Frequently Asked Questions - Heavy Duty Checkweigher FAQs

Not enough test weight on scale.
The heavy-duty checkweigher is typically provided with a 500 Lbs. load cell. The average weigh station conveyor weighs approximately 250 Lbs., using up ½ of the load cell’s capacity (this is called dead-load). A test weight of at least 5% (25 Lbs.) should be used to calibrate the scale. A weight less than this will not create enough of a difference (span) between no test weight and the applied test weight for the controller to recognize as a valid weight. Keep in mind that the controller’s calibrated operating range is 2 times the size of the test weight. If the unit is calibrated with 25 Lbs., the controller can only weigh up to a 50 Lbs. product weight, and then will display an “Over Scale” error.

Too much test weight on scale.
The standard heavy-duty checkweigher has a full-load capacity of approximately 150 Lbs. If the weigh station were calibrated with 100 Lbs. of test weight, the low-end weights (partially filled or bags broken open) weighing less than 5 Lbs. may not be detected by the checkweigher. Calibration test weight size is typically 50 Lbs., which yields a maximum weighment size of 110 Lbs., and can easily detect weights down to about 2 Lbs.

Load cell damage in scale base.
It is possible for a load cell to receive two types of damage; mechanical overload and shock overload. Mechanical overload, such as applying 200 Lbs. to a 100 Lbs. scale, simply distorts the elements within the load cell to a point where they do not return to their previous state, and calibration cannot be done reliably or at all. Shock overload, such as dropping a 50 Lbs. bag onto the same scale. The 50 Lbs. bag itself is not enough to overload the scale, but the acceleration force of the dropped bag is. Calibration with a damaged load cell will present drift, erratic weights, and non-repeatable calibration checks. Load cell replacement is recommended.

Flexure damage within scale base.
The scale base, below the weigh station conveyor, consists of two parallel sets of flexure plates suspending the “live” portion of the scale from the “fixed” portion of the scale, and a load cell. The flexure plates are precisely positioned to provide very linear and repeatable weights over the range of the weigh station conveyor. If a flexure plate becomes bent or knocked out of alignment, the scale will loose its ability to function properly. Usually a visual inspection by a scale technician can find this type of problem. Replacement flexure plates are available from Thompson Scale’s stock.

– Not enough test weight on scale. The heavy-duty checkweigher is typically provided with a 500 Lbs. load cell. The average weigh station conveyor weighs approximately 250 Lbs., using up ½ of the load cell’s capacity (this is called dead-load). A test weight of at least 5% (25 Lbs.) should be used to calibrate the scale. A weight less than this will not create enough of a difference (span) between no test weight and the applied test weight for the controller to recognize as a valid weight. Keep in mind that the controller’s calibrated operating range is 2 times the size of the test weight. If the unit is calibrated with 25 Lbs., the controller can only weigh up to a 50 Lbs. product weight, and then will display an “Over Scale” error.

– Confirm power supply is working properly. All DC voltages are generated by the 472, and are used to power the 474 Load Cell Digitizer card. If any LED on the front face plate of the 472 Power Supply Card is not lit, remove power from the controller, then remove the 472 card and replace the failed fuse with one of the on-board spares.

– Check for 10 VDC output to load cell. Using a digital electrical meter set to VDC, and with the scale controller on, place the leads on the 474 Load Cell Digitizer card terminals 1 and 5. The output should be 10 VDC, plus or minus less than 1%. If the voltage is lower or higher than this value, it may indicate a defective Load Cell Digitizer card or Power Supply card.

– Check Load Cell. Using a digital electrical meter set to mV (millivolts), and with the scale controller on, place the leads on the 474 Load Cell Digitizer card terminals 3 and 4. A load cell rated at 2mV/V (total output of 20mV since we excite the load cell with 10 VDC) should have a reading 2 to 12 mV with nothing on the scale (except the weigh station conveyor). The mV reading should increase by about 2 to 4 mV when the test weight is applied.

A load cell rated at 3mV/V (total output of 30mV since we excite the load cell with 10 VDC) should have a reading of 3 to 20 mV with nothing on the scale (except the weigh station conveyor). The mV reading should increase by about 3 to 5 mV when the test weight is applied.

Filter Frequency in the configuration menu has been set to zero (0). Enter the configuration menu, as described in the manual, and confirm this setting. Normally it should be set to 5.

