Nitrox/Trimix & CO analyzer

[lidas123]

I also noticed as another user, that keeping it on, after a while He starts and begins to mark me that he is there even if he is not there, and then I have to intervene on the potentiometer and bring it to zero, it is normal or it can be improved ? or what should I do Thanks again

 

[Miyaru]

I also noticed as another user,

I already expected your new account to be a sock puppet...

 

[lidas123]

I already expected your new account to be a sock puppet...

????? I don't know what you mean, maybe you are getting confused

 

[lidas123]

Hello I ask for your help because I don't understand, I installed the md62 sensor with the resistances as close as possible, the 500ohm potentiometer but I can't understand why I don't reach zero, with potentiometer all on one side he still marks me 65mv, You know how to help me please

 

[stepfen]

1. When you rotate the potentiometer, does the output voltage change in an expected manner?? Starting from one end of the potentiometer the voltage should be initially high and decreasing then reach zero and continue decreasing now negatively (unless you are measuring the absolute value) until you reach the other end of the potentiometer for a high (but negative) value. If not there is something wrong with your circuit.

2. What kind of potentiometer are you using??
I think this kind

is too course for this application. I use something like this:

and for this I need to rotate it quite a bit to get it close enough (but not exactly) zero.

3. Also you can calculate the expected resistance values (approximately) you should get at the 4 wires of the connector. Then verify them with a multimeter in your circuit. If your values are not as expected probably there is a mistake in your wiring.

As I have told you few times already the more specific things you ask the more specific answers you will (probably) get.

 

[Miyaru]

The MD61 and MD62 sensor work in a Wheatstone Bridge:

In the above drawing, Rx is unknown. When R2 is adjusted to the exact same resistance, the resistance over the left and the right path is exactly the same.
The voltmeter between D and B will show a difference of zero Volt.

In the schematic for the analyser

• the resistors R1 and R3 are inside the MD61/MD62
• the variable resistor (potentiometer) is at point C, and this must be a 10-turn potmeter.
• two equal resistors are at the R2 and Rx positions
• the voltage supplied to A and C must be 3.00 Volt
• the measurement between D and B is done by the ADS1115 component

The above is simply physics and has nothing to with the analyser yet.

In the sensor, the left pair of pins is connected to a sealed compartment which is the reference.
The right pair of pins is connected to the open compartment - this is the unknown resistor and the resistance varies with the gas analyzed. As described in the datasheet, the sensor must be given time to warm up.

Since a Wheatstone Bridge is extremely sensitive, all connections must be soldered and not placed on a breadboard. This introduces too much variable resistance.
The sensor is very sensitive to carbondioxide, so if you are breathing out over the sensor while turning the little screw on the potmeter, it will keep varying.

Read the link about the Wheatstone Bridge and apply the theory.
Use a multimeter to check that there's a stable 0V and 0mA between D and B.
Check that the power supply is 3.00V.
Using 3.3V will sooner or later kill the sensor.

 

[lidas123]

Found the problem, potentiometer that didn't work

I changed potentiometer and now it works

 

[lidas123]

doing tests with the sketch that has posted Miyaru, thanks to him for what he has shared, I noticed one thing when I do calibration he, starts the calibration from OK and then restarts the calibration alone, what can I do

 

[kelemvor]

For what it's worth, I found the winsen ze15-co for less (including shipping) from China than the winsen ze07-co. Only differences I see on the datasheet appear to be faster warmup/response/recovery on ze15. I got them here: aliexpress two of them with fedex ip shipping to Florida, USA was $40.

Too bad the winsen sensors aren't available on Amazon.

P.S. I have nothing to do with aliexpress, the seller I linked to, or winsen.

 

[Miyaru]

For what it's worth, I found the winsen ze15-co for less (including shipping) from China than the winsen ze07-co. Only differences I see on the datasheet appear to be faster warmup/response/recovery on ze15. I got them here: aliexpress two of them with fedex ip shipping to Florida, USA was $40.

Too bad the winsen sensors aren't available on Amazon.

P.S. I have nothing to do with aliexpress, the seller I linked to, or winsen.

...same output voltage range on the same pin. Can be exchanged with the one I described, without any software changes.