Technical Tricks and Tips
Not sure how to calibrate your scale? Having trouble wiring your kit? Scale not working as it should? Read through this section on Technical Tricks and Tips to see if you can find information that will help you.
ON THIS PAGE:
> How do Digital Scales work, anyway?
> All About Load Cells?
> Wiring your Junction Box?
> Calibrating your Scale?
> Troubleshooting a Problem?
1. How do Digital Scales Work, Anyway?
I. All About Load Cells
a. What is a load cell and how does it work?
For most of our applications, we use what is called a single‐ended shear beam load cell. This type of load cell measures the downward force directly above the load cell foot. When force is applied to a load cell, it deflects (bends), which causes it to generate an analog electrical current. This current is then transmitted through the junction box and on to the indicator. Once this analog signal reaches the indicator, an internal Analog/Digital conversion board converts the analog electrical signal to digital “counts.” These “counts” can then be converted to pounds, kilograms, etc. (see calibrating the indicator for more about digital counts). There are numerous types and sizes of digital load cells, each one designed for a specific application. Some of the most common types include:
> Single-ended shear beams:
These are most commonly used in platform scales, such as floor scales, small cattle scales, vet scales, etc. They are typically available in capacities ranging from 500lbs to 20,000lbs.
> Double-ended shear beams:
These are most commonly used in larger platform scales, such as truck scales, group animal scales, etc. They range in capacity up to about 75,000lbs each.
> S-Type or S-Beam Compression/Tension Load Cells:
These are used in numerous applications, from hanging scales, to in-line tension/compression applications. They range in capacity from 100lbs to several tons.
> Compression Load Cells (not shown in image):
These are typically used for very large scales such as truck scales, rail scales, etc. Their capacity can go well in to the hundreds of thousands of pounds.
> Single-point load cells (not shown in image):
These are used for fine and very fine weighing applications such as lab scales.
b. How do I mount the load cells?
If you purchased one of our scale kits, it probably came with a shim/spacer or a mounting block that is used to create a small gap above the foot‐side of the load cell. This is important. In order for the load cell to generate a measurable electric current, the load cell must be able to flex directly above the foot. Generally, 1/20th of an inch is all of the space that is required, but if you mount the entire load cell flush with the underside of your scale base, it will not work.
c. Where on my platform should I mount the load cells?
No two platforms are the same, but as a general rule, you want the FOOT of the load cell as close to each corner as possible. Try to mount the load cells as far apart from each other as possible.
d. The load cells have a lot of extra cable. Can I cut it to make it easier to manage?
NO. Never cut the load cell cables. When load cells are manufactured, the electrical resistance along the length of the cable is factored into the overall signal strength. If you cut the cable, you will have too little resistance and, therefore, too much signal. Two words: ZIP TIES.
e. What if I cut the cables all the same length?
In theory, it will work. However, we do not recommend doing this, and cutting the cables will void your warranty.
f. Is it okay if the load cells get wet?
Generally, yes. Our standard load cells are made of hardened tool steel (we also have stainless steel load cells, but they’re very expensive and most people do not need them. If you’re in a Caribbean climate, you may want to consider them). However, we do not want the load cells to sit, submerged in standing water. Washing them down when you’re done weighing won’t hurt them but leaving them submerged will corrode the seals and they’ll start to pit.
II. The Junction Box
a. What is the junction box and why do I need one?
The junction box contains a simple summing card that aggregates the analog signal from each of the four load cells and passes it along to the indicator. In theory, you could also just twist all of the load cell cables together and wire them directly to your indicator, and the scale would work just fine. However, the advantage to using a j‐box is that they contain a tuning port that allows you to amplify or de‐amplify the signal from each load cell. This is normally not necessary, but as your scale ages and the load cells begin to wear down, the j‐box will extend the life of your scale considerably.
b. How do I wire the j‐box?
We source our j‐boxes from several different suppliers so you may find that the wiring diagrams on our website do not match the j‐box that came with your scale or kit. Not to worry. All j‐boxes are wired the same: EXC+ or E+ is RED EXC‐or E‐is BLACK SIG+ or S+ is Green SIG‐or S‐is White GND or SHLD is Yellow. If your J‐Box has SEN+ and SEN‐, you can ignore them. These are used to control more advanced scale indicators. Frequently, the signal wires (Green and White) need to be reversed. For example, if you installed your load cells with the arrow sticker pointing DOWN, you’ll need to flip the Green and White wires to reverse the signal. See Troubleshooting for more about this.
