Load Cell Ready For Calibration

To support the Airtripper Extuder Filament Force Sensor and other projects involving load cells, this article will cover some ideas in acquiring load cells and getting them ready for calibration. The article will cover the pros and cons of buying a bare load cell or buying electronic digital scales for the load cell inside. To finish off the article there are three electronic kitchen scale teardowns to reveal the load cells inside, and you will see how the scale enclosure & platform can be re-assembled for calibration purposes.  A separate article will follow to cover load cell calibration.

You might think the best way to get a load cell unit is to buy it from Ebay. The obvious reason for this is that you know the dimensions of the load cell in advance and the price is, in most cases, cheaper than buying electronic digital kitchen scales for the load cell inside. However, buying the scales for the load cell inside has advantages because you get a ready made weighing platform for calibration and you’re able to test the load cell in advance with the scales manufacturer electronics.

If using load cells for the first time, it is important to note that you might not get the same resolution, accuracy or stability that you get with electronic digital scales. Microcontrollers such as the Arduinos are limited to a 10bit analog to digital converter and the higher the load cell rating the less resolution per 1kg you will get. The microcontrollers can’t read from the load cells directly so an instrumental amplifier is required.  There will be more about this in the calibration article to follow.

Bare Load Cells Or Electronic Kitchen Scales

DIY Load Cell Weighing Platform & Stand

Unfortunately, just buying a bare load cell could make it difficult to calibrate without a suitable weighing platform and stand. Buying electronic scales for the load cell inside could provide a suitable weighing platform & stand for the load cell calibration. A weighing platform & stand could be custom designed and printed on a 3d printer or, with a few DIY tools, a platform could be made out of wood. However, depending on the type of load cell & application, the platform design will have to be strong enough to support the maximum calibration weight being tested. A calibration test load can be as much as 5kg or more and the weighing platform needs to be strong enough to support it.

Buying a bare Load Cell

5kg Load Cell Bought from Ebay

Bare load cells can be purchased from Ebay in a variety of load ratings and are usually described with full specifications. In order to make use of the load cells you may require a suitable weighing platform and stand for calibration depending on the application.

PROS

• You know the load cell dimensions in advance.
• The load cell can be cheaper than buying electronic kitchen scales.
• You can plan your designs with known dimensions while waiting for the delivery of the load cell. Also, other supporting fixtures can be ordered.
• A custom made calibration weighing platform can be adapted to fit different sizes of load cells.

CONS

• The weighing platform and stand will have to be custom made for calibration purposes.
• A custom made weighing platform and stand could add to the cost of the load cell.
• Ordering the load cell on its own means the rated load range can’t be tested in advance. Buying electronic scales will allow you to test the load cell inside in advance of removal.

Buying Electronic Scales For The Load Cell Inside

Electronic kitchen Scale With Weighing Platform Removed From Load Cell

There are plenty of kitchen scales to choose from on Ebay but you may also get them at a good price from your local discount store. Using the discount store will give you a chance to inspect the scales before purchase to determine the approximate dimensions of the load cell inside, and returning the scales would be much easier after purchase.

PROS

• Digital kitchen scales could be bought cheaply from car boot sales or discount stores, or Ebay even.
• The Scales can be tested quickly after purchase or delivery.
• You have a ready made weighing platform and stand for calibration purpose.
• The kitchen scales can be used to test the weighing range of the load cell inside before adding custom electronics.
• The load cell wire assignments can easily be identified from the circuit board labels.

CONS

• The build quality of the cheapest electronic scales might not be strong enough to weigh loads up to the maximum load rating.
• The dimensions of the load cell inside the kitchen scales is unknown until after purchase and the enclosure is opened.
• You may find the load cell dimensions are not suitable for the application intended after opening the electronic kitchen scales enclosure.
• Buying electronic kitchen scales could cost more than buying just the load cell unit itself.

Digital Kitchen Scales Teardown For Load Cells

WH-B05 Electronic Digital Kitchen Scales

The WH-B05 electronic scale was purchased from Ebay and was available from more than one Supplier. It’s a very compact unit measuring approximately 16.3 x 12.8 x 3.5cm and has a 5kg load capacity. The build quality is good with a strong sturdy weighing platform and the scales can be had for less than a bare load cell unit.

The load cell inside is a very compact 5kg version and the smallest I’ve seen for the load rating. The scales base and weighing platform can be saved for calibrating the load cell with test weights of up to 5kg. For the smaller weight sensing projects these scale could be ideal.

5kg Load Cell From WH-B05 Electronic Digital Scales

SF-400 Electronic Digital Kitchen Scales

This is another purchase from Ebay and this one has a load capacity of up to 10kg, the unit measures 24 x 17 x 3.5cm. I found the build quality to be too low for the 10kg load rating and there was some buckling in the weighing platform when tested with a 5kg load.

Getting a better quality scale of the same rating as the SF-400 will be a lot more expensive unless you can pick one up second hand. If you can make your own custom weighing platform & stand then buying a bare 10kg load cell could be a better option instead.

10kg Load Cell From SF-400 Electronic Digital Scales

KENWOOD DS800 Electronic Digital Kitchen Scales

The KENWOOD DS800 is a high quality digital kitchen scale with a more sophisticated and cleaner looking load cell inside. The glass weighing platform and the metal chassis ensures, when weighing, the maximum rated weight is kept steady and stable. These scales are now discontinued but found that they are sold under a new label; James Martin by Wahl ZX774 Digital Scales.

The load cell in these scales was used for the airtripper extruder filament force sensor prototype. The electronic scale is a bit expensive to buy just for the load cell inside but if you see one of these used going cheap, snap it up. The platform and chassis can easily be used to calibrate other load cells; like the bare ones sold on Ebay.

5kg Load Cell From KENWOOD DS800 Electronic Digital Scales

On closing

Load cells are one of my favourite sensors and I’ll be using them in a few projects I’ve got lined up. All the load cells in this article have been calibrated and tested successfully using an Arduino and an instrumental amplifier. I’m currently testing different load cells in preparation for the load cell calibration guide write-up. The OpenScad files for the Airtripper extruder filament force sensor will be edited to allow user configuration to fit variant load cell sizes.

Hope you enjoyed the Electronic Kitchen Scales Teardown Versus Load Cells.

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