Building a pressure reader

[Vlady]

As far as using the data the answer would be defiantly yes because this is manufacture calibrated and they guarantee on the accuracy as long as I use this points. I know this just gives me more work but it has to be done

 

[Vlady]

"Method 1
Produce a linear equation (y=mx+c) for the data between each pair of calibration points. You will have six equations for your seven data point pairs.
Write the software, which will:
Read the ADC
Compare the voltage with the calibration points
Calculate the pressure using the appropriate equation
Display the pressure."[/quote]

Jim
I looked at the website and I like Method 1 it seems to fit best of what I am looking for. Now it is the software part :cry: This is going to be something that I will need lots of help with [/code]

 

[Nigel Goodwin]

It's still rather unclear what you want?, from your later postings I 'presume' you only want to use a small portion of the sensors range?, and expand it to read full scale on your output device?.

If so, it's best (and easiest) to do it in hardware, you can use an opamp between the sensor and A2D, and convert the required span of the sensor to 0-5V (or whatever the A2D needs). You adjust the DC offset of the opamp to provide 0V out at your minimum reading, and adjust it's gain to give 5V out at your maximum reading.

Personally I've always thought 16 bit A2D's rather a waste of time?, their extra resolution is likely to disappear into circuit noise - plus where do you get the rest of your components to such a degree of accuracy?.

 

[Gorgon]

When manufacturer calibrates this they calibrate it to this pressure setting and the voltage readings at that pressure. Never mind this all I need to know how to program it so it will do:
“HPA Vdc(output)
800hPa -0.004Vdc
820hPa 0.2857Vdc
850hPa 0.7151Vdc
870hPa 1.0009Vdc
900hPa 1.4294Vdc
920hPa 1.7152Vdc
950hPa 2.1436Vdc
Example:
Lets say it is reading current voltage of 0.14285Vdc so what I want it do is give me a read out based on looking only at 800 to 820hPa instead of the hole range of 800 to 1150hPa. So when it gets the voltage reading of 0.4932 it will give me corresponding readout based on looking only from 820 to 850hPa and so on”

Does this makes much since??

Hi Vlady,
I just wonder, is the first value in your table correct? Or are you missing a digit? If you say that the correct value is -0.0004Vdc it makes more sense!

If we assume the value to be -0.0004 the m in the equation (y=mx+c) would be in the range of 69.866554 to 70.028011 or 0.25% absolute or +/-0.125% ove the range. The value 69.979006 is the most common, repeated 2 times. Using this should qualify over the full range. To make the equation and measurement simpler you could adjust the offset point to 0.000000. The 'c' value should then be 800.02799.

The big question is then how you would implement all these digits to an 8bit micro. Using the formula and the reference values the 950hPa will be 950.034987. Not too bad and well inside the +/-0.05% FS. It is even well inside the +/-0.05% range accuracy.

TOK

 

[Vlady]

Hi Gorgon
Thanks for your response…
Yes the value -0.0004 is correct calibrated in the factory. This is the accuracy that I have to work with actually it can be only to 4 digits should be accurate. To lot of people it may seem strange but it is not for what I am doing. And I don’t think I want to use 8bit I agree with JimB and others on using 16bit. I think this would give me plenty of digits but will be more difficult to work with..

 

[Nigel Goodwin]

Hi Gorgon
Thanks for your response…
Yes the value -0.0004 is correct calibrated in the factory. This is the accuracy that I have to work with actually it can be only to 4 digits should be accurate. To lot of people it may seem strange but it is not for what I am doing. And I don’t think I want to use 8bit I agree with JimB and others on using 16bit. I think this would give me plenty of digits but will be more difficult to work with..

Most PIC A2D's are 10 bit, this gives potentially just over 0.1% resolution, but where are you going to find your 0.1% resistors to build the rest of the circuit?.

 

[Gorgon]

Hi Gorgon
Thanks for your response…
Yes the value -0.0004 is correct calibrated in the factory. This is the accuracy that I have to work with actually it can be only to 4 digits should be accurate. To lot of people it may seem strange but it is not for what I am doing. And I don’t think I want to use 8bit I agree with JimB and others on using 16bit. I think this would give me plenty of digits but will be more difficult to work with..

Hi Vlady,
I think we talk about two different things. I did not mean the AD converter, here you need at least 12 bits or more. What the increased value of a 16 bit converter gives you is very dependent on the hardware layout of your design. You can easy loose the last 2-3 bits accuracy in noise from the sourounding electronics.

I was talking about the processor you select for this project.

TOK