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Lower cost sensor and pump alternatives?

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Joined: 7 months ago
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I'm curious if anyone here has recommendations for lower cost sensor and pumps? I, like many it seems, really want to emulate what Kyle was able to achieve with his NFT system, but I'm stunned at the cost of some of the Atlas parts (specifically the EC sensor - $250?!). While I search around for various alternate sensors and pumps, curious if there are some I should avoid or any lower cost alternatives someone might suggest? If my first experiment goes well, I plan on expanding it, so at that time I may invest in higher quality parts - but right now, for this experiment, I'd like to keep it lower cost!

Kyle Gabriel
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Joined: 7 years ago
Posts: 612

You can use any pump, really. There's a Generic Pump Output that powers a pump for a duration in order to move fluid, rather than supplying a volume to dispense. Furthermore, if you measure the dispense rate of your pump and set the Rate setting of the Output, you can supply a desired volume to be dispensed and the Output will calculate and power for the duration required to dispense that volume.

Check out this forum post from a user who sourced the same pumps Atlas Scientific uses, without the controller board:

For the water sensors, there are a few users who have used cheaper alternatives, with varying success. Often you will have to work with an ADC or develop your own Input module. I haven't used any other pH or EC sensors, so I haven't made any alternate Input modules myself. Additionally, many of the cheaper probes are not lab grade and cannot reliably be left in solution for long durations like the Atlas Scientific/lab grade probes can. 

Mycodo Developer

aka liked
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Joined: 9 months ago
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You can just use some cheaper probes than the atlas ones and use them only with the atlas ezo circuits for ph, ec... and voltage isolators for each circuit. Because every ezo circuit is suitable for any Probe Brand, according to the description on their Website.

So ph, temp, ec and orp probes (20-50€ each) with all required boards from atlas will cost about 300€. 

I will do so and let you know the result. 

Im expecting a short life span of the probes but i also dont have the money to go crazy. 

This post was modified 7 months ago by danton

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First I'll say that Kyle's NFT Hydroponics post was a huge inspiration, and was the main impetus for me to jump onto the Mycodo bandwagon after struggling with writing my own Arduino code to control all the disparate systems in my hydroponics setup. But being cheap, I just couldn't deal with buying the Atlas Scientific EC and pH kits that he'd recommended, and I really wanted to figure out cheaper alternatives.

I ended up purchasing cheaper ones from DFRobot to prototype my ideas: the Gravity Electrical Conductivity Sensor V2 K=1 and the Gravity Analog pH Pro Kit V2 (appears identical to the cheaper Gravity Analog pH Kit V2 but upgrades the "lab grade" probe to "industrial" and doesn't come with pH calibration solutions). Each of these kits comes with a probe and a sensor board to drive the probe and convert the signal into an analog voltage that can be measured with an ADC. I chose the Gravity ADS1115 for this purpose. Everything came together in the mail about a week after my order.

As I half expected, using both probes simultaneously results in problems (this is why Atlas charges a premium for their sensor boards!). For example, if I measure the pH of my hydroponic reservoir with the EC probe also inserted, a non-physical pH of 21 is reported. As soon as I remove the EC probe either from the reservoir, or disconnect the immersed probe from the analog sensor board, the pH jumps back to the correct value. Essentially, the solution closes a galvanic circuit between the EC and pH probes, so that one measurement perturbs the other.

The Gravity EC board puts out an oscillating square wave at the BNC output centered around 0V, and measures the AC electrical conductivity of the solution. I suspected that the EC sensor board had some kind of DC bias relative to the pH sensor board, so I tried to measure this with an oscilloscope on the EC. However, there was no detectable DC offset that could be measured against either terminal of the Gravity pH BNC output in air. I suppose that one forms due to electrochemical potential differences when immersed in solution, but I didn't investigate further.

What I did next was simply to snip the leads of the EC board's BNC output connector where it connects to the board, and spliced in 1uF capacitors I had lying around. I used the Murata GRM21BR71A105KA01L which are 1uF 10V X7R MLCCs, and anything similar should work. The series capacitors simply block any DC bias at the board input and allow AC signals to be passed, and I can now run both EC and pH sensors simultaneously. Seems like an easy fix for the folks at DFRobot to implement, if they're reading this...

So there you have it: a way to save some money without shelling out the big bucks for the Atlas kits. All I have left to do is to calibrate the pH sensor -- I'm waiting on $17 worth of pH standards to arrive in the mail -- and in hindsight the regular Gravity pH kit with the included calibration standards might have been a better deal. Next step for me is to figure out how to duty cycle the EC and pH measurements (turning off the sensor boards between measurements) in order to minimize electrochemical side-reactions and preserve probe lifetime. Then I'll have to replicate this setup for each reservoir I have...

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@dookaloosy Thanks for the detailed write up, that was very insightful!


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