Products for USB Sensing and Control
Products for USB Sensing and Control

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## Weighing Scale Kit

ID: KIT4007_0
Recommended for new designs: This product (or a similar replacement with a compatible form, fit and function) is estimated to be available for ten years or more.

An aluminium-framed weighing scale kit with three different load cells.

## $40.00 Quantity Available: 6 Qty Price 5$38.00
10 $36.00 25$32.00
50 $28.00 100$26.00
250 $24.00 500$22.00
DAQ1500_0 $30.00 VINT 2 #### Load Cells This kit comes with three different load cells with varying sensitivity. If you need extras, the following load cells are compatible with this kit: Product Sensor Properties Physical Properties Image Part Number Price Sensor Type Weight Capacity Max Screw Thread Size FRC4114_0$7.00 Shear Load Cell 780 g M4
FRC4115_0 $7.00 Shear Load Cell 5 kg M4 FRC4116_0$7.00 Shear Load Cell 25 kg M4

#### Product Specifications

Physical Properties
Material Aluminium, anodized coating
Length 156 mm
Width 156 mm
Customs Information
American HTS Import Code 8423.90.10.00
Country of Origin CN (China)

For characteristics of the included load cells, please visit the specifications section of their respective product pages:

## 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. You can learn more about the sensors in the load cell on our Strain Gauge Primer. 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:

Code Sample
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.