How Do You Know if Your Load Cell Needs Calibration Service or Repair?
Ever wondered if your load cell needs repair or just routine calibration? You’re not alone! This is a common question for many users. Here, Justin Smith, manager of Interface Calibration and Repair Services, offers valuable insights to help you determine if your load cell needs a visit to our shop.
One of the initial signs that your load cell might require servicing or repair is a shifted zero balance. This balance, the reading displayed when no load is applied but the load cell is still installed, can significantly shift over time. This could indicate that the load cell has undergone some event that altered its readings. In such cases, our Interface Services Department, known for its precision and reliability, can adjust and recalibrate the load cell to restore its functionality. However, in rare instances, the cell may be damaged beyond repair and will need to be replaced.
If the load cell has been subjected to an overload beyond its specifications, a prompt evaluation can determine if the sensor is still in working conditions. Even if the degree of overload is not severe, the load cell may still be used at the user’s discretion. However, it’s crucial to note that some performance parameters may violate specifications, and the cyclic life of the load cell may be reduced. Therefore, timely evaluation is key to prevent further damage and maintain load cell performance.
Unstable readings can indicate that your load cell has a setup, electrical, or mechanical issue. Some change in readings, known as creep, is expected when applying a load, but any excess instability should be noted, and if it continues, Interface Services should evaluate it.
For load cell indicator systems all the previously mentioned apply. If there are inconsistencies in the readings, then it could mean that the indicator parameters have changed or that the indicator has been reconfigured. Also, a cable change in the system could lead to a shift in the output due to a difference in the resistance in the cable.
Another indication that your load cell may need service is that the output varies significantly when the same load is applied. This may indicate that the cells had a mechanical issue that Interface Services can evaluate and, in some cases, repair. In the event that the cell cannot be repaired, we can recommend a suitable replacement for your application.
Last tip from Justin, if your load cell has been exposed to an impact force event, like dropping the cell or a sudden and/or violent load being applied to the cell, then it may have experienced a shock overload and should be sent in for evaluation.
If the load cell is defective for reasons other than overload, return it to the factory for a detailed evaluation. The factory evaluation may show whether the cell is repairable or non-repairable and whether repair or replacement will be under warranty. If non-warranty, the customer will be contacted with the cost of repairs and recalibration and delivery date after receipt of authorization to proceed.
Technical Tips Regarding Load Cell Evaluations
As outlined in our essential Load Cell Field Guide, the evaluation process starts with visual inspection. Here are the processes we follow at the Interface factory. It is quite easy to make a quick diagnostic check of a load cell. The procedure is quite simple, and a minimum of equipment is required. Should it be determined that the load cell is at fault, the unit should be returned to the factory for further evaluation and repair as may be required. Many of the checks may be performed with an ohmmeter.
Check Bridge Circuitry And Zero Balance
- Instrument required (numbers apply to standard 350-ohm bridges): Ohmmeter with 0.1 ohms resolution in the range of 250-400 ohms.
- Bridge Input Resistance: RAD should be 350 ±3.5 ohms (unless the cell has “standardized output,” in which case the resistance should be less than 390 ohms)
- Bridge Output Resistance: RBC should be 350 ±3.5 ohms
- Bridge Leg Resistances: Comparing the leg resistances at no load permits evaluation of the cause of any permanent damage in the load cell flexure. The “computed unbalance” of the bridge shows the general condition of the cell.
- The computed unbalance, in units of “mV/V,” is determined as follows: Unbalance = 1.4 • (RAC – RAB + RBD – RCD)
- The Zero Offset, in units of “% of Rated Output”, is determined as follows: Zero Offset = 100 • Unbalance ÷ Rated Output
If the ohmmeter resolution is 0.1 ohm or better, then a computed Zero Offset of greater than 20 percent is a clear indication of overload. A computed zero balance of 10-20% is an indication of probable overload. If the load cell has been overloaded, mechanical damage has been done that is not repairable, because overloading results in permanent deformation within the flexural element and gages, destroying the carefully balanced processing that results in performance to Interface specifications. While it is possible to electrically re-zero a load cell following overload, it is not recommended because this does nothing to restore the affected performance parameters or the degradation of structural integrity.
Insulation Resistance Tests
- Insulation resistance, shield to conductors: Connect all the conductors together, and measure the resistance between all those wires and the shield in the cable.
- Insulation resistance, load cell flexure to conductors: Connect all the conductors together and measure the resistance between all those wires and the metal body of the load cell.
The tests described above can be performed using a standard ohm meter, although the best results are obtained with a megohm meter. If resistance is beyond the standard ohmmeter range, about 10 megohms, the cell is probably okay. However, some kinds of electrical shorts show up only when using a megohm meter or with voltages higher than most ohmmeters can supply.
CAUTION: Never use a voltage higher than 50 VDC or 35 VRMS AC to measure insulation resistance or a breakdown of the insulation between the gages and the flexure may result. Low resistance (below 5000 megohms) is often caused by moisture or pinched wires. The cause and extent of damage must be established at the factory to determine if the load cell may be salvaged.
Interface provides more than a million calibrations every year. We calibrate every load cell we build and provide calibration services. As a load cell manufacturer, Interface recommends an annual calibration service to keep your measurement devices operating at peak performance.
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