At this point we need a precise voltage amplifier. A digital component that fit at our needs is INA125.

You can get the data-sheet from here : ina125

From the data-sheet we can get a generic schematic

Notice that INA125 have a gain range from 4 up to 10.000

This gain is regulated by the resistor R_{G }located at the pins 8-9

The gain formula for this device is :

**Gain=4+(60000/R _{G})=>R_{G}=60000/(Gain-4)**

In our case for a 5V arduino reference we need

R_{G}=60000/(896-4)=>**Rg=68 Ohm**

In the 1,1V reference case we have

R_{G}=60000/(196-4)=>**Rg=312 Ohm**

The connection with the arduino will be like below

The same procedure will be followed with a **20Kg** load cell.

The load cell manufacturer once again says that the rated output is 1,0946 mVout/Vin

This means that 1,0946 mVout/Vin

So if we drive the loadcell with arduino (5V) we have

Vin = 5V and mVout-max=1,0946*5=5,473mV(full loaded with 20Kg)

So at 20000gr we have a mVout-max=5,473mV

At 1gr we have x mV

So **x=0,00027365 mV/gr**

By default arduino uses an analog reference of 5V this means that you have a resolution of** 4.9 mV/step **as we said before.

We need a precise amplifier that can increase the electrical signal in order to get **0,00027365 mV/gr**

So we need a gain like this :

** Gain = (arduino mV/Step) / (loadcell mV/gr)**

Gain (5v) = 4,9 mV/step / 0,00027365 = 17906

The maximum gain of INA125 is 10000, so a gain that high, using 5v as analog reference, is not possible.

Let's see what happens at 1,1V using **analogReference(INTERNAL)**;

We know that **1.1v/1024 steps = 0,00107421875 v/step = 1,07421875 mV/step**

So :

Gain (1.1v) = 1,07421875 mV/step / 0,00027365 = 3925

This time the gain is reachable! We need a resistor :

R_{G}=60000/(Gain-4)=>**Rg=15,3 Ohm**

All the schematics can be downloaded from here : loadcell-schematics

Now we are ready to start constructing **Liquid Filler !!!**

See you in the next article!

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