Simply stated, a draft weighing system is best defined as a device used to add or disperse, in discrete steps, a measured amount of product to another containment vessel or in-stream receptacle. Each measured amount, either added to or discharged from the primary containment vessel is called a “draft.”
Depending upon the precision required for a specific application, the amount of product needed in each “step” can be measured in volumetric, timed or weight units. For non-weight-based applications, a containment vessel is simply filled to a specific level (or fed for a specified amount of time) and then discharged. For applications demanding greater precision, a myriad of additional strategies are employed to achieve the required weight precision.
Draft weighing systems disguises
Many systems employed in the commercial and industrial marketplace to achieve specific goals (for example, box or bag filling systems) are, in fact, draft weighing systems. These types of systems are simply called upon to accomplish their mission in a single step (for example, fill a small hopper to precisely 50 lb of walnuts and then discharge the product into a waiting sack). Technically, however, only systems that provide a measured amount of product in multiple steps are traditionally referred to as “draft weighing systems.”
A draft weighing system can perform as a gain-in-weight as well as a loss-in-weight device. A gain-in-weight system fills a containment vessel to a precise weight and then performs a discharge. On the other hand, a loss-in-weight system starts with a containment vessel that is at least partially filled and then discharges a measured amount of product with each cycle (step).
Evolution of the draft weighing system
Prior to the development of load cell technology, draft weighing systems were primarily mechanical systems; a containment vessel was filled to a desired level and when that level was achieved a lever was switched that disabled continued vessel feeding and simultaneously opened a discharge gate to release the product into another vessel or directly into a “stream.” With these types of systems, target volume or weight precision was less than perfect and an operator was needed to actually count the number of drafts that may have been needed before stopping continued draft weighments.
Along comes the PLC
The advent of the modern Programmable Logic Controller (PLC) tremendously helped make draft weighing systems more accurate and less labor intensive. Logic controllers enabled the integration of independent components (sensors, circuits to toggle motors, actuators, alarms, etc.) to both monitor and regulate the system in a more tightly controlled manner. Independently attached timing devices
were used to “count” each discharge and then provide an output to the PLC when the target number of draft weighments were completed informing the PLC to terminate continued draft weighments. For example, simple capacitive sensors were employed to detect a specific product level in the main containment vessel. When the capacitive sensor detected the presence of an introduced product and provided a positive signal return, the PLC toggled the feeding element (belt conveyor, vacuum, vibrator, etc.)to an open state and the discharge element (release gate) to a closed state, initiating product discharge. However, weight-based control over the elements via these newly introduced logic controllers was a bit more complicated. Eventually electronic weigh indicators were introduced to the industrial marketplace that provided an attached PLC with an analogue (and later a digital) signal to help in applications where achieving precision weight-based targets were required.
Along comes the digital weigh controller
The perfection of the Wheatstone Bridge, load cell and digital weigh controller with optional input and output connections put the evolution of the draft weighing system into hyper drive. However, the manufacturers of these earliest weigh indicators only provided firmware control to better manage the feed/weigh/discharge functions of the system. Although these instruments are still widely used, they are somewhat limited in scope due to the necessity to “burn” a new EPROM to affect even the most insignificant modification. These earliest systems simply permitted the user to key in a set-point value to better control the in-feed/discharge functions of the system. An integrated cycle counter was an added bonus.
The evolution moved into full gear with the introduction of the programmable weigh controller in the early 1990s. The programmable weight-based controller not only enabled the user/integrator to make rapid modifications to the system when needed but additionally gave the user an opportunity to effectively communicate with a wider in-house network of controllers, personal computers and data collection devices.
Retrofitting legacy systems
The modern programmable controller created a bonanza for the nation’s independent scale dealers and systems integrators. Legacy draft weighing systems were relatively easy to retrofit by simply re-wiring the existing system components (for example the in-feed carrier, high-level sensor and discharge gate). A competent systems integrator could retrofit a legacy system in very short order. With the introduction of weight-based controllers the need to add a weigh indicator and analogue input to the PLC became a moot issue.
