One day, a customer called and said, “I’ve bought a lot of A&D load cells and the indicator value varies just by touching the load cell cable on several of them”. We carefully inspect the operation of our load cells before we ship them so we doubted that there could be that many defective items in one place.
We asked the customer how the indicators were being used. Even after extensive questioning, we couldn’t determine the cause of the problem so we asked one of our service engineers to go and check things out.
The service engineer reported that, “The indicator value definitely changes just by touching the cable. What’s surprising is that it doesn’t seem to happen when you touch far away from the terminal”.
“When the load cell is connected to the terminal directly without using a crimp-on terminal, it’s fine,” he continued. “It’s weird. The crimp-on terminal shouldn’t affect the indicator value…”
After a little more investigation, we finally found out what the problem was. The crimp-on terminals being used didn’t match the thickness of the conductor of the load cell cables. That’s why touching the cable changed the contact resistance and negatively affected the indicator value. If the contact resistance of the EXC terminal applying power to the load cell increases, the sensitivity of the load cell drops similarly.
The input resistance of many load cells is 350Ω. Even if the contact resistance increases by a mere 35 mΩ- or 1/10,000th – the load cell output drops by 1/10,000th. This shows why it’s dangerous to use ill-fitting crimp-on terminals like this.
When troubleshooting in the field, it’s important to get the correct information right from the start. However, the cable size of the crimp-on terminal wasn’t one of the first things to come to mind and we wasted a lot of time solving this problem.
Crimp-on terminals can be convenient for making stable connections. However, if they are used incorrectly, things such as the oxidation of metal surfaces can cause unexpected problems, even after years of trouble-free use.
There are many types of crimp-on terminals with different shapes and hole diameters to suit connection cables. Be sure to choose crimp-on terminals that are suitable for your cables and use appropriate crimping tools.