Unit in S-Type sensor

Sensors Force & Weight
1

Hello,

I am confused between whether to use mass in [kg] or weight in [N] when choosing a sensor to measure force.

From Physics, we know that Weight (downward force) = mass * 9.81 m/s^2

Supposing the objects of interests are under 30kg. Should I consider a S-Type sensor of capacity slightly larger than 30 kg or

30kg * 9.81m/s2 ~ 300 Newton?

From the data sheet, it seems that there are four different versions including 5-50kg, 100-500kg, etc.

Load Cell - 200kg, S-Type (TAS501) - SEN-14282 - SparkFun Electronics

Also, if I use a 200kg sensor to measure the force generated by objects under 30kg, will the precision/accuracy drop?

2

Units you use are up to you, just calibrate the sensor using your preferred units.

For your other question, the weight you want to measure should be close to the range of yor selected load cell. A tiny weight on a large rating won’t be that accurate.

3

An equivalence between weight and force is assumed in the load cell specs.

One kilogram-force is the force applied by one kilogram of mass at rest, near sea level, in the direction of gravity.

Of course, the load cell is just a chunk of metal bending and it can’t distinguish between force due to mass at rest & gravity, a mass accelerating/maneuvering, an externally applied force (like a hand pushing down), angled orientation, etc.

Just add -force to the end of all your load cell specs.

Capacity = 200 kilograms-force.

All else equal, the response to a given load is proportionally reduced as the load cell capacity increases. Double the capacity reduces resolution by half. You could expect a more accurate weighing of your 30 kg item with the 50 kg load cell than with the 200 kg one since the 50 kg cell has four times the signal. On the other hand, after someone steps on the lower capacity scale and busts the load cell, you’ll wish you’d gone with the bigger one.

4

Just add a “-force” at the end is a good idea to avoid confusion. How come load cell does not have a hardware limit to prevent it from getting busted by heavier weight?

How come on the datasheet, it mentions that there are only four cables? As far as I know, S-type Load Cells have five cables which is also mentioned in the Load Cell Amplifier HX711 Breakout Hookup Guide.

5

rgbpearson:
How come load cell does not have a hardware limit to prevent it from getting busted by heavier weight?

Some force measuring devices like scales have overload stops, usually check bolts/rods, set screws or other hardware to help prevent the load cell from bending beyond a certain point. It’s up to the designer and, later, operator.

The fifth wire is probably a shield and used to provide some measure of RF and static protection.

Shield isn’t electrically required for the measurement. It’s like a chassis or earth ground.