A lot of people have the morning routine of stepping on a scale to check their weight. What most people don’t know is that they are using a very simple “load cell” to find out their weight. Old scales used to use weights to try and level out the two sides of the scale; now we have methods that measure weight automatically.

The first thing we need to know about a load cell is a definition of what we are talking about. A load cell is a force gauge that consists of a transducer that is used to create an electrical signal whose magnitude is directly proportional to the force being measured. There are four common types of load cells. They are pneumatic, hydraulic, strain gauge, and capacitance.

Let’s begin by looking at how a pneumatic load cell works. Since it is pneumatic, we know that it will deal with air pressure. A pneumatic load cell consists of an elastic diaphragm which is attached to a platform surface where the weight will be measured. There will be an air regulator which will limit the flow of air pressure to the system and a pressure gauge. Thus, when an object is placed on a pneumatic load cell it uses pressurized air or gas to balance out the weight of the object. The air required to balance out the weight will determine how heavy the object weights. The pressure gauge can convert the air pressure reading into an electrical signal.

The next type of load cell we will discuss is the strain gauge. This is the most popular style of the load cell. A strain gauge load cell is a transducer that changes in electrical resistance when under stress or strain. The electrical resistance is proportional to the stress or strain placed on the cell making it easy to calibrate into an accurate measurement. The electrical resistance from the strain gauge is linear therefore it can be converted into a force and then a weight if needed. A strain gauge load cell is made up of 4 strain gauges in a “Wheatstone” bridge configuration. A Wheatstone bridge is an electrical circuit that measures unknown electrical resistance by balancing two legs of a bridge circuit, one of the legs contains the unknown component. The “Wheatstone bridge” circuit provides incredibly accurate measurements. The strain gauges that are in the Wheatstone bridge are bonded onto a beam which deforms when weight is applied.

The last type of load cell we are going to discuss is a capacitive load cell. Capacitive load cells work on the principle of capacitance, which is the ability of a system to store a charge. The load cell is made up of two flat plates parallel to each other. The plates will have a current applied to them and once the charge is stable it gets stored between the plates. The amount of charge stored, the capacitance, depends on how large of a gap between the plates. When a load is placed on the plate the gap shrinks giving us a change in the capacitance which can be calculated into a weight. 