Dissolved oxygen data acquisition

[Capega]

Hello 

I am trying to connect a dissolved oxygen sensor to the 9219 module in the NI cDAQ-9178 chassis using Labview software (DAQ Assistant). The manufacturer of the sensor is applisense and the sensor comes with a BNC cable. Is it possible to acquire a signal wiring only the BNC cable to the 9219 module HI and LO ports in voltage mode? or do I need a driver to meke possible the acquisition?

I hope you can help me.

Thanks

[Ardent]

Hi Capega!

Could you please provide some more information about your setup?

To be more precise:

-Do you have the output range of your dissolved oxygen sensor?

-And s your signal source (sensor) grounded or ungrounded (floating)?

If you have the 9219 up and running, you can acquire the signal by just wiring the BNC cable to the 9219 HI and LO ports, if the range criteria is met. Then however you still have to convert your sensor input in a meaningful format (you should know the shape of the output signal, so that you can convert it later on).

Hope this helps!

[Capega]

Hi Ardent!

Actually I don't have the info of the output range of the sensor in Volts, the manufacturer uses a controller in which has an analog output of 0/4-20 mA. Do you think I could make the acquisition in mA? 

The only wires I connected were from the BNC connector (sensor) to the HI and LO ports (9219), I believe I need a known scale as ypu said because when I configure the DAQ Assistant with min an max values -1 and 1, respectively, the connection diagram appears without wires connected to the port, but when I use -4 and 4 as min and max the wires in the diagram get connected. So Ia start the data adq., The problem is that I don't see a meaningful change in the signal measured by the sensor when I change from a solution with 0% of dissolved oxygen (DO) to a solution with a higher percentage of DO. Could something be wrong with my connection?

Thank you for your answer! 

[lfelias]

Hi Capega!!

I have been reading about this issue, and I have to tell you that we did that in Mexico with a PH sensor, if you don´t have the values, what you have to do is to measure known oxigen levels in order to make the scale of Oxigen vs mA.

Also it is important to see if the sensor is linear, if not, you will need to make this compensation programatically.

As long as you know the device pin out you can do it. It will take longer than if you know the signals but it will work.

Also you might be able to ask the company that makes that sensor how it works, if you are lucky they will let you know!!

Regards,

[lfelias]

Also, what you can do is to send to your input a known current level to see if it works, it should be able to see that level, even goes from 0 to 20 mA.

Here is the link of the manual http://www.ni.com/pdf/manuals/374473e.pdf in which you can find the connection diagrams and other useful stuff.

I will be taking looks of this forum if you have any doubt.

Regards,

[Capega]

Hello

I tried what you told me last time, I send to the 9219 a known current and it responded well. But still I am not able to acquire a good signal from the sensor, when I connect the BNC cable to the 9219 (current mode, 1 sample, 10 hertz) it doesn't appear to show any significant change as I put several solutions with different values of dissolved oxygen. It seems like nothing conneted to the 9219

I have forgotten to tell you that the sensor needs a polarization voltage of -675 mV during 6 hours to give an accurate value according to the manufacturer, so in order to polarize it I connected the BNC cable to the 9264 module and I set the required constant value of voltage. 

I have also tried to measure with a multimeter the current of the sensor when disconnected but it doesn't show signal. Do you know if it is normal? or is the current too low?

Thank you for your last answer

Regards

Capega

[lfelias]

Hi Capega,

Well, I don´t know that specific sensor; however, if you are using a Multimeter with a suficient resolution, that low current signal shouldn´t be a problem. We might be not giving the sensor the right exitation or something like that.

Do you have the connection diagram so we can take a look at it to try to figure out how to connect and exite it??

Regards,

[Capega]

Hello

Actually I don't have the connection diagram of the sensor, the only I know is that is has two lines to connect and it must be polarized with a voltage of -675mV. The brand is applisens.

The last test I made was to connect the sensor directly to a multimeter and see the signal in V, it showed a signal around 20 - 150 mV but when I connected the sensor to the NI 9219 I could observe in the screen a reduced signal of 0.01 - 0.05 mV. I tested the resistance of the wires of the sensor with the multimeter and the sensor itself and they gave me a 0 ohm signal.

I have tried other voltage sources like batteries and the NI 9219 responded well

Do you know why is being reduced the signal?

how can I acquire the whole signal from the sensor? 

Thank you 

Regards

[Capega]

Hello

Actually I don't have the connection diagram of the sensor, the only I know is that is has two lines to connect and it must be polarized with a voltage of -675mV. The brand is applisens.

The last test I made was to connect the sensor directly to a multimeter and see the signal in V, it showed a signal around 20 - 150 mV but when I connected the sensor to the NI 9219 I could observe in the screen a reduced signal of 0.01 - 0.05 mV. I tested the resistance of the wires of the sensor with the multimeter and the sensor itself and they gave me a 0 ohm signal.

I have tried other voltage sources like batteries and the NI 9219 responded well

Do you know why is being reduced the signal?

how can I acquire the whole signal from the sensor? 

Thank you 

Regards

[lfelias]

Hi, sorry for the delay, I was out of the office last week.

The module 9219 is a Whistone bridge module, which means, the sensor you are conecting to should be a resistor variator. If you are trying to measure the voltage itself, It might not be able to measure the voltage.

You should be using a voltage input module.

Regards,