Number of system photo-eyes set incorrectly. Enter the configuration menu, select item #8 (Get Next Menu), then select “P) Photo-Eye Configuration”. This setting should be equal to the actual number of photo-eyes connected to inputs within the controller. If there are no photo-eyes associated with your checkweigher, this value should be zero (0). If there are no photo-eyes, but this value is set to 1 or 2, the checkweigher program is awaiting an input from the photo-eye to trigger termination of the weighment, and will never display a weight value.

Weigh station installation is critical to proper and repeatable weights. The product entering and leaving the weigh station should be as smooth as possible. In-feed and discharge conveyors surrounding the weigh station should be at the same level and speed. Gap between conveyors should be kept to a minimum, since too large a gap will allow the product to “nose dive” into the head roller of the weigh station.

Support structure for the weigh station must be isolated and stable. A structure tied to in-feed or out-feed/reject conveyors will induce low-frequency vibrations into the weigh station scale, affecting weight accuracies.

Conveyor belt tension is very important. Running the belt too tight will cause excessive loading on the weigh station rollers and excessive friction of the belt against the slider bed. The belt should be loose enough to lift at the center between the head and tail rolls, and you should be able to slide your hand between the belt and slider bed, touching the V-guide on the underside of the belt. Another way to adjust this tension is to place your heaviest package on the weigh station conveyor and then start the conveyor. The belt needs to be only tight enough to start without slippage of the belt.

In a perfect world, the weigh station conveyor and scale would be totally isolated from all surrounding structures and electrical connections. Unfortunately this isn’t possible, and electrical connections for the weigh station conveyor motor and any accessory photo-eyes or other devices must be made. These connections MUST have enough slack (we usually recommend a 6″ service loop) between the connection point on the conveyor and the fixed structure near the conveyor. A motor connection made with flexible conduit (Seal-tite) or S.O. cord that is tightly tied down will restrict the movement of the conveyor, and will adversely affect weight.

The weigh station bearings must be properly maintained to provide accurate and repeatable weights. If the bearings are worn or have partially failed, the drag and mechanical “noise” generated by the bearing will affect weights. Between the head and tail rolls on the weigh station, and located on the underside of the conveyor, is an idler roll. This roll holds the belt up and away from the weigh station scale lid. If the bearings fail in this roll, it will cause drag and mechanical “noise”. Check the roll to make sure it spins freely.

The head and tail rolls on the weigh station are precision balanced prior to installation. This balance allows the rolls to turn at any speed without generating low-frequency vibrations (kind of like an out-of-balance tire on your car). If the shaft of the roller becomes bent, due to mechanical impact or over-tightened belt, the roller will generate low-frequency “noise” and affect the accuracy of the scale.

The weigh station conveyor length and speed have been specifically configured for your application. These two factors, along with your product length, provide Thompson Scale with enough information to formulate the highest possible production rates at the right speed, and provide enough weighment time for each package. If however the conveyor speed is changed or a new longer package is introduced into production, sufficient weighment time may not be possible. Contact Thompson Scale with information on maximum product length, conveyor length, and conveyor speed. We will quickly calculate minimum weighment times, and make recommendations on possible solutions to your application problem.

Along with the minimum weigh time, described above, it is also very important that only one product is on the scale at one time. For systems without an exit-end photo-eye (called our Early Weight Terminate photo-eye option), the leading edge of the package must be about 1/4 of its length off the weigh station before we terminate the weighment cycle on that package (20″ long package would have to be 5″ onto the down-stream conveyor) If the weighment cycle of the first package is not completed prior to entry of another package, weight from the second package will influence the final weight of the first. Make sure that packages are properly spaced.

Scale damage will also affect weight. The scale provided below the weigh station conveyor should provide reliable, accurate, and linear weights across the weigh station conveyor. Place the controller into the Calibration Check mode, and then place a test weight at the center of the weigh station. Record the displayed weight, then move the test weight to each corner of the weigh station. The values in each corner should be +/- 2 increments of the center weight value. If one or more corners are out of tolerance, then the scale base may require service. Contact Thompson Scale to discuss your readings and possible solutions.

The checkweigher controller monitors the load cell output as its only source of weight data. If excessive vibration causes high weight readings to occur while the checkweigher is trying to take a weighment, over-weight values will be generated. It is possible to enter a TARE or container value in the product setup for each product you run. Make sure that the TARE on the static scale is the same as that of the product setup.

