How a Load Cell Works

Torque measurements

What is a Load Cell?

First, what is a load cell? They are all around us, and are instruments usually used in measuring weight. There are actually five types, though some are more common than others. The five types include the strain gauge, hydraulic load cells, diaphragm load, ring type load cells, and the spool type. A strain gauge load cell is a series of strain gauges placed on a spring made of steel or aluminum that will slightly deform under weight, but return to back the starting position when the load is removed. This is the most common type of load cell. The strain gauge type has been around for more than 40 years.

How Accurate Are They?

Naturally, if you’re judging weight it’s essential to get things right. If properly calibrated, a load cell will be accurate to around 0.03% to 1%, depending on the exact type of load cell. ISO9000 standards require re-calibration as often as every two years, and more if the instrument has experienced deterioration. Calibrating involves comparing the measurements of two instruments, one with a known correctness, to see if the unknown machine is accurate and by how much it may be inaccurate.
Much load cell calibration today is done with a torque wrench and torque sensor. The torque wrench was invented in 1918 by Conrad Bahr for use in the New York City Water Department. The torque wrench turns a part along the axis of a fixed point, allowing much tighter fastening of connections than would be possible with direct force. The torque sensor today experiences the full torque load that a load cell is under and reports the amount via an electric signal for calibration comparison.

Why Are There So Many Different Types of Load Cells?

Load cells are so critical to so many applications in the modern world that they must be able to work in a wide variety of situations. For example, if located outdoors they must not be so sensitive to temperature changes that they expand or contract too much. Hydraulic load cells can operate at temperatures as cold as -76 degrees F, for example, where a strain gauge load cell would be useless. They also need to be able to withstand force coming from a direction other than the load, such as from the side or below, though they are installed carefully to minimize this kind of pressure.

What Kind of Situations Demand This Measurement Tool?

Load cells measure your cheese at the deli counter, and the gravel before it leaves a rock yard. They measure the resistance of water over high-performance swim wear, and the weight of loaded trailer trucks. They measure the amount of concrete being poured for a specific use, and calculate the exact weight of every bottle of soda or bag of chips leaving a plant. They are used in furniture and computer testing, to measure water tank levels, in telescope alignment, and they tell us if a helicopter rescue hoist is in good shape. In short, precise tension load cells are essential to almost everything in our modern world.

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