user10738: I have an mpx12DP pressure sensor that I want to use to measure the depth of water in a 1m tall water tank.
But it only measures up to 55mV of voltage change, which the arduino can't really detect changes in. (The sensor measures 0 to 1.45psi- 1m of water should give me 1.4psi, so I should be able to get really good accuracy).
Is this as good sensor to use for this purpose? How do I amplify the signal from the sensor? I have tried an LM324 op-amp, but couldn't get it to work (this may be entirely the wrong thing to use! And I was utterly unsure of the kind of circuit to use it in). Any advice on parts.. schematics.. a suitable already-amplified sensor that reads only up to 1.45psi?
Thank you,
Pressure sensor datasheet: http://www.digikey.co.uk/product-detail/en/MPX12DP/MPX12DP-ND/684659
Op amp datasheet: http://www.ti.com/lit/ds/symlink/lm124-n.pdf

user3917: It is reasonable to use  analogReference(INTERNAL);, as suggested by Alexander M, to set the range of the ADC to 1.1 V.  
Another alternative is to use two resistors to make a voltage divider, to make an external reference of 1 V (min.) at the AREF pin, and then use  analogReference(EXTERNAL);, but the Arduino's supply voltage is likely to be noisier than the internal reference.
Regarding tiggun's suggestion about grounding the V- pin, it isn't clear to me that the mpx12DP will function correctly with V- and ground connected together.
In any case, you can use two or three of the op amps in the four-op-amp LM324 package to make a differential amplifier or an instrumentation amplifier.  See figures 38 and 39 on page 20 of the LMx24 datasheet mentioned in the question.  Figure 38 shows a differential amplifier; you could, for example, replace R3 with a 50KΩ resistor and R4 with a 1MΩ resistor to get a voltage scaleup of about 21, via the formula shown: Vₒ = (1 + R4/R3)·(V₂-V₁).  Alternately, you can use a small variable resistor as in figure 39 to make a variable-gain instrumentation amplifier.

user4664: You could try using analogReference(INTERNAL);, to reduce the upper range of the ADC to 1.1volts, thus giving you better voltage resolution.

10738: <p>I have an mpx12DP pressure sensor that I want to use to measure the depth of water in a 1m tall water tank.</p>

<p>But it only measures up to 55mV of voltage change, which the arduino can't really detect changes in. (The sensor measures 0 to 1.45psi- 1m of water should give me 1.4psi, so I should be able to get really good accuracy).</p>

<p>Is this as good sensor to use for this purpose? How do I amplify the signal from the sensor? I have tried an LM324 op-amp, but couldn't get it to work (this may be entirely the wrong thing to use! And I was utterly unsure of the kind of circuit to use it in). Any advice on parts.. schematics.. a suitable already-amplified sensor that reads only up to 1.45psi?</p>

<p>Thank you,</p>

<p>Pressure sensor datasheet: <a href="http://www.digikey.co.uk/product-detail/en/MPX12DP/MPX12DP-ND/684659" rel="nofollow">http://www.digikey.co.uk/product-detail/en/MPX12DP/MPX12DP-ND/684659</a>
Op amp datasheet: <a href="http://www.ti.com/lit/ds/symlink/lm124-n.pdf" rel="nofollow">http://www.ti.com/lit/ds/symlink/lm124-n.pdf</a></p>
 3917: <p>It is reasonable to use  <code>analogReference(INTERNAL);</code>, as suggested by Alexander M, to set the range of the ADC to 1.1 V.  </p>

<p>Another alternative is to use two resistors to make a voltage divider, to make an external reference of 1 V (min.) at the AREF pin, and then use  <code>analogReference(EXTERNAL);</code>, but the Arduino's supply voltage is likely to be noisier than the internal reference.</p>

<p>Regarding tiggun's suggestion about grounding the V- pin, it isn't clear to me that the mpx12DP will function correctly with V- and ground connected together.</p>

<p>In any case, you can use two or three of the op amps in the four-op-amp LM324 package to make a differential amplifier or an instrumentation amplifier.  See figures 38 and 39 on page 20 of the <a href="http://www.ti.com/lit/ds/symlink/lm124-n.pdf" rel="nofollow">LMx24 datasheet</a> mentioned in the question.  Figure 38 shows a differential amplifier; you could, for example, replace R3 with a 50KΩ resistor and R4 with a 1MΩ resistor to get a voltage scaleup of about 21, via the formula shown: Vₒ = (1 + R4/R3)·(V₂-V₁).  Alternately, you can use a small variable resistor as in figure 39 to make a variable-gain instrumentation amplifier.</p>
 4664: <p>You could try using <code>analogReference(INTERNAL);</code>, to reduce the upper range of the ADC to 1.1volts, thus giving you better voltage resolution.</p>
