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

PHIDGETS Inc.

Unit 1 - 6115 4 St SE
Calgary AB  T2H 2H9
Canada
+1 403 282-7335

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
1000 $20.00

This convenient kit contains three micro load cells of different strengths and an aluminum shell that can be assembled into a simple weighing scale. The top and bottom of the aluminium shell is installed on either end of the load cell, which creates shear force in the load cell when an item is placed on the surface of the scale. The force measured is proportional to the weight of the item, so with proper cailbration you can calculate the weight. See our guide on Calibrating Sensors for more details.

Connection

Load cells require a power source and produce a very small voltage differential when under strain. In order to read this signal, an amplifier or a board with a high-precision analog-to-digital converter is required. For more details and a list of Phidgets that will connect to this load cell, see the Connection & Compatibility tab.

Warning

warning Load cells can typically withstand weights up to 150% of their specified measurement range without damage, but the 780g cell has thin joints where the two halves of the cell are attached, and it is vulnerable to bending or breaking if loaded above 780g. Take care to avoid overloading this load cell.

Comes Packaged with:

Parts
  • 1x Weighing Scale Aluminium Shell
  • 1x Hex Key
  • 4x 12mm Screws (M4)
  • 1x FRC4114_0 - Micro Load Cell (0-780g)
  • 1x FRC4115_0 - Micro Load Cell (0-5kg)
  • 1x FRC4116_0 - Micro Load Cell (0-25kg)

Bridge Interfaces

Load cells and strain gauges produce a very tiny voltage differential and must be read by a high-resolution analog-to-digital converter. You can connect this kit to a bridge interface as described in this table:

Function Load CellBridge Interface
+5V Red 5V
+ Green +
- White -
Ground Black G
Functional Functional
The following bridge interfaces can be used with this scale kit:

Product Board
Image Part Number Price Controlled By Number of Bridge Inputs
1046_0B $90.00 USB (Mini-USB) 4
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
Canadian HS Export Code 8423.90.00
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:

Documents

Assembly

KIT4007 Assembly.jpg
  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.

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 Primer.