A compensation circuit would require a temperature sensor mounted on the pressure sensor. The temperature signal would then be used in the compensation circuit to provide analog compensation, or by the micro to provide compensation in software. I doubt that you can make such a compensation circuit cheaper then just paying the extra $13 for the compensated device.
Likely the most important parameters for your application are rail-to-rail outputs, low input offset voltage, and low input offset voltage drift.
That's a great deal of work to temperature compensate the sensor just to save a few bucks, especially if you factor in the time/cost of calibrating each sensor over temperature. Why the great concern about cost?
Hi vne147,
why don't you use a Motorola/Freescale pressure sensor type MPX2010DP?
It's a laser trimmed differential pressure sensor with temperature compensation.
The MPX201DP has a range of 10KPa and should be well within the range you want to measure. 1PSI=6.97475793KPa
Here is a circuit with in- and output offset correction and adjustable gain with factors 50 to 1,000.
The schematic uses the MPX2050DP (range 0 - 50KPa), but works with any of these sensors out of the MPX-series.
Regards
Boncuk
Unfortunately, we do not recommend all our sensor (MPXV2010DP) to become in
direct contact with any kind of liquid substance which would damage the gel and
cause the sensor to become out of specification.
The strain gauge and the electronic circuitry for calibration and compensation
are protected by a nitride layer but the aluminum bonding pads which provide
electrical connections between the leadframe and the gauge are not protected, in
order to make the bonding feasible. The complete die is also protected with a
silicone gel. This gel is not fully hermetic, although we use much better gel
for our newer types of pressure sensors, water or any other fluid can penetrate
the gel and can reach the die. When the sensor die is in contact with water
e.g., oxydoreduction reactions between Al/Al3+ and water start as soon as the
sensor is biased. After some working hours, or maybe days, the aluminum pad of
the supply pin is definitely destroyed, so that an open circuit on the Vcc pin
occurs. This is an aluminum corrosion phenomena. But the corrosion phenomena is
stopped when the sensor supply voltage is switched off. There are also some
other failure causes like galvanic corrosion, but the Mean Time To Failure
(MTTF) due to these other causes much longer than the MTTF caused by
electrocorrosion of the Vcc pad. Therefore electrocorrosion is the major failure
cause, and they are permanent.
Eliminate the op amp. I'm using an ADS1100 AD converter which is 16 bits. It costs $4.50. It has a resolution of .1mv per bit. It also has a prescaler that will multiply your input by 8. Feed your pressure sensor into into the AD and then to the microcontroller. With proper design you eliminate all those Op amp trimmers and the calibration process. If you need to you can drive the AD on one input and use a "Zero adjust" trimmer on the other input for simple calibration. This gives you 15 bits of resolution.
Even if you only need 8 to 12 bits of resolution, having 15-16 allows you to select a narrow range of the AD span--no need for op amp scaling or offset.
Also look at the ADS1110 converter.
If I'm missing something obvious that can be done to account for the differences in full scale span from sensor to sensor please let me know. But, the only workaround I see right now is software based.
The air trapped in the system prevents acid to enter the pressure chamber.
Regards
Boncuk
I may be off with this as their data sheet is a clear as mud to me on the sensitivity but I see it this way. If you use 10 Volt excitation on the bridge the F/S output will be 45 mV. The output of the bridge will be a function of your excitation voltage. When using a circuit like this, it is very important that your excitation voltage be stable.
Typically with a bridge transducer the output is specified as mV/V at F/S out. If for example I have a 1 PSID transducer and apply 1 PSI to the high side port I will get a F/S output based on my excitation voltage. The sensitivity could be specified as 2 mV/V so with 10 V excitation at full scale I would get 20 mV out from my bridge.
Looking at their spec it confuses me because where they say typical and mention 10 V it would tell me that at 10 Volt excitation the output would be 45 mV which is 4.5 mV/V. The max excitation is 12 V but they say:
Supply Voltage 10.0 Vdc typ., 12.0 Vdc max.
Followed by:
Sensitivity 45 mV typ.
Full Scale Span 25 mV min., 45 mV typ., 65 mV max.
I would think at 12 Volt excitation you would get 54 mV out for F/S?
Since these things are pretty cheap I would order a few and experiment a little. Apply a precise 1 PSI to the high side port and see how it behaves with various excitation voltages.
Ron
You DON'T need to "excite" a sensor with the rated voltage, unless it's got a built-in amp.
It's just a differential Whetstone Bridge of 4 resistances. The voltage rating is only a limit where the I^2*R heating begins to affect calibration. You can use it on a 2v Vdd if you like.
Note that this is a differential signal, and the differences are small. Most PICs don't have a true differential ADC, and only 10 bits. The offset error due to temp can be very significant if your signal is small. The offset of a differential amp (and the offset may vary with temp) may not only be an accuracy problem, it can cause the output to saturate at + or - too soon and not achieve the intended scale.
Hi vne147,
normally a pressure hose is connected to the sensor's port. The liquid, even sulfuric, won't enter the diaphragm chamber since you only compress air within the pressure hose and the chamber.
Just take care the sensor is elevated above the max liquid level.
You are going to measure static pressure - no dynamic pressure!
The air trapped in the system prevents acid to enter the pressure chamber.
Regards
Boncuk
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