensors that measure the weight on the scale. In most modern scales the weighbridge is supported by the load cells themselves. 4. Terminal: sometimes referred to as an indicator, the terminal is the control panel for the scale. It displays the weight value to the operator, and often serves as the connection point for other scale peripherals. 5. Cables: the signal from the load cells must be transmitted to the terminal. In most cases this is done with cables.

Steel and concrete decks should provide equal weighing performance because both are built to the same design specifications. However, there are some differences that could make one deck type more advantageous for your application. Steel-deck truck scales have a quick installation time of one day and they are lighter weight. Concrete deck scales have a 28-day installation time, as the concrete must cure for this allotted time. They are also much heavier. Concrete-deck scales have better traction than steel decks, which can be slippery when wet. Additionally, concrete scales are better suited for point loading, as it is not recommended on a steel deck. Maintenance, service life and total project cost for both are about the same. Business owners will have to determine what is best for their application and needs.

Junction boxes (also called "J-boxes") are used with analog load cells, as well as most digital load cells. Junction boxes combine the signals of multiple load cells and then transmit the summed signal. A typical truck scale using analog load cells will have 2 to 4 junction boxes.

A pit scale, as the name implies, is built over an excavation, and the surface of the weighing platform is flush with the surrounding ground. The depth of the pit is subjective, but can affect the ease of service from any maintenance crew. Pitless, or open-sided scales, are built up from a grade and have a profile about 28 to 51 cenimeters or 11 to 20 inches. They may have one or both sides open. While the choice between a pit or pitless installation is often based on preference, there are a few instances that may require the use of pit—such as if there is not enough room for an approach, if there is a height restriction, or if safety regulations require it as there is no chance a truck can fall off the sides. The choice will have to be specific to the company and installation requirement.

Replacing a truck scale is an expensive solution that might not be necessary if your load cells are failing. When a scale, especially a relatively new one, experiences frequent downtime and repairs, the problem can often be traced to some part of the load-cell system (load cells, cables, connectors or junction boxes). Upgrading these components with a POWERCELL^® PDX^® load-cell system is a cost effective way to turn a troublesome scale into one that performs better than new. METTLER TOLEDO is able to easily retro fit even competitor scales with new load cell technology.

The accuracy of a scale easily can be taken for granted. But, if you buy or sell goods with the use of a truck scale, accuracy is critical to your transactions. The accuracy of the scale can influence profit margins, inventory levels and quality management. The size of the legal accuracy tolerance for your scale depends on your location and whether your region recognizes NIST/NTEP, OIML or other standards. The tolerance is a function of the scale's capacity being utilized and the size of the scale's weighing increment. For example, according to NTEP regulations, a truck scale with 200,000-pound capacity weighing 70,000- to 80,000-pound trucks can have 160 pounds of error (positive or negative). While that is the approved level of accuracy, METTLER TOLEDO continues to offer superior accuracy with POWERCELL PDX.

Most truck-scale providers offer weighing systems with numerous grounding features. A single-point grounding system with surge protectors can be the first line of defense for the scale and scale house. These passive systems offer some level of protection. Active systems are able to re-route the current of a lightning strike away from major components. POWERCELL PDX load cells are equipped with a proactive lightning system called StrikeShield. These load cells actively divert a lightning strike away from the terminal and have been tested up to 80,000 amperes in a laboratory to prove their effectiveness.

Most scale owners expect a scale to last 10 to 20 years, but lifespan depends on several factors. First, selecting the appropriate scale option and highest quality manufacturer will set your scale up for either a long life or a short one full of regret. There is a large variance in manufacturing quality that should be considered when shopping for a truck scale. Environmental factors and traffic volume also play a part. A scale that weighs 200 trucks per day will wear faster than a scale weighing only 20 trucks per day.  METTLER TOLEDO has scales in the field still operating that are 35 years old. Many competitors have only been manufacturing for ten years. Make sure you consider proven performance when shopping for a truck scale.

Most legal-for-trade applications require the scale to gain certification of compliance from local weights and measures authorities at certain intervals (annually, semi-annually, etc.). Another factor that should be considered when determining how often to calibrate is how many trucks per day will be weighed on the scale. The demands of heavy traffic volumes often cause the scale's accuracy to drift outside of tolerance levels faster; putting the business at risk of losing additional profits. If your vehicle scale experiences high volumes of trucks per day, you should consider increasing the frequency of calibration. And finally, environmental factors will affect the calibration of a truck scale. As temperatures fluctuate, the large-scale structure will expand and contract. These subtle changes will adversely impact the calibration of the scale. A few of the most advanced scales, such as POWERCELL PDX scales, can protect and even compensate for these effects, but most do not.