III. The Indicator
a. How do digital scale indicators work?
As we said at the very beginning of this document, in question I‐a, a load cell measures force and sends an analog current to the indicator; the stronger the force, the stronger the current. A digital scale indicator converts that analog signal into digital counts. When you calibrate a digital indicator, you are essentially telling the indicator that the amount of counts that are generated with NOTHING on the scale is ZERO pounds. And, the amount of counts that are generated when there is, say, 200lbs on the scale, is 200lbs. Once you define these two values for the indicator, the internal logic board is able to do the math and figure out that the calf that is standing on the scale is causing the scale to generate a whole bunch of digital counts that equal 74lbs, for example.
i. The indicator menu has two values that must be calibrated: Zero and Span. (Span means there is weight on the scale.)
This is true for all digital indicators (some indicators have more than one Span Calibration, but it is generally not necessary to calibrate all of the Span points). In the BB8100SS Indicator and the TI‐500E Floor Scale Indicator, F16 is your Zero Cal and F17 is your Span Cal. In the BB7510 Indicator, C5 is your Zero Cal and C6 is your Span Cal. In the TR1NK indicator, it’s P5 and P6. The actual menu depends on your indicator (consult your manual) but the principle is exactly the same. Zero Cal and Span Cal.
i. The scale weighs unevenly. I put 50lbs on the scale and it weighs 50lbs in the middle, but it weighs 30lbs on one side and 90lbs on the other.
This is generally a signal error, and it is frequently caused by a mechanical obstruction on one side of the scale. It is not your indicator. Try the following:
a. Make sure the scale is reasonably level and that all four feet are on the ground. (It doesn’t have to be perfectly level).
b. Make sure each load cell is installed properly and there is at least a 1/20th of an inch gap above the foot so the load cell can bend.
c. Make sure all four arrows on the load cells are pointing the same direction.
d. Check the connections in the junction box. Make sure the j‐box is wired properly and none of the wires are loose.
e. Check all the load cell cables for damage.
f. You may have a bad load cell. You can try to use an ohm meter to measure the signal at each load cell. They should all be about the same reading. If one is way off, that could be the issue. Having said this, however, it is possible for a load cell to fail without a change in signal strength, so this is not 100% foolproof.
g. You can try to turn the tuning screws in the junction box to increase or decrease the signal from each load cell. This generally only works if you’re only a pound or two off. If you have a 50lb difference from one end of the scale to the other, this won’t work.
h. You may have a bad junction box. The odds are greater that you’ll win the lottery than that you’ll have a bad junction box, but it does happen every once in a while.
i. Disconnect all four load cells from the j‐box. Reconnect one of the load cells and recalibrate the scale with only one load cell. If it weighs accurately, add another load cell and recalibrate. Repeat until all four load cells are reconnected. If you have a bad load cell, this will identify it.
ii. My indicator jumps all over the place.
This can be caused by a faulty Analog/Conversion board in the indicator or by a signal error from the load cell.
a. Try steps a through e above. If that doesn’t work…
i. Turn the indicator off.
ii. Disconnect the cable that connects the junction box to the indicator.
iii. Flip the calibration/programming switch on the back of the indicator to put it in programming mode.
Turn the indicator on and navigate to your ZERO CAL menu (F16 on the BB8100SS, C5 on the 7510).
Push the down arrow to view the raw counts. Is it stable or does it jump around? (it should be stable). Push the zero key to zero the value. Does it stay at zero or does it climb up or down? (it should stay at zero) With your bare finger, simply touch the load cell port on the indicator. Does the value change? (it should). If it is not stable, you may have a problem with your indicator.
iii. The scale works but it is not accurate.
Recalibrate the scale. If it calibrates ok but is still not accurate, you may have a signal error. See Question 1, above. Generally speaking, if your scale calibrates ok, your indicator is working fine.
iv. I get ERR2 when I calibrate the scale or I’m seeing a negative number on the display.
This is signal polarity error. Open the j‐box and swap the Green and White wires on the home run cable (the cable that goes to the indicator). In general, ERR2 is any sort of signal error, and you may have to go through the steps in Queston 1. However, if you’re seeing a negative number, swap the signal wires. This is almost always the problem.
Finally, if you have any questions that aren’t covered in this document, feel free to contact us.