What is the max connection between a I2C sensor and the Prop pin.

msiriwardena

I am woking on a mechanical ventilator(Respirator) and I require very accurate pressures values( 0- 20 cm H2O) to be read at the patients mouth.
The pressure sensor is mounted on the board which is about 5-6 feet away from the tube that onters the patients mouth,since air is compressible
there is some loss of pressure at the receiving end(sensor) - so the sensor readings are lower compared to real.

Of the possible solutions are : 1.To mount the sensor close to the patient as possible (in the tube) and then rune the wires from the I2C sensor to the
prop - SDA,SCL and power
2.Come up with a formula to calculate the drop in pressure/foot of the sensor tubing.
If the distance is not an issue the I2C pressure sensor can be placed in the tubing but that will be very expensive as the patient tubings are disposable
hence will be very costly.

Any help will be appreciated.

Siri

P.S: The sensor data sheet is attached.

N8VHFBrian

I ran into a similar problem for a temperature sensor.
Take a look at this IC from NXP. The datasheet gives some insight into the reasons behind the distance limits as well as info on the chip.

http://www.nxp.com/documents/data_sheet/P82B715.pdf

Hope this helps a bit.
-Brian

Mike Green

You're using a relatively expensive sensor with the A/D converter and a calibration microcontroller built-in. At 100KHz clock speeds, it should be able to work 5-6 feet from the Prop. You'll need twisted pair wiring or shielded wiring over that distance.

If the elasticity of the tubing is reasonably consistent, you should be able to calculate the drop in measured pressure per foot in the tubing. It would be temperature sensitive and probably sensitive to the pressure as well. You'd have to do some experimenting there to develop calibration data. You'd want to look at different manufacturers' tubing as well.

SRLM

The I2C bus specification has this to say:

17.3 Wiring pattern of the bus lines

In general, the wiring must be so chosen that crosstalk and
interference to/from the bus lines is minimized. The bus
lines are most susceptible to crosstalk and interference at
the HIGH level because of the relatively high impedance of
the pull-up devices.

If the length of the bus lines on a PCB or ribbon cable
exceeds 10 cm and includes the VDD and VSS lines, the
wiring pattern must be:

---

SDA

---

VDD

---

VSS

---

SCL
If only the VSS line is included, the wiring pattern must be:

---

SDA

---

VSS

---

SCL
These wiring patterns also result in identical capacitive
loads for the SDA and SCL lines. The VSS and VDD lines
can be omitted if a PCB with a VSS and/or VDD layer is
used.

If the bus lines are twisted-pairs, each bus line must be
twisted with a VSS return. Alternatively, the SCL line can be
twisted with a VSS return, and the SDA line twisted with a
VDD return. In the latter case, capacitors must be used to
decouple the VDD line to the VSS line at both ends of the
twisted pairs.

If the bus lines are shielded (shield connected to VSS),
interference will be minimized. However, the shielded
cable must have low capacitive coupling between the SDA
and SCL lines to minimize crosstalk.

I2C Specification

msiriwardena

Thank you.For all the expert info regarding I2C bus.
Mike I plan to measure the pressures at both ends with the range of pressures I am going to deal and come up with a formula to calibrate the readings as this will eliminate the I2C bus issues.

Thanks again for the info.

Siri

Duane C. Johnson

Hi msiriwardena;

Pressure drop calibration is a bit more