For a workshop I had initially planed to order the 100 kg version of the Mavin NA1 via Maritex / Poland, see also Welche Wägezelle für low-cost-Waagen? - #10 by clemens. I had ordered some test pieces before, I did not test the load cells extensive but they seemd to be ok.
Maritex could not deliver 40 pc. timely, so I searched for alternatives and found the one from Omega. In the end I had not time to check the cells against temperature stability before and I ordered the needed amount for the OpenHive ESP32 / TTGO T-Call SIM800C workshop.
Beginning of the measuring periode you may find hives with a constant weight. In the meantime some beekeeper had put the scale under the hive and the temperature sensor in the hive so we can get no comparison data from this hive.
I would not say closing that the Omega E120A is not usable. We have used the load cell in a primary not intended setting. So it is a single point load cell that “expect” a single load on top, fixed only on one point and apart from that “floating”. We use this single point cell in a two point setting and so forces from other directions than top to bottom can occur. So my conclusion on this point of time would be: Checking the load cell in a single point setting and see if the temperature effect is still there and maybe then trying to minimize this side forces, e.g. by a thick rubber layer and check temperature effects then in a two point setting again.
I did a lot of tests and collected much data with the Bosche H30 for this setting: Open Hive "Scale Bar" for Single Side Weighting and I evaluated the load cell – or at least the pieces I got as reliable and less temperature related by using this cell in a single point setting. But using this load cell in a double setting, in this case it was a scale on the side of a bee hive and on the other side a piece of wood, leads to unexpected steps in the weight graph. It was not the picture of a 1:1 temperature related effect more a sudden shift in the graph. So It may be related to suddenly moving material triggered by heat influence or you have two independent effects, side forces and temperature.
This may be a nonsatisfying answer for you but it is a more “scientific”, we have to do more research or “it belongs” … :-)
Even if we judge the Bosche H30 as a good load cell 50 EUR plus tax is far too much for a low budget DIY project, especially in a setting with two load cells. So Bosche is imo no option for a really cheap scale. But perhaps you may tell us more about your plans and how related a cheap price is to that.
You are asking about the commercial scales. Years ago you could not find any scale with tow load cells, as far as I remember Wolff Waagen was the first commercial scale with two load cells. All other used single point settings. But in the meantime you have our colleague from https://easyhive.org/ or https://www.beehivemonitoring.com. The last one has scales with a quite broad variance for the bathroom cell we should not discuss ;-) but also the XS with +/- 10 g! But I would not count on it. Perhaps you get weight scores in 10 g steps but I fear temperatur effect is not extraordinary better than we have seen. I think the luck of the commercial scale is that nobody put a static load on it over a long time with changing temperature. There will be always a bee hive / rain / snow / a beekeeper and so the temperatur effect is not obvious.
The cheap load cells we are using are produced for a grocer’s shop setup. You unload a scale tare it, put apples on it, measure the weight and so on. There is no load over time and without a tare procedure intended. So I fear we have to search for load cells that fit our needs by chance.
Honestly, I’m trying to do a reliable solution with a limit of 100€ (micro, load cell, sim, etc…) it could fit my needs.
I exactly agree with you.
Reading many topics, articles and talking with different persons (I was at an exhibition in Italy a month ago to scout it) I’ve arrived at the same conclusion.
These days I thought to contact someone that sells a commercial scale to test one with a fixed weight. Then I could buy it if it will report the correct value for at least a week (but as you said I’m not so confident that commercial-scale works well).
In that case, I’ll go to x-rays the scale bought to understand all the secrets.
I fear that the Zemic BM11 with “stainless steel, hermetically sealed” will blow the budget of 100 EUR by far. I would expect the Zemic alone will cost 100 EUR or more.
Have a closer look at the NA151 you have linked. On this load cell you have only one big screw thread for a single screw feet per load cell. So you have to use 4 load cells for this setup, see Comparison of Different Load Cells for a 4-Point Scale and Füße für Waage mit 4 Wägezellen - #15 by clemens. For a setting with two load cells you have to use cells with 2 threads each so you can fix a L- or U-profile on the top and bottom side. 200 kg is also a bit high. It is better to have a oversized scale than a too week one, but with a 100 kg load cell x2 you have still 200 kg it can measure and in the most cases you have 150 % “limit load” and 300 % “breaking load” so you are still save with 100 kg Emax!
A lot of belongs to your scale design. especially the mechanical part will eat up some money. It is also difficult to switch from one to an other load cell if the form factor is changing. So I would first decide / fix a scale design (1 load cell, 2 load cells or more), then fix the form factor and then choose a load cell. My current favourite design is the 2 bars variant as descibed on Waage OpenHive 2bars [für T-Call] I found for this load cell form factor also many cells, see Waage-Konstruktion mit 2 Wägebalken und 130 x 30 x 22 mm Wägezellen but I did not keep up with a systematic test for this load cells. So in case you are also interested in this it would be a welcome support and motivation to go on in this direction. On (or under :-) my desk are the 130x30x22 cells from Omega, Marvin (as I remember both 100 kg) and a pair of CZL601 but only 50 kg so it could be a gola to do some systematic tests till Christmas.
Btw will it be an open source project you are planig or a commercial one?
As promised yesterday, I have deeply read what you wrote.
The Zemic’s sales contacted me and we planned tomorrow a brief call (when they contact by phone the price is high )
I’m a practioner and you are the expert.
You have more experience than me about it. I take into account your suggestions.
I admit to having not considered the mechanical part in my budget. I have always thought the most difficult part of the project was the electronic side, whereas, once a time I would have finished load cell, micro, etc… then I will go ahead with the mechanical part. Reading your words this approach it’s not the correct strategy and I’m available to change it.
My goal is to find a stable solution to use next season and I’m available to invest time and not much money to reach it.
Let me know how you want to proceed and If I could help you in some way.
A systematic check of different load cells regarding temperature would mean to build scales with one “scale bar” (not two), put a constant load on it, add a temperature sensor and set it up outside exposed to the current day and night temperatures. Log all data and compare weight and temperature over time.
You will always have to use a load cell amp, but this is not for the excitation voltage but amplifies the signal from the load cell to the microcontroller. The load cell signal is in the mV range, so a normal Arduino / ESP can not handle such a small signal with the built-in ADC. You need an external PGA and the load cell ICs also have an ADC with higher resolution.
I’m an employee of Maritex. And I’m the PM responsible for the load cells. To follow the topics on this forum, I registered to answer some questions and comments you add here.
Unfortunately, as you know, there are very big problems with the availability of electronic components all over the world. This problem also applies to load cells. MAVIN - of which we are a distributor, since this year produces only ordering products, has very small quantities available immediately from the warehouse.
That’s why we have problems with some models (mentioned by @clemens - model NA1-100kg). Load cells that are not very popular, MAVIN produces only under order and requires a large amount to start production. Therefore, we are having availability issues, for which I apologize. Please contact me on PM, if you are interested in any model, I will try to help.
We have a large beekeeping market in Poland, so bee scales are a popular projects. My experience and purchases of our clients show that they use different models: NA2-100kg, NA3-150kgml, NA4-200kg. Also NA151-60kg- 4 pcs. in one scale. NA151 have recently been very popular in our clients’ projects. If you have any questions, please let me know and I’ll try to help.
As compensation for accessibility problems, I can offer a -10% discount code for our online store. If anyone would like such a code, please contact me on PM - I’m not sure if it complies with the regulations, so I don’t want to paste it here. If admin agrees - I will add the code in this message )