Hi everyone,

I am trying to get correct values for measuring 03 concentration through MiCS-2610 03 sensor but I have some doubts after reading the datasheets.

A) You advise me in other post that in the output of the sensor we get voltage values in voltios, so suposse we obtain VS value, isn't it?

So, with VS value, Vcc value and RL (R load) y can get the value of Rs.

B) I supossed that Vss is the value that I give for my sensor board, and I am using a 3,7V battery but when I habe been reading your datasheet I have readen that this sensor is powered trough a 2,5V regulator, which is the correct value to use??

C) After ir I have continued with datasheet sensore and I see we have a RL resistance between pines 4 and 1, which is the RESISTOR parameter in thenext function:

SensorGas.configureSensor(SENS_SOCKET2B, GAIN, RESISTOR);

In GasSensor datasheet of e2V put that air resistance takes values between 3 and 60 kohms (typically 11kohms), but in your datasheet put it takes values betweenAir resistance: 100 ~ 1000kΩ, what's the correct specification?, is air resistor the value of RL, how can I get it? Should I suposse it is 11kohms??

D) If I obtain RS value ok I have to interpretate it with the graphics RS/R100ppb-ppb, what's the value of R100ppb??

Thanks,


MIMG

I would like if you could provede us with the formula of the graphic interpretation, since there is no way to get an exact value if we only have it on paper.

Thanks you so much,

MIMG

Hi MIMG,

I answer here to your questions:

A) All you mention is correct, but take into account that the Vs measured is not referred to the supply voltage, but to ground. Use the formula provided in section 2.0 for the conversion.

B) You should take 2.5V as Vss for the O3 sensor, since it is powered from the regulator. Anyway, the 3.7V from the battery are never directly used in the mote, they are regulated at 3.3V or boosted up to 5V.

C) Yes, RL is the load resistor used to form the voltage divider, you can configure it with the function you mention.

There is a little difference between the terminology of the data sheets. R0 is the resistance at 100ppb, which is taken as a reference to plot the graph and may take the values between 3k and 60k, but is typically of 11k. Using this resistance as reference and calculating the resistance of the sensor at the operation conditions you can get the gas concentration from the graph.

I haven't found that 100 ~ 1000k resistance you mention, could you tell me where did you see those values?

We haven't a formula and we cannot extrapolate it for a general application since there is a great variability between sensors of the same model. If you don't need a great accuracy you may assume typical values and try a linear approximation (with logarithmic concentration) by sections in your working range.
For a better accuracy, you will have to calibrate the sensor.

Regards.

I have a similar inquiry too: -



a. does the value GAIN = RS/RO or is it something else?

b. does the value RESISTOR = RL or is it RS ?

If so, during calibration what are the values recommended for GAIN and RESISTOR ? For example if I need to calibrate the methane sensor. I am required to find the Sensor resistance in 5000 PPM of methane (R0) and corresponding to this value I get a voltage (Vout) then I can find Rs using the formula in section 2.0. But to work out R0 I need the correct RL value but so how do we define what R0 is ?

Hi AmroQuandour,

A. No, that is the gain of an amplification stage used in certain applications when working near the extremes of the range.

B. With RESISTOR you configure the value of the load resistor RL, Rs is the resistance of the sensor in a given concentration.

During calibration, I'd advise to set the gain at 1. For the load resistor, look at the graph the resistance of the sensor for the middle of your working range (in your case, since you have a specific value for which you want the calibration, 5000ppm) and calibrate it using that value, so you can calculate the exact resistance of the sensor from the voltage you get.

Regards.

yes I understand. But it becomes an issue because you have 2 unknowns and 1 equation. I mean for example the methane sensor: - I got a relationship for the graph in log scale.

let Rs/R0 = y

so : -

y1 = 1 , y2 = 2
x1 = 1500, x2 = 1000

the equation expressed is: -

log(y) = mlog(x) + log(b)

so to find m : -

m = ( log(y2) - log(y1)) / ( log(x2) - log(x1) )

for the above values: -

m = log ( 2/3 ) >> slope

to find the b (intercept) : -

log(y) = Log(2/3)log(x) + log(b)

log(1) = log(2/3)log(1500) + log(b)

log(b) = -log(2/3)log(1500)

so the whole equation is

log(y) = log(2/3)log(x) - [ log(2/3)log(1500) ]

or

log(Rs/R0) = log(2/3)log(x) - [ log(2/3)log(1500) ]

Now when I need to configure the sensor

SensorGas.configureSensor(SENS_SOCKET4A, GAIN, RESISTOR)

I must choose the right RL to get the right Rs

because if I try to solve for Rs from the above original equation, I don't know what R0 is ... do you understand what I mean?

During calibration I have to set an RL value but how do I choose it if I don't know what the real R0 is ! !!!!

Hi AmroQuandour,

I know what you mean, let me explain myself. The idea is that you get an estimation for Rs from the graph provided, then you use this approximation as RL to calibrate, and after calibrating, from the RL (estimated Rs) and the output voltage, you get the real Rs value.

Regards.

Ah I think I figured it out: - The method is divided into 2 parts: - First part is non-exposure (I.E theoretical calculations ) and the second part is gas-exposure (measured values)

NON-EXPOSURE : -
--------------------------------------------------------------------------------------------------------------
suppose that my values are as follows: -

SensorGas.configureSensor(SENS_SOCKET4A, GAIN, RESISTOR)

let's say I used : -

GAIN = 1 , RESISTOR = 1 Kohms .

I obtain a voltage of approximately: - 3.3 Volts

then using the following relationships which I derived: -

let Rs/R0 = y >>>>>>>>>>> (1)
y1 = 1 , y2 = 2
x1 = 1500, x2 = 1000
m = ( log(y2) - log(y1)) / ( log(x2) - log(x1) )
log(b) = -log(2/3)log(1500)

log(Rs/R0) = log(2/3)log(x) - [ log(2/3)log(1500) ] >>>>>>>>>>> (2)

: . Using the equation in 2.0

Rs = ( ( Vc - VL)RL ) / VL

if I replace this equation into (1) : -

y = Rs/Ro = [ ( ( Vc - VL)RL ) / VL ] / R0 >>>>>>>>>>>>(3)

Now for a concentration of 5000 ppm => Rs/Ro = 1

[ ( ( Vc - VL)RL ) / VL ] / R0 = 1

Vc = 5 V
VL = 3.3 V
RL = 1 Kohms

Therefore: -

Ro = (Vc - VL)RL/VL = (5-3.3)/3.3 = 0.515k ohms

Now theoretically Rs @ 5000 ppm should equal to 0.515k ohms at RL = 1 kohms

So when I go to calibrate using these values RESISTOR = 1 GAIN = 1 then using 5000 ppm of methane gas then theoreically I should get : -
RS = 0.515k Ohms.

and based on this value I can then get the sensitivity.

Is this correct?

Sorry

m should be equal to = log(2) / log(2/3) I made an error