Inaccurate calibration or loss of valid calibration. The checkweigher controller uses values defined in calibration to determine product weights crossing the scale. If this calibration was performed improperly, such as using a 2 kg. weight but entering a 5 Lbs. test weight value in the controller, the package weights would not be calculated properly. If a significant change occurs to the scale’s load cell, such as overload damage, hitting the weigh station conveyor with a forklift, removing the weigh station conveyor and then replacing, interference from motor or photo-eye connections, it will affect calibration.

Operating Zero can be affected by removing built-up product on the weigh station conveyor and scale. The operating zero is maintained by an automatic zero tracking program within the checkweigher’s program. However, this feature can only zero out a maximum of ½% of the load cell capacity at one time. If more than ½% is added or removed at one time, it will be necessary to enter into the Calibration Check screen and manually zero the scale. If this is not done, it may take several package weights to fully re-zero the scale.

Differences in the static scale and the checkweigher’s scale. It is quite possible that the static scale and checkweigher just differ in what is displayed on a package weight. This difference is almost always a small amount, such as 1 or 2 increments different. The static scale may not have the same accuracy or resolution as the checkweigher. If this is the case, it is dangerous to try and compare an accurate scale with one having less accuracy. The static scale should have at least the same accuracy and display resolution as the checkweigher, and best case, should be about 2 times as accurate as the checkweigher.

Weighment cycle too short. A package must be on the checkweigher for a minimum period of time to generate an accurate and repeatable weight. Run the package across the scale 10 times, writing down each weight. Are the weights the same, +/-0.5%? If not, most likely there is not enough time for the checkweigher to properly weigh the package. Contact Thompson Scale for further assistance.

It is possible to enter a TARE or container value in the product setup for each product you run. Make sure that the TARE on the static scale is the same as that of the product setup.

Inaccurate calibration or loss of valid calibration. The checkweigher controller uses values defined in calibration to determine product weights crossing the scale. If this calibration was performed improperly, such as using a 2 kg. weight but entering a 5 lb. test weight value in the controller, the package weights will not be calculated properly. If a significant change occurs to the scale’s load cell, such as overload damage, impacting the scale conveyor, removing the conveyor and then replacing incorrectly, interference from motor or photo-eye connections, it will affect calibration.

Operating Zero can be affected by removing built-up product on the scale conveyor and scale. The operating zero is maintained by an automatic zero tracking program within the checkweigher’s program. However, this feature can only zero out a maximum of ½% of the load cell capacity at one time. If more than ½% is added or removed at one time, it will be necessary to enter into the Calibration Check screen and manually zero the scale. If this is not done, it may take several package weights to fully re-zero the scale.

Differences in the static scale and the checkweigher’s scale. It is quite possible that the static scale and checkweigher just differ in what is displayed on a package weight. This difference is almost always a small amount, such as 1 or 2 increments different. The static scale may not have the same accuracy or resolution as the checkweigher. If this is the case, it is dangerous to try and compare an accurate scale with one having less accuracy. The static scale should have at least the same accuracy and display resolution as the checkweigher, and best case, should be about 2 times as accurate and the checkweigher.

This LED is provided as a “heartbeat” indicator. As long as the LED blinks on and off at a steady pace, the controller’s CPU is operating properly. If this LED is continuously ON or OFF, consult the factory for more in-depth trouble shooting assistance.

If an updated program EPROM was installed into your controller, it is possible that a feature has been moved or modified as part of the updated program. Contact us for more information on your specific program.

Password is required, but either don’t know it or don’t have one. Press front panel ESC key until display shows Main Menu. Select item 5 (Select/Edit Product), then 3 (Edit Password). When asked for password, type in the word “scale”. The next prompt will be for entry of a new password. If desired, enter a new password. To disable password protection, press the Space Bar once, then press Enter.

The keyboard provided on the 4693 controller is read by the #476 card located at the bottom inside of the controller door. In the bottom left corner of the 476 card are two (2) red LED’s. One is dedicated to the processor chip on this card, and blinks on/off like a heartbeat to show proper operation of the processor. The other LED provides a visual indication that a key press on the keyboard is properly read. Press the suspect key(s) while watching this LED. If the LED does not blink each time the key is pressed, the key has most likely failed. If the LED does blink, but the controller does not properly respond, the 476 card may be defective. Contact Thompson Scale for further assistance and to purchase replacement parts required.