How accurate are smart scales?

Are smart scales accurate weighing instruments or just simple bathroom scales with expensive connectivity features? To find out, I tested the Withings WS-30, Fitbit Aria and A&D UC-324NFC using class M1 weights.

A&D UC-324NFC Fitbit Aria Withings WS-30
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Preparation

Each of the scales had not been used before and was configured as instructed by the manufacturer. This included signing up for an account with Withings and Fitbit. In the case of A&D’s UC-324NFC, I took advantage of a function available in the Wellness Connected  app which let me “calibrate” the scale by entering the local gravitational acceleration.

The scales were tested under similar conditions with a stable temperature of around 22°C and were placed on a flat, hard surface. The Withings and Fitbit scales were left alone for a night after the initial configuration as I wasn’t quite sure how they determine the zero value: unlike older electronic scales, you don’t have to switch them on and wait for them to show ‘zero’, instead, you step right on (the UC-324NFC determines the zero point after you step off).

Test procedure

I used 20kg weights to test the scales at 40, 60, 80, 100 and 120 kg. This procedure was repeated 3 times for each scale (I did a lot of weight lifting that day).

test
Creative Commons LicenseThis image by digitalscalesblog.com is licensed under a Creative Commons Attribution 4.0 International License

Results

The first column gives you the nominal weight (in kg) and the next three are the values as shown by the scales.

Withings WS-30

40 40.2 40.2 40.2
60 60.3 60.3 60.3
80 80.4 80.3 80.3
100 100.3 100.4 100.4
120 120.4 120.4 120.4

Fitbit Aria

40 40.2 40.2 40.1
60 60.2 60.2 60.2
80 80.2 80.2 80.2
100 100.3 100.3 100.3
120 120.3 120.3 120.4

A&D UC-324NFC

40 39.95 39.95 39.95
60 60.00 59.95 60.00
80 80.00 80.00 79.95
100 100.00 100.00 100.00
120 120.00 120.00 120.00

As you can see, A&D’s UC-324NFC was the most accurate scale, never deviating by more than 50g from the true value. Its 50g resolution and gravity compensation function certainly paid off.

However, the Fibit and Withings scales did a great job, too. Looking at the measurement results in each row, you’ll see that they are either identical or off by just one scale interval (100g). We can therefore say that these scales were slightly less accurate, but still very precise* (or that their repeatability was still excellent). As far as your body weight is concerned, you’ll generally be less interested in absolute values and more in tracking changes (“am I loosing or gaining weight”) and these scales would be perfectly suitable for this purpose. Nevertheless, I was a bit surprised that Fitbit and Withings didn’t use the location I had entered during account creation to adjust the scale for the local gravity acceleration. This would most likely have improved their accuracy.

Overall, the results are very good. As far as I can tell from this test, you’re not just paying for the “smart” features, you get accurate scales, too.

Further information / where to buy:

Disclosure:

  • Some links above are affiliate links.
  • My company used to sell A&D’s UC-324NFC in our German scales shop, but we don’t anymore. The Withings WS-30 and Fitbit Aria were purchased at retail prices, A&D’s UC-324NFC was purchased with a dealer discount.

*Meaning of accuracy and precision (as used in this article):
Accuracy and precision explained


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New Scales and Balances coming in 2014

A&D

A&D has confirmed that the SJ-WP series of compact bench scales is going on sale in Europe in the first quarter of 2014.

A&D SJ-30KWP Food Scale

To understand what’s so ingenious about this scale, you should know that until now, compact scales were either fast or they had a high degree of ingress protection. It was not possible to have both a very short stabilization time (<1s) and resistance to dust and liquids in a compact instrument. The reason for this is that the air pressure inside the housing of the scale changes when a load is applied (or removed). For accurate measurements, the air pressure inside and outside has to be equalized. Of course, this could be achieved by simply building a few holes into the housing, but then you would allow dust and liquids to enter. The conventional solution has been to use selectively permeable membranes which let air through and block water. However, air diffusion through these membranes takes time, slowing down the measurement process.

A&D’s new approach is remarkably simple and effective: By mounting the load cell outside of the housing, the SJ-WP scale achieves a stabilization time of 0.5s or less in combination with IP67 protection (dust tight and water proof up to 1m):

A&D SJ-WP externally mounted load cell

Fast, compact, resistant to harsh environments and easy to clean, A&D’s SJ-WP could be a game changer in the food industry.

You can already find further information on A&D’s website.

Adam Equipment

While I have no information on new products coming in 2014 yet, I was told that Adam is planning to add USB host functionality to a number of balances. This would allow weighing data to be written directly on a memory stick, greatly simplifying data logging and data transfer to a computer. This functionality is already available on Adam’s PMB moisture analyzer:

PMB Moisture Analyser

Kern

The first pages of the 2014 catalog show Kern’s continued focus on touchscreen scales. The new models include analytical and precision balances as well as platform scales. While Kern does not indicate who the ODM is, I’m pretty sure these devices are made by Radwag in Poland.

Kern Touchscreen Scales

A cooperation between the two companies would make sense: Kern’s strengths are sales and marketing, while Radwag is good at actually developing and manufacturing weighing instruments.

Further information will soon be available on Kern’s website.

Ohaus

The new Valor 2000 and 4000 are aimed at the food industry and are meant to improve productivity even in harsh environments. There are still some inconsistencies in the available documents but it appears that Ohaus is claiming a stabilization time of 0.5s with an IPX8 rating (no dust ingress protection; protected against submersion beyond 1m).

Ohaus Valor 4000 XW

If you’ve read this post from the very beginning, you might be wondering how Ohaus has been able to achieve this with a load cell that’s placed inside the housing. The answer is called “Flow-Through Design” and simply means that there are indeed some holes in the housing which allow air and liquids to enter and exit relatively unhindered. The electronic components are silicon sealed to protect them from fluids and condensation:

Ohaus Valor 4000 flow through design

I’m looking forward to comparing this approach with A&D’s external load cell solution.

Update March 2014: Ohaus has released a video featuring the new scales:

Smartlux (my company)

I want to take this opportunity to announce that our RS-232 communication software “232key” is going to be released this year:

Rs232 Software

Our software is designed to make it easier to transfer measurement values from any instrument with an RS-232 interface to any application which accepts keyboard input. It receives data from a scale, balance or other device via RS-232 (COM port), extracts the numeric value, formats it and “types” it into another program (“keyboard wedge”-functionality). I’ll announce the release on this blog.

A look inside the HC-i counting scale

One of the interesting features of A&D’s HC-i counting scale is its modular construction. The indicator can be separated from the base:
A&D HC-i counting scale

The following image lets you have a look inside the base of the scale. You can click on the tabs to highlight different elements and read a short description:

HC-i counting scale inside base
HC-i load cellThe load cell is attached to the bottom plate on one end and to the top plate on the other. It will slightly bend when a load is applied.
Strain gagesThe deformation of the load cell is measured by tiny sensors called strain gauges. If you look closely (or click on the image to see it in full size), you can see two of them on top of the load cell while two more are located on the bottom side.
Overload stopsThe overload stops prevent the load cell form deforming too far when a load beyond its capacity is applied. The two screws on the top are used to slightly tilt the top plate in order to adjust the overload gap between the plate and the 4 corner stops.

Please don’t hesitate to leave a comment and let me know if you’d like to see more interactive images.