Project Specification

Open-Source Laboratory-Grade Scales

Open-source digitally replicable lab-grade scales that are cheap, precise and accurate.

Specifications

FilamentMaterial: polylactic acid (PLA)
Diameter: 2.85 mm, Lulzbot TAZ 6
Optional Cover FilamentMaterial: translucent glycol modified polyethylene terephthalate (PETG)
Diameter: 2.85 mm, Lulzbot TAZ 6
Print Settings PLALayer Height: 0.14mm
Wall Thickness: 2 mm
Top/Bottom Thickness: 2 mm
Infill: cubic, 20%
Support Material: Zigzag, Touching Bedplate Only, 30%, 50 degrees
Bed Adhesion: Skirt
Nozzle Temperature: 205C
Print Speed: Infill: 40mm/s, Wall: 30mm/s, Support: 60mm/s
Print Settings PETGLayer Height: 0.18 mm
Wall Thickness: 1 mm
Top/Bottom Thickness: 1 mm
Bed Adhesion: Skirt
Nozzle Temperature: 230C
Print Speed: Wall: 20mm/s
Load CellsType: strain gage, single-point parallel beam
Model: TAL220 and TAL221
MicrocontrollerArduino (nano)
Load Cell AmplifierHX711
Push-Button Normally Open, Momentary
Solder BreadboardSize: 40 x 60 mm
Requires: Solder & Soldering Iron, Female header pins, Male header pins
Power supply5V USB

Overview

This tech spec was submitted by Joshua Pearce as part of the University Technology Exposure Program.


Problem / Solution

With the emergence and success of free and open-source designs, digitally fabricated scientific equipment made from these designs is gaining popularity because of its customizable to cater to scientists’ needs, lower cost, and equal or even superior performance to commercial products. 3-D printed parts made by self-replicating rapid prototype (RepRap) fused filament fabrication (FFF)-based 3-D printer is with adequate mechanical strength and quality for final functional components.

All laboratory requires digital scales for measuring mass in a wide range of scientific applications. The precision and accuracy of laboratory scales can significantly affect the outcome of various processes. Open-source 3-D printed digital scale designs satisfy scientists’ requirements while being inexpensive and easily replicable. The design is for a precision balance with a measurement resolution of 0.001g and features an easy-to-read automatic liquid crystal display. The mechanical components are manufactured with open-source RepRap-class material extrusion-based 3-D printers, while the electronics are also readily available in open-source designs, including the Arduino Nano.

Design

The open-source scales are made up of 3-D printed components and electronic components. The 3-D printed components include the base, cover, and bed. While the electronic components include Arduino microcontroller (nano), USB-A to mini-B USB, HX711 load cell amplifier, push-button, jumper wires, solderless or solder breadboard, optional 16x2 LCD using 10 k Ω potentiometer and 220Ω resistors,  optional 5V USB power block, and lastly the TAL 220/TAL 221 load cell with fasteners.

These designs of open-source scales are based on load cell devices. Strain gauge load cells used in the design convert strain of a material into an electrical signal relative to the force applied by the object being measured, thus determining the weight or mass.

The 3-D printed base features bosses and snap-fit joints that are designed for the fit placement of the electronic components. Only the load cell requires fasteners to hold it in place. The assembly of base and cover components completely encloses all electronics to limit airflow that affects the output of the load cell amplifier.

The scale designs accommodate two sized of single-point parallel beam load cells: TAL220 and TAL221. These models are inexpensive, has a wide selection of weight ranges, and are insensitive to moment loading for measurement repeatability. However, the base and bed design can be modified for other load cell models.

The open-source scales can be used in two ways: as an independent digital scale with a displayed output that can be tared and calibrated without a computer, or as a serially connected logging instrument for a laboratory setting and data-logging.

A 5V USB power block or computer USB supplies all the components of the entire scale to achieve independent functionality. Each component’s power needs are small enough to allow Arduino’s low-current digital pins to supply them independently. The single-push button controls the scale’s tare and calibration function: short press for tare and long press for calibration.

The serial capability is for automated weighing or situations where a computer will always be used as the power supply and does not require an LCD. The microcontroller is configured for serial interface with a computer as a logging scale. The API for serial communication allows for continuous tracking of the scale readout for data-logging, which includes the capability to serially query the scale output, command tare, calibration, and provide basic scale information. 

The open-source scale yields repeatable results within 0.05 g with multiple load cells and precise results within 0.005g depending on load cell range and style. The scale produces accurate results by achieving performance standards compared to branded commercial lab-grade scales. The smallest load cell version of 100g yields precise results comparable to commercial digital mass balance

References

A research paper describing the challenge, design, and outcome of the research

Benjamin R. Hubbard and Joshua M. Pearce

Wevolver 2022