Alternative to the traditional truck or rail-car load-out system
Without the use of a truck or rail scale, filling large vehicles to a precise weight would be a difficult task. This is where a draft weighing system really comes in handy. All a user must do is fabricate a weigh hopper to which material can be fed, capture each target weighment and then discharge the product into the waiting vehicle. For example, a vehicle that required a 53,500 lb load could easily be filled using a hopper capable of accurately measuring a 1000 lb target draft weight by filling it with 53 and ½ (+ or -) draft weighments in a short amount of time.
NTEP, Legal-for-Trade, certified draft weighing systems
Legal-for-Trade (L-F-T) certification of a draft weighing system is critical if the system will be used in a commercial environment, i.e. where money is exchanged for goods.
Customized, NTEP Certified, L-F-T draft weighing systems are few and far between. This is due to the fact that submitting a system for NTEP, legal-for-trade certification entails a cost of anywhere from $3000 to $5000 – a cost that’s simply too prohibitive for most scale service providers. Add to this cost the extra fees required for periodic re-certifications and the price tag definitely is out of range for the average scale dealership.
Additionally, NTEP, L-F-T certified systems also require the use of additional peripherals. For example, two printers capable of swapping between each other if either paper has run out on one of them or has gone off-line. If only one printer is used, the system must be prohibited from making continued draft weighments until the printer problem is resolved.
Agriculture and fishing industries: two hot markets
Draft weighing systems, both gain and loss-in-weight varieties, are used extensively in the agricultural marketplace. The fresh nut industry relies heavily on the use of gain-in-weight systems. For example, when a semi-trailer pulls up to an offloading area to discharge a load of almonds freshly harvested from the field, the product must move through several zones (pre-cleaning, shelling and grading) before a determination can be made regarding fair payment to the grower. Processors don’t want to pay for rocks, branches, twigs and dirt that were collected along with the fresh almonds. A Legal-for-Trade, gain-in-weight,draft weighing system is typically used at both the pre-cleaning level (removal of rocks, gravel and larger twigs) and the shelling level (removal of smaller rock particles, shells, dirt and, after being run through a laser sorter, damaged or genetically inferior
Figure A depicts a 3-stage draft weighing system currently in use by a California almond processor. In this application, almond meats are fed to the top-most hopper to a target weight of approximately 200 lb. This 200 lb draft weight is then discharged to the transitional hopper below it. The transitional hopper in turn feeds (by gravity) the feed hopper belo it. The feed hopper’s discharge aperture deposits the almond meats directly on to a vibratory pan which in turn feeds the bucket elevator shown at the right
of the vibratory feeder.
Industrial seed processors rely heavily on non-legal-for-trade,gain-in-weight and loss-in-weight draft weighing systems for seed coating purposes. For example, a seed processor may want to apply 2.75 lb of a liquid insect repellent chemical to every 300 lb of corn seed to help ward off insect damage after the seed is planted. In order to do this with any precision, a draft of 300 lb (+/-) must be delivered to a hopper and then discharged to a centrifuge. With the centrifuge rapidly spinning, a loss-in-weight application (residing in the same controller) gradually releases precisely the amount of repellent needed to coat the amount of seed that was measured in the gain-in-weight cycle.
The commercial fishing industry is another heavy user of gain-in-weight draft weighing systems. Without their use, harbor buyers of fresh squid and shrimp, for example, would be forced to empty the hull of a ship with buckets, pour their contents into smaller containers and then weigh each container in order to accurately determine the amount of product extracted from the vessel. An ideal solution for this application is a Legal-For-Trade, gain-in-weight, draft weighing system coupled with a strong vacuum top pneumatically extracting product from the ship’s hull.
Draft weighing systems of many different varieties are found throughout the industrial and commercial marketplaces. Many of them are, in fact, legacy systems in need of a major upgrade. This is great news for the independent scale dealer and systems integrator as well as the end-user. An upgrade to a weight-based, programmable controller is really all the end-user really needs to extend the life of their legacy systems for many years into the future and at the same time gain enormous benefits from their information gathering and reporting capabilities.
About the author
Al Blazo is an independent consultant with twenty years of experience in the weighing, measurement and process control industry. Al specializes in applications development for the GSE® line of process control instrumentation. He can be reached at firstname.lastname@example.org.