NO3 readings returned for voltage outside of calibrated points

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some_dev
Posts: 2
Joined: Thu Aug 08, 2019 3:46 pm
Company: Net Feasa Ltd.

NO3 readings returned for voltage outside of calibrated points

Post by some_dev » Fri Aug 09, 2019 1:29 pm

I have a Smart Water Ion board monitoring NO3 levels in seawater. It appears as if the sensor is not suited for seawater. However, my question is regarding how voltage readings are used to calculate NO3 concentrations based on the given calibration points.

Given the calibration points:

Code: Select all

// Calibration concentrations solutions used in the process
#define point1 132.0
#define point2 660.0
#define point3 1320.0

// Calibration Voltage values
#define point1_volt_NO3 2.736
#define point2_volt_NO3 2.651
#define point3_volt_NO3 2.615
I would expect ANY point below the lowest voltage point and above the highest voltage point to result in an invalid (-1) NO3 concentration result.
However, some of the reading we are getting don't fit this assumption.
An example below:

Code: Select all

Voltage      NO3 Concentration
2.1819	       -1
2.213	       -1
2.2656	       -1
2.3839	      106322.1
2.3459 	      219111
2.32          358355.7
2.4085	      66656.59
2.409	      65984.44
2.367	      146713.64
2.3826	      108947.72
2.3915	      92031.95
My observation here are twofold,
1) The device is returning concentration results for voltages outside the calibrated voltage points
2) The device is returning results for concentrations that do not lie within the calibrated concentration points.

Is it expected behaviour for the device to try to give values for concentration larger or lesser than the calibrated points?
If so, what is the % margin above and below?

libelium-dev
Posts: 27967
Joined: Mon Sep 28, 2009 1:06 pm

Re: NO3 readings returned for voltage outside of calibrated points

Post by libelium-dev » Mon Aug 12, 2019 8:59 am

Hi,

The readings obtained by the sensor could be lower or higher than the calibration points. The points are used to draw a straight line and the API calculate the concentration values working with this line. If the values obtained is far from the calibration point the result will be less accurate.

Regards

some_dev
Posts: 2
Joined: Thu Aug 08, 2019 3:46 pm
Company: Net Feasa Ltd.

Re: NO3 readings returned for voltage outside of calibrated points

Post by some_dev » Mon Aug 12, 2019 11:35 am

Thanks for that.

Based on you response I have two follow up questions:
1) Is there a recommended offset be between calibraion points? We are looking at using different calibration solution concentrations than the standard.
2) Would there be a way to change the software to require more than three calibration points?

libelium-dev
Posts: 27967
Joined: Mon Sep 28, 2009 1:06 pm

Re: NO3 readings returned for voltage outside of calibrated points

Post by libelium-dev » Mon Aug 12, 2019 3:19 pm

Hi,

There is not recommended offset between points but it's recommended to use solutions with a similar order of concentration of NH3 as the liquid where you're going to use the sensor.

The API has been developed to use 3 points but you can modify it to improve it.

Regards

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