Raspberry Pi: Connecting a scale can be very easy…

…as long as you choose the right scale! The photo below shows an A&D EK-2000i compact balance connected to a Raspberry Pi 3 Model B using a compatible serial cable and an FTDI RS-232 to USB converter:
Scale connected to Raspberry Pi

Python and pySerial make it easy to read weight data from a scale

On the Pi, I used Python with pySerial to read the weight from the scale. The script I put together is trivially simple. You can download it here or just type it on the Pi:
Script to read weight sent from A&D scale

By pressing the PRINT-Button on the scale, I then sent a few weight values to the Pi and the script wrote them to the terminal (b’ is added by Python):
Weight printed to terminal

In a more useful application, you would want to parse the data sent from the scale to get the weight as a number and then do something with it (maybe write it to a database). You could also do interesting things with the GPIO on the Pi like sounding an alarm when a certain weight is exceeded (though a microcontroller might be better suited for this).

I then switched the scale to “stream mode” (continuous transmission) where it sent 10 values per second (stable or not). The output you see below is the result of placing a single object on the scale and waiting for it to stabilize:
Weight data in continuous mode

I hope this example shows that it can be really easy to transfer the weight from a scale to a Raspberry Pi.

Notes on scales and the Raspberry Pi

One advantage of A&D scales is that (almost) all of them use the same data format and interface parameters and that you’ll find a good description in the manual:
A&D EK-i balance data format

However, you could use also use scales made by other manufacturers as long as they come with an RS-232 interface and the ability to send the weight in ASCII format. Available data output modes vary, so please read the product description carefully before buying.

A converter like the one I used is already built into some scales, which means that you can directly connect them to the Raspberry Pi via USB (you won’t need a serial cable). You’ll find many Ohaus scales with this interface option and the ones I’ve tested worked fine. However, cheaper scales with a USB interface can often not be used with pySerial as they do not emulate a serial port. They can also present other challenges (e.g. data that is not transmitted in human-readable ASCII format).

You do of course not have to use Python, just make sure that your preferred programming language can communicate over a serial port on the Pi. The book Raspberry Pi with Java: Programming the Internet of Things (IoT) contains projects using Ohaus scales and, well, Java.

Finally, please don’t try to use the serial interface already provided by the Pi on the GPIO pins without a logic level converter (RS-232 voltage could reach up to 15V). Excellent further information on serial ports on the Raspberry Pi can be found here.

Hydraulic Press Channel uses compression load cell for crush resistance measurement

For most videos featuring weighing instruments, a few thousand views would be considered a success. The Hydraulic Press Channel regularly achieves millions of views and included a compression load cell with an indicator in one of the latest videos:

How Much Weight Can a Soda Can Hold? Hydraulic Press Test

If you’re wondering why these types of videos are so popular on YouTube, this article might offer some insights. As far as the measuring instrument is concerned, it appears to consist of an AnyLoad 266AH-5t load cell connected to a Rinstrum R320 indicator (with peak hold function).

Photo: Ohaus Navigator product range

Ohaus Navigator (NV), Navigator XL (NVL) and Navigator XT (NVT) scales – what’s the difference?

Physical dimensions

While all scales in the Ohaus Navigator product range are fairly compact, their dimensions vary according to the series to which they belong. In our photo, you’ll see the Navigator XT (NVT) on the left, the narrow and lengthy Navigator XL (NVL) in the middle and the smaller Navigator (NV) on the right:
Ohaus Navigator scales

The Navigator (NV) is the only one with different platform shapes within the series: The NV212 – the only scale with 0,01g readability in the entire range – has a round platform as shown in the picture while higher capacity scales have a platform which is similar to the other series.

Dimensions (width x length x height in mm)

Navigator XT (NVT): 240 x 249.5 x 69.8
Navigator XL (NVL): 204 x 282 x 74
Navigator (NV): 204 x 212 x 58

Options

All scales can use the same interface options (RS-232, USB or Ethernet) as they have the same option slot on the bottom. The photo provided by Ohaus shows the Navigator XL (NVL) with installed USB interface:
Ohaus Navigator XL (NVL) with USB interface installed

However, only the Navigator XL (NVL) and XT (NVT) series have room for an optional rechargeable battery:
Ohaus Navigator XL scale with installed optional rechargeable battery

The scales can also be used with non-rechargeable batteries (4 size AA batteries for the NV and 4 sice C batteries for the NVL and NVT).

Performance

All scales stabilize quickly (within one second according to Ohaus) and perform similarly:

Model Repeatability Linearity OCL
NV212 2 d ± 2 d ± 3 d
NV511 2 d ± 2 d ± 3 d
NV1101 2 d ± 2 d ± 3 d
NV2101 2 d ± 2 d ± 3 d
NV4101 2 d ± 2 d ± 3 d
NV5101 2 d ± 2 d ± 3 d
NVL511 2 d ± 2 d ± 3 d
NVL1101 2 d ± 2 d ± 3 d
NVL2101 2 d ± 2 d ± 3 d
NVL5101 2 d ± 2 d ± 3 d
NVL10000 2 d ± 2 d ± 3 d
NVL20000 2 d ± 2 d ± 3 d
NVT1601 2 d ± 2 d ± 3 d
NVT3201 2 d ± 2 d ± 3 d
NVT6401 2 d ± 2 d ± 3 d
NVT10001 2 d ± 2 d ± 3 d
NVT16000 2 d ± 2 d ± 3 d

The values are expressed in scale intervals (d, difference between two successive indications, often also referred to as “(display) division” or “readability”). OCL stands for off center load. To minimize the OCL error, place the weight in the center of the platform. Approved models are not included in the table as their availability varies by region.

Further information

www.ohaus.com