Tank Scales – Left Coast Scales

Booting Your Hopper

I was recently called out to a customers because their hopper scale kept filling after the filling auger was turned off. I performed a thorough inspection and couldn’t find anything mechanically wrong with the scale. I had the customer run through the batching process so that I could observe the problem from the customer’s perspective. I watched the indicator slowly fill the hopper and after the filling equipment turned off, the scale slowly and erratically added an additional 200 to 300 pounds to the batch. I then went out and watched the batching equipment fill the hopper. As I watched, I saw the boot on the left slowly start to suck in until it was displacing only half the volume it had when the batch started. Somehow or another the boot was being put under vacuum and the boot was pulling on the scale. When the batch was done the boot slowly relaxed and returned to normal. With the customer’s consent I put a pipe into the top of the boot to relieve the vacuum. We ran another batch and the problem went away. I told the customer to figure out where the vacuum was coming from and fix it, but the problem was fixed for now. Rubber boots can cause some weird and perplexing problems.

If you have a tank or hopper that is being filled with powder or something similar, you have seen boots. Boots can be made of many different materials from cotton fabric to silicone rubber. They often are the culprit in weighing issues on hoppers and tanks that had been working fine and all of a sudden aren’t. They are used when you have materials that need to be transferred into a hopper or tank but the material will cause a lot of dust or want to flow out of the filling aperture. Since hard piping to a scale is a no-no, boots were created to allow the scale to move freely but still confine the material being transferred.

All of the issues that will occur with boots come down to some kind of bind. Here are some of the problems that you will see:
1. Material builds up inside of the boot and bridges across the gap between the fill pipe and the pipe on the hopper.
2. The boot is installed without any give, or too tightly.
3. Material works its way between the receiving pipe and the boot.
4. The material sticks to the boot and hardens.
5. The boot stretches like a rubber band as the hopper fills and moves.
These issues all prevent the hopper from moving freely. Without free movement the scale will often not zero correctly or weigh non-linearly.

These problems can be very difficult to find and diagnose. A full inspection on tanks and hoppers is not complete without examining the boots and piping. When problems are found it is always best to have your customer clean, repair, or replace suspect piping and/or boots to prevent problems from developing.

Examples of Impacted Boots on Various Hopper Scales

A Sticky Situation

I overheard one of my customers, a large manufacturer of corrugated boxes, complaining about a spill on the production floor. The spill cost them 4 hours of production down time because of the clean up. I later asked my contact about the problem and was told that their glue dispensing system had overflowed again. They use a liquid glue similar to Elmers Wood Glue to hold the boxes together. The glue is kept in a bulk storage tank and dispensed to each box forming line. Each line has a surge tank to keep a local supply on hand for immediate use. They had attempted to use many different methods to maintain a set level in their surge tanks, from floats to ultrasonic level sensors. Everything they tried failed. The glue would always end up coating the sensor and the tank would stop working or over flow. Even with regular maintenance these systems had an unacceptably high failure rate, with up to one spill a quarter.

I offered to help them with their overflow problem. The solution needed to be out of the box, literally and figuratively, since having the sensors inside the container meant they were susceptible to contamination. The surge tanks are made out of polypropylene and have flat bottoms. The first suggestion was to put the tanks on floor scales, but due to the overflow problems they have had they wanted the load cells to be protected. I designed a table for the floor scale that raised it 2 feet off the floor, then put a skirted cover over the deck that hangs down below the cells and feet. So, in the unlikely event that a spill occurs the load cells and feet are protected.

I chose the Rinstrum R420 for this application. The R420-K401-A is the base unit in its line. As a base unit it is still very powerful, with up to 32 digital I/O, and a set-point engine to run a process. After discussing the application with the customer we defined the following specifications:\

An alarm needs to go off if the weight exceeds 80% of the capacity of the tank
Another alarm needs to go off if the tanks weight goes below 25% of capacity.
The tank level needs to be kept between 45% and 65% of capacity so that the glue is always above the heating coils in the tank.
If the tank level is above 35%, an agitator needs to be turned on.

The R420 was set up with three free running set-points. Set-point 1 was set up for the high level alarm, and would activate if the gross weight went over the programmed level. Set-point 2 was set up as the low level alarm and activated under the programmed weight. Set-point 3 was used to turn on the agitator whenever the level was above the weight. Set-point 4 and set-point 5 turned the filling pump on and off to keep the tank at the optimal level.

The customer was delighted with the new system. They had been struggling with this problem for a long time, and no-one had ever considered using a scale to solve the problem. As scale salesmen and service technicians we think in terms of scales for every application. To our customers, scales are a very small portion of their lives, and as such may not be considered for simple applications such as this one. By keeping my eyes and ears open while visiting the customer I was able to find a great sale, and able to provide a simple solution that had been eluding my customer for years.

The Load Cell Went Out and Blew Up Your Indicator

I picked up a customer recently who was tired of the high cost of repairs with their old scale company. It seemed that whenever one of their scales would go down it would have multiple components fail. The load cell, a j-box and the indicator would all go at once. I have only had multi component failures in very few instances and only in severe environmental conditions, like high pressure wash down and immersion or where the scale was abused. I find that when a technician has found multiple failures it is more likely that he is not sure where the problem lies and is throwing everything at it until the problem is fixed. He arrives on site and the scale is drifting. Aha! He thinks it has a bad load cell. He replaces the load cell and it is still drifting. Ok, well it must be the indicator. Now the indicator gets replaced, oh no, the scale is still drifting. Well, it must be the j-card; ah shoot the home run cable is smashed next to the j-box. Well, he got the ok to replace the j-box, might as well replace it as well as put in a new cable. Then he can look good by telling the customer that he’s not going to charge them for the cable. This seems to be a fairly common scenario, because I have heard it many times. I don’t think it is dishonesty that makes it happen; just the technician’s desire to look good in front of his customer and not understanding proper troubleshooting practices. When proper troubleshooting practices are followed the above scenario would not and could not happen.

