Pressure sensor circuit - explanation?

I was looking for different ideas for measuring water depth for my espresso machine and thought maybe the easiest way to measure water would be to measure the water pressure at the bottom of the water tank. Anyways, I came across this site: http://www.practicalarduino.com/projects/water-tank-depth-sensor

The schematic for the circuit is: |500x346

I have some generally understanding of the circuit, but I'm wondering if someone would be willing to take a few minutes and explain how the circuit works. I really can't figure out U1/4 is doing (maybe part of calibration), why the circuits go through the op-amp again at U1/3, etc.

thanks

Google "instrumentation amplifier". The 1k resistor connected to the output of U1/4 is normally grounded. U1/4 is used to inject a small voltage for offset/trim.

The MPX2010DP sensors is also available with inbuild temp compensated insrumentation amp. I think it's the MPX5010DP. Leo..

Thanks, knowing that term helps.

The MPX5010DP can connect directly to an analogue input (plus 5volt and ground).
Theoretically you have ~40 digital values per 10cm of water with this sensor.
Leo…

For ~$3 more it's definitely worth using the MPX5010.

I'm not aware of any Freescale MPX... sensors that are rated for water immersion. My own experience is that they fail within a few days of getting wet.

If you are determined to use one of these sensors because of the price, then you will need to use a bubbler arrangement so that the sensor always remains dry.

Unfortunately I don't believe there is such a thing as an affordable water rated pressure sensor. Clearly there is a gap in the market, but I don't see any signs of this changing in the near or distant future, but I would like to be proven wrong.

mikb55, you don’t need a bubbler arrangement for a tank that empties semi-frequently (like OP’s espresso machine)

Chagrin: mikb55, you don't need a bubbler arrangement for a tank that empties semi-frequently (like OP's espresso machine)

theoretically, you are correct, as long as the tube can be drained. however, the humidity will be 100% and the surface of the sensor will potentially be saturated. I would advise against this approach if there are any other viable sensing methods.

Is a bubbler a setup where a tube containing the pressure sensor is lowered into the water tank and air is passed down through the tube to ensure that water does not rise up inside the tube? I was thinking about using a closed ended tube but I hadn't considered humidity.

I'm now thinking that I'm better off using a water level conductivity sensor, if I can find a long enough one that will fit through the tank's opening.

bcphysics: I'm now thinking that I'm better off using a water level conductivity sensor, if I can find a long enough one that will fit through the tank's opening.

I wouldn't recommend that for any substance that is for human consumption. There can be electrolysis from impurities in the water which will leach potentially toxic metal from the electrodes and dissolve it in the water. A capacitive sensor avoids this by using insulated electrodes.

dave-in-nj:
theoretically, you are correct, as long as the tube can be drained.
however, the humidity will be 100% and the surface of the sensor will potentially be saturated.
I would advise against this approach if there are any other viable sensing methods.

Freescale references AN1950 right in the MPX5010 datasheet – how to use it for water level sensing. One of its target applications is for use in washing machines.

You can make a very simple and remarkably accurate external capacitive water level sensor, as shown here.

Thanks for the replies. I've been thinking about this for a while and wonder if a decent solution would be as explained here: http://www.electroschematics.com/9964/arduino-water-level-indicator-controller/

My primary problem with my espresso machine is that I don't want to run out of water while making a shot of espresso. So I really only need a binary reading of water level - I need to know when the water falls below a certain level.

A float switch is good for that. There's lots of different types available. Most are "automotive" sized, which makes them a bit big for a coffee machine but there's no reason why it shouldn't be possible to make it fit.

Hi,

Anything you put in your machine will have to be "Food Grade" for health reasons.

google food grade float switch

You will be amazed at what is available.

Tom.... :)