| When your application needs to measure static or | | | | in one direction – either in Tension or in |
| quasi-static loads, forces or weights, you would need | | | | Compression |
| to use a load cell. A load cell is a device that converts | | | | * The reason for this is that –calibration systems |
| the applied force into an electronic signal that can be | | | | and calibration processes are designed for one |
| used to compute the magnitude of the force applied. | | | | direction. |
| Although relatively simple in concept, you’ll find | | | | * If your application needs forces to be measured in |
| that there are many complexities that might affect | | | | both directions i.e. Universal loads – then the |
| measurements when you actually implement your | | | | accuracy of the system may be lower than it is in |
| load cell system. One needs to consider a number of | | | | just one direction. |
| factors before you decide which load cell to use. | | | | * Some load cells such as the S-Beam load cells are |
| These include: | | | | better for such universal applications and if your |
| 1.Magnitude of force/load/weight being measured | | | | application permits the special mechanical configuration |
| * This is important to know because typical load cells | | | | of the S-Beam load cell – then you should |
| offer overload protections of about 1.5 times the | | | | consider using that kind |
| rated load capacity. If you apply loads – even | | | | * Manufacturers of load cells may charge you an |
| temporarily – beyond this maximum load – | | | | extra fee for calibrating the load cells in both |
| you can seriously damage your load cell and might | | | | directions |
| need to recalibrate it to ensure accurate readings | | | | 7.Mounting Options |
| * If you suspect that there could be an occasional | | | | * This may be one of the most often overlooked |
| spike in loading conditions, be conservative in picking | | | | aspect of load cells! Designers, engineers or users |
| a load cell with adequate capacity to handle the | | | | often pick a load cell and then discover they have to |
| highest load condition with at least 50% margin to | | | | spend hundreds of dollars more to design fixtures or |
| avoid exceeding the elastic limit of the load cell | | | | other accessories to actually use it in practice! |
| 2.Accuracy needed for application | | | | * Pay special attention to the mounting options |
| * Accuracies of load cells are generally rated as a | | | | offered by load cells on the top as well as bottom of |
| percentage of full scale. So a load cell with 0.1% | | | | the load cell. Threaded holes are easier to work with |
| accuracy rating for a 100 lb load cell would offer +/- | | | | and are generally lower in profile and less expensive |
| 0.1 lb accuracy, but the for a 1000 lb load cell offer a | | | | to use than if you have to drill a hole and thread it on |
| +/- 1 lb accuracy. So you need to pick the right | | | | your end! |
| capacity load cell for your application | | | | * Also pay special attention to off-center loading |
| * Certain test and measurement applications may | | | | conditions when you design your application. Some |
| need the accuracy as a percentage of reading so | | | | load cells such as pancake, button style and other |
| you may need to pick a load cell with adequate | | | | circular load cells are more accurate when used axially |
| accuracy to address those applications | | | | along the center of the load cells. When off center |
| * You also need to consider the accuracy of the | | | | loading conditions exist – then certain load cells |
| complete system – not just the load cell. | | | | such as cantilevered single point load cells offer |
| Depending on how you plan to use the load cell, the | | | | better performance than others. |
| accuracy of the system could be a function of the | | | | 8.Output Required (Analog or Digtital) |
| load cell, the signal conditioning equipment, the | | | | * This is one of the most important factors to |
| resolution of the digitizer, the variation of the power | | | | consider when picking a load cell since the type of |
| being supplied and other environmental factors | | | | output you want for your application is one of the |
| 3.Operating Temperature Conditions | | | | biggest determining factors in the final cost of your |
| * Although most load cells offer certain degree of | | | | solution |
| temperature compensation, you need to consider the | | | | * The most common output available from |
| operating conditions before you pick a load cell. | | | | conventional load cells is mV/V. When you apply 10V |
| * Is your application in a well controlled, room | | | | DC power – you get about 20 milliVolt for full |
| temperature or outdoor, harsh environments with | | | | scale outputs. This is a low level analog signal which |
| changing humidity and temperature? If it is outdoor | | | | cannot be input into general purpose DAQs or PLCs |
| – you should look for load cells with IP68 rating * | | | | and requires amplification before it can be used in |
| Even with temperature compensated load cells, there | | | | that mode. Look for load cells with amplified outputs |
