Hi everyone,

I am trying to use some simple thin film resistive weight sensors. These sensors output a certain voltage given the pressure applied. However, the pressure/voltage curve is not linear, it is a 2nd degree polynomial function with a flat beginning of the curve (see picture below). I am using two sensors (their average voltage) to weight one shelf.

I have chosen a very stable high quality power supply for my circuit so that the voltage is quite stable.

Now comes the issue: estimating the weight based on the voltage. At first I have tried to use the filter calibrate_linear, with very scarce results. Then I remembered that the most similar approximation of the curve is a 2nd or 3rd degree polynomial function (see pic below), and I have tried that as a filter. The results, however, are very poor especially with low weights (the initial part of the curve).

This is the latest code I have tried:

```
i2c:
sda: 4
scl: 5
ads1115:
- address: 0x48
id: ads1115_1
- address: 0x49
id: ads1115_2
sensor:
- platform: ads1115
ads1115_id: ads1115_1
multiplexer: 'A0_GND'
gain: 6.144
name: "Rack1_Sensor1"
update_interval: 1s
id: "Rack1_Sensor1"
- platform: ads1115
ads1115_id: ads1115_2
multiplexer: 'A0_GND'
gain: 6.144
name: "Rack1_Sensor2"
update_interval: 1s
id: "Rack1_Sensor2"
- platform: template
name: "Rack1 sensors average"
id: "Rack1_Sensors_Average"
lambda: |-
return (id(Rack1_Sensor1).state + id(Rack1_Sensor2).state) / 2.000;
update_interval: 5s
accuracy_decimals: 3
unit_of_measurement: V
- platform: template
name: Rack1 weight
id: "Rack1_Weight"
lambda: |-
return (id(Rack1_Sensor1).state + id(Rack1_Sensor2).state) / 2.000;
update_interval: 5s
accuracy_decimals: 3
unit_of_measurement: kg
filters:
- calibrate_polynomial:
degree: 3
datapoints:
# Map 0.0 (from sensor) to 0.0 (true value)
- 4.775 -> 12.785
- 4.768 -> 11.476
- 4.757 -> 10.124
- 4.742 -> 8.772
- 4.723 -> 7.31
- 4.711 -> 5.848
- 4.638 -> 4.386
- 4.57 -> 2.924
- 4.44 -> 1.462
```

I tried using ranges (I could estimate the # of items placed on the sensor instead of their weight, since they are standard bottles), but it doesnâ€™t allow me to differentiate if the bottles are full or not, so I really need to measure the weight.

What else can i try to approximate the curve better? Is there a way to define a more accurate *f(x)* and describe it in a lambda filter (I would actually like to write the mathematical equation in the lambda filter)?

Hereâ€™s the curve the computer calculated:

3rd degree polynomial function:

4th degree polynomial function:

Exponential function: