There are two types of Load Cells, they are

Basic Priniple of Hydraulic Load cell

When a force is applied on a liquid medium contained in a confined space, the pressure of the liquid increases. This increase in pressure of the liquid is proportional to the appilied force. Hence a measure of the increase in pressure of the liquid becomes a measure of the appilied force when calibrated.

The main parts of a hydraulic load cell are as follows

A dirphragm
A piston with a loading platform (as shown in figure) placed on top of the diaphragm.
A liquid medium which is under a pre-loaded pressure is on the other side of the diaphragm.
A pressure gauge (bourdon tube type) connected to the liquid medium.

The force to be measured is applied to the piston.
The appilied force moves the piston downwards and deflects the diaphragm and this deflection of the diaphragm increases the pressure in the liquid medium (oil).
This increase in pressure of the liquid medium is proportional to the applied force. The increase in pressure is measured by the pressure gauge which is connected to the liquid meduim.
The pressure is calibrated in force units and hence the indication in the pressure gauge becomes a measure of the force applied on the piston.

As the hydraulic load cell is sensitive to pressure changes, the load cell should be adjusted to zero setting before using it to measure force.
This hydraulic load cell have an accuracy of the order of 0.1 percent of its scale and can measure loads upto upto 2.5*10^5 Kgf
The resolution is about 0.02 prcent.

Basic Principle of Pneumatic Load Cell

If a force is applied to one side of a diaphragm and an air pressure is applied to the other side, some particular value of pressure will be necessary to exactly balance the force. This pressure is proportional to the applied force.

The main parts of a pneumatic load cell are as follows:

A corrugated diaphragm with its top surface attached with arrangements to apply force.
An air supply regulator, nozzle and a pressure gauge arranged as shown in figure.
A flapper arranged above the nozzle as shown in figure.