| could be transient temperature conditions that affect | | | | to reduce the headache associated with this step |
| the load cell readings. Generally speaking the | | | | and the additional cost incurred |
| compensation works best when used under constant | | | | * If the distance between the load cell and the DAQ |
| temperature conditions | | | | PLC is large – you may need to use the 4-20 mA |
| 4.Duration of measurement – Short Term vs. | | | | type of analog output. Better still – if your |
| Long Term | | | | application works with a PC or a DAQ/PLC with digital |
| * An important consideration in picking the right load | | | | inputs then look for load cells with Serial/RS-232 |
| cells is to understand the stability of a load cell under | | | | RS-485/USB or Wireless outputs |
| short term vs. long term test conditions | | | | 9.Total cost of Ownership |
| * Most scales work under the scenario where | | | | * The cost of the solution depends on many |
| someone “Tares” the load cell and then | | | | factors:i.Cost of load cellii.Cost of signal conditioning |
| takes a reading. Accuracies are generally very high | | | | equipment needediii.Cost of digitizers/control |
| for these applications especially under room | | | | systemsiv.Cost of Programming or applying calibration |
| temperature conditions | | | | to get readings in lb/Kg/Nv.Cost of periodic |
| * But when temperatures are changing or when a | | | | recalibration |
| constant load is placed on a load cell for a long time, | | | | * You need to consider the total cost of solution |
| and one cannot “Tare” the reading | | | | before making a decision about which load cell to |
| periodically there could be drift or Creep in the | | | | purchase |
| baseline reading. If your application cannot tolerate | | | | * In addition, there could be significant unit to unit |
| this kind of drift then you need to purchase a load | | | | variation in performance and accuracy – so you |
| cell with good long term stability and low creep | | | | need to consider the cost of calibrating the system |
| * Process control applications that cannot be | | | | – if you are an OEM designing a system to be |
| periodically tared and still require very high resolutions | | | | used by an average consumer |
| and accuracy with temperature over long periods of | | | | 10.Certifications Needed |
| time – are some of the most demanding | | | | * If your application is going to be used commercially |
| applications to outfit with a load cell | | | | to transact business on the basis of weight, there |
| 5.Data Update Rate | | | | are rules and regulations that you’ll need to |
| * How fast do you need the reading to be? 1 Hz, 10 | | | | follow |
| Hz, 100 Hz or faster? | | | | * For example if you are selling coffee beans |
| * This is important to know because load cells are | | | | packaged in 1lb bags, then you need to use NTEP |
| generally speaking “quasi static” devices. | | | | certified load cells for measuring the coffee bags |
| That is they are not meant to measure the dynamic | | | | during packaging |
| forces acting on the sensor – rather just the the | | | | * In Europe and other areas this certification is called |
| static loads | | | | “OIML”. |
| * When you need to measure forces at a very high | | | | * In general R&D, Education and Test and |
| frequency to study the dynamics of a system – | | | | measurement applications do not require such NTEP |
| you need to purchase a force sensor with those | | | | OIML certifications, but there might be industry |
| characteristics | | | | guidelines and recommendations you need follow |
| * The response time of a load cell is generally of the | | | | In general, our Capacitive Load Cells are an excellent |
| order of 3-5 milliseconds – BUT this does not | | | | integrated solution for measuring forces/loads for R |
| take in to consideration the mass of the body | | | | & D environments. They offer very high sensitivities |
| attached to the load cell. Depending on what is | | | | in a very rugged package with easy mounting options |
| attached to the load cell – the response time | | | | to incorporate into your application. Our sensors offer |
| could be much slower than what you expect | | | | direct connectivity to PCs and make it effortless to |
| * Furthermore, the response time is generally | | | | write software applications. These are suitable for a |
| indicated as that of the load cells’ analog output | | | | wide variety of general purpose applications. |
| – and not that of the complete digitized system. | | | | When you need very high accuracies, NTEP/OIML |
| You need to consider the data rate of the data | | | | Certifications and need long term stability, for those |
| acquisition system, the PC or PLC being used etc. to | | | | applications we recommend you use our resistive |
| get very high data rates | | | | load cells with either the DI-1000 Series Digital Load |
| 6.Direction of loading | | | | Cell Interfaces or the AI-1000 Series Analog |
| * Load cells generally are designed for measurement | | | | Interfaces for your application. |