A technician should always have the tools necessary to trouble shoot the scales he will be working on. To be properly equipped he should have a good load cell simulator, a multimeter capable of reading up to 5000 mΩ or displaying nanoSiemens (nS), a measure of conductance (mΩ = 1000 ÷ nS), a 9 volt battery, and the necessary technical manuals for the equipment. He should also have a thorough grounding in how to use them.

By following the steps outlined below a technician should be able to trouble shoot most scales in a relatively efficient manner. The steps should be followed until the problem is resolved or the problem can’t be found. Many times an intermittent problem will not repeat while you are troubleshooting the scale and everything will appear normal.

It is dangerous for the novice troubleshooter to fall into the trap of familiarity. Just because a problem looks like something you have seen before doesn’t mean the cause of the problem is the same. After a while familiarity will help your troubleshooting but learn to do it correctly first before taking short cuts.

Troubleshooting Steps:

Determine from the operator and/or observation what the problem is. Without a thorough understanding of what is going wrong the technician can spend fruitless time searching for the solution to an imaginary problem. Sometimes the problem lies with customer expectations, and the equipment is incapable of performing the expected task or the customer is not properly trained in the use of the scale.
Thoroughly examine all of the components of the scale noting any discrepancies and deficiencies. For example debris under the scale, the hole in the keypad, the abraded cable, any of these could be the problem or an additional problem that could be confusing the issue further. Next, correct these deficiencies where possible and re-examine the symptoms.
Check the scale setup, dip switches, jumpers and program to ensure that the indicator is set up correctly. The following are real samples of problems that I have seen in the field.

The GSE 250/255 has a program soft switch to select 4 or 6 wire load cells. If the unit is set for 6 wire and a 4 wire load cell is used, the scale does not respond to weight.
On the old Fairbanks H90-5200, the one that has dip switches. If the dip switches are set incorrectly the scale can display underload/overload or even drift.
Several manufacturers have indicators that have a 2mV/V – 3mV/V jumper that can prevent an indicator from going to full scale if set incorrectly. I had one that would weigh perfectly to 72,380 lbs but then stop going up over that weight. I spent hours hunting for that problem.
The Mettler Toledo Jaguar/JagXtreme relies on an internal backup battery and when it dies the setup can be completely lost.

Scale controllers have too many variations to list the possible problems in set up, however if the instrument is set up correctly the problem most likely exists on the electrical side.

If the problem is not in the set up of the scale then remove all of the accessories, printers, scoreboards, computers, chart recorders, set point boards, etc…. Any of these accessories could be causing the problem with the scale, through feedback, shorts, miscommunication or numerous other possible ways. If the problem goes away it was in the accessories. Re-connect the accessories one at a time until the problem starts again; once it does you have found your problem component.
If the problem does not go away; put the indicator on a simulator. Check for stability and linearity. The indicator should be stable and by clicking the simulator up and down should repeat and increase and decrease in a linear fashion. If the indicator does not perform correctly then it should be repaired or replaced.
If the indicator passes, the home-run cable should be re-connected, and the simulator should be connected at the j-box. This test is to determine if the home run cable is good. The cable can look fine but still be damaged inside the jacket. If the scale fails and has transient voltage protection (surge voltage protection, SVP) in line with the indicator, bypass the transient voltage protection (SVP), if this works then replace it. If it doesn’t, replace the home run cable and retest the scale.

For Single Load Cell Systems:

Check the wiring at the load cell, perform load cell tests or if possible replace the load cell. If this fixes the problem then it was the load cell. If it doesn’t work go to step number 11.

For Multi Load Cell Systems:

Remove all of the load cells from the J-Box and wire the Load Cell simulator to the J-card and test the test the Indicator for stability and linearity. Does it work? If the indicator doesn’t work replace the J-card and retest it.
If the j-card is good, test the load cells by using the following tests:

Tie the signal, excitation, and sense wires together and test their resistance to shield. The resistance should be at least 1000 megohms, or less than 1 nanoSiemen. If it isn’t registering in the correct range then the load cell should be replaced.
If the load cell passes the first test, then wire the load cell to an excitation source (A 9 volt battery works fine.) The output of the load cell with no load should be approximately zero. If the load cell can’t be unloaded then you need to know the maximum output of the load cell, for example, a 3 mV/V load cell with 9 volts of excitation would be 27mV. Now a determination must be made to see if the output is above what you would expect and if it is then it needs to be replaced.

** An alternative to the above tests is to re-install the load cells one at a time until the bad load cell is found. This method does not always work due to indicator issues.

After the load cells have been tested and if necessary replaced, the cells need to be re-wired to the j-box and the scale re-evaluated.
Does the scale now work? If it doesn’t work re-evaluate the above tests to make sure they were done correctly. If everything was done correctly and the problem was not resolved call technical support.

Many manufacturers are willing to help you troubleshoot their equipment, because if the equipment can be maintained and repaired it doesn’t get replaced with their competitors. Some of the larger manufacturers will not help because they are trying to protect proprietary knowledge. If you are in a situation where the manufacturer can’t or won’t help, there are several companies that will help if your company has developed a working relationship with them. These companies are the after market repair companies such as National Scale & Control, Debac, RL Ziemba and others. They not only can provide support but often can provide the repair parts for the equipment, which is why they help. Contact them before you need them and find out what they need from you before calling for help. Nobody likes a freeloader and the time they work with you is an investment that they want a payback on.

By following a troubleshooting technique, whether mine or another, your technicians will find the problem faster and more efficiently. This makes for happier customers and less frustrated technicians.