Assembly

1. Select a load cell (780g, 5kg or 25kg) and bolt it into the bottom plate with the arrow on the end of the cell pointing down toward the plate
2. Guide the load cell cable out of the notch in the back of the plate
3. Bolt the top plate onto the load cell

Calibration

The load cell inside the scale produces a small voltage differential based on how much strain its metal body is experiencing. How do we convert this small voltage into a useful unit of weight or force? The load cell needs to be calibrated.

The simplest way to calibrate the load cell is by using linear interpolation using two known weights and their corresponding voltage measurements. The output of a load cell is linear, so once we find the line that connects those two points, we can use it to convert any voltage from the load cell into a weight or force in the same units as our known weights.

Here's the process:

• First, clear the scale and take a voltage measurement. This will be our first data point.
• Next, find an object of known weight. You'll either need to measure the weight of something with another scale, or use a weight from a lab weight set. If possible, try to get a weight that will be close to the maximum weight you expect to be measuring for your project.
• Take a second voltage measurement with this object.
• Now we have two points, so we can calculate the slope of the line:

• And we can use the slope and our first point to solve for the y-offset:

• Now, we have a formula that we can use to convert any voltage from the load cell into a useful value!

Click below for a code sample in python that shows how calibration could be done at the start of your program:

from Phidget22.Phidget import *
from Phidget22.Devices.VoltageRatioInput import *
import time

calibrated = False
m = 0
b = 0

def onVoltageRatioChange(self, voltageRatio):
	if calibrated:
		# Calculate calibrated weight with y = mx + b
		sys.stdout.write("\rWeight: " + str(round((m*voltageRatio)+b,2)) + "g      ")

def main():
	global calibrated
	global m
	global b

	voltageRatioInput0 = VoltageRatioInput()
	voltageRatioInput0.setOnVoltageRatioChangeHandler(onVoltageRatioChange)
	voltageRatioInput0.setChannel(0)
	voltageRatioInput0.openWaitForAttachment(5000)

	try:
		input("Clear the scale and press Enter\n")
	except (Exception, KeyboardInterrupt):
		pass

	v1 = voltageRatioInput0.getVoltageRatio()

	try:
		w2 = input("Place a known weight on the scale, type the weight in grams, and press Enter:\n")
	except (Exception, KeyboardInterrupt):
		pass

	v2 = voltageRatioInput0.getVoltageRatio()

	# Calculate slope 'm'
	m = (float(w2) - 0) / (v2 - v1)
	# solve for b using zero point : b = y-mx
	b = 0 - ( m * v1)

	print("Calibration Complete: y = " + str(m) + "x + " + str(b))
	calibrated = True

	try:
		input("Press Enter to Stop\n")
	except (Exception, KeyboardInterrupt):
		pass

	voltageRatioInput0.close()

main()

Measurement Considerations

The measurement from load cells are susceptible to drift due to temperature changes in the wires. You may find that even when measuring the same object with no movement, the weight measurement may drift significantly. There are two ways to avoid this kind of error:

• For a long session of constant measurements, allow a 30-minute warm-up after the voltageRatioInput channel is enabled. The drift, due to self-heating of the wires, will eventually stop. Once stable, you can calibrate and measure.

• For brief, occasional measurements, you should enable the channel, immediately take the measurement, and then disable the channel. By doing this, there is so little current moving through the wires that the device does not get a chance to heat up. You should also follow this process when calibrating.

Overloading

Usually load cells can safely be loaded at 150% of their maximum weight. For example, you could put a 7.5kg object on the scale when the 5kg load cell is inside. The measurement would saturate (see the Saturation event in the Phidget22 API for your Bridge Phidget for details), so you wouldn't be able to tell how much heavier than 5kg the object is, but you don't need to worry about damaging the load cell.

However, you need to be careful with the 780g load cell- its high sensitivity makes it vulnerable to damage if loaded beyond 780g. If there's a chance your objects will go above this maximum, we recommend using the 5kg load cell instead.

Further Reading

For more information about how load cells work, see our Load Cell Guide.