Hi, I am trying to take measurements between 0 and 20kg and I need minimum resolution of 1 or 2gr. I am trying to calculate if I’ll be able to do that with a 20kg micro load cell (RB-Phi-119) and a HX711 Load Cell Amplifier (RB-Spa-1359) but I am not sure of how to do this. Can you help me?
Thanks in advance
Hi,
The resolution you will get when measuring weights with load cells will depend on several factors :
]The electrical specifications of the load cell you will use such as : rated output (mV/V), non linearity, hysteresis, etc./:m]
]The noise rating of the instrumentation amplifier./:m]
]The resolution of the ADC. /:m]
**Rough Calculation **:
For example, if you use the 20 Kg Micro Load Cell with the HX711 Load Cell Amplifier and the load cell is 5V powered, you would get something like this:
According to its datasheet, this load cell has a rated output of 1.0 ± 0.15 mV / V. This means that with a 5V DC supply, this load cell will have an output that varies by a total of 4.25mV - 5.75 mV.
From the datasheet of the HX711 Load Cell Amplifier :
Once amplified (gain = 128), you should have a voltage that can be read by the ADC, in the range of 524-756 mV DC of variance.
With all this information, we can now map the ADC’s reading to the force (weight) measured.
In this case, the ADC of the HX711 Load Cell Amplifier has 16777216 steps (24 bit resolution), or roughly 1 steps of ADC value per ~0.0003 mV DC (5V/16777216 steps).
Therefore, you will get a variance of about 1746666 (524mV/0.0003mV) to 2520000(756mV/0.0003mV) steps for the full scale of 20 kg.
Therefore, your final resolution will be roughly: ~8-11 mgrams/ADC Step. .
Please note that this calculation assumes that the entire range of the ADC is used (usually the 2 LSBs of the ADC are ignored due to noise) and the DC voltage source for the ADC and amplifier is perfectly stable (well filtered, without
noise).