# Based on my project do I need to calibrate load cell

Hello everyone - I am building a bottle filler that will fill a bottle and shut off when a bottle is full. The product is fairly thick so today we lightly pressurize a 300 gal tote and turn a value on manually to fill a 20oz bottle. What I was thinking about was still pressurizing the tote, having the fluid go into a manifold with 4 electromechanical valves that would turn on with a push of a button and then shutoff based on a reading from 4 respective load cells weighing 4 bottles. The load cell only needs to know one point or value accurately. I was thinking on power up to place a full bottle on each load cell and pressing a button to record the load cell value. Then each valve for a given load cell will shutoff when it hits the right load cell value.

Will that work?

Anthony

That could work. Under-fill the reference bottle because there will be a time lag between triggering the valve and the last drop falling in.

Yes! You need to calibrate your load cell against two known loads (A(W1, C1), B(W2, C2)) in order to know the accurate value of the unknown load (C(W, C)). Here, Ws are the weights and Cs are the respective responses of the load cell as raw Counts. From these 2-point responses, you will find the equation for W in terms of C as:

``````W = kC + p
``````

k and p are computed constants and are called gain and offset.

Calibrate and store the values in EEPROM or even hardcode in your sketch.

Otherwise if you have a power outage halfway filling all subsequent bottles get filled halfway. Having to recalibrate every time you switch on power (and remembering to place your calibration bottles - indeed you have to find out how much in advance you have to switch) is quickly going to get old.

Thank you Everyone for your inputs. If all 4 load cells are calibrated reasonably well, I could then put a switch or a potentiometer on the system to adjust volume for different container sizes and apply it to all load cells.

In general, how long will a load cell stay calibrated? This will be in an uncontrolled factory environment so temperature and humidity will swing. The load cell will be mounted to a surface that does not absorb moisture. I have read they drift but have not seen a statistic of time or amount. I know our pallet scales only get calibrated once a year or so.

Thank you,

Anthony

Check the datasheet of the load cell you have in mind. There must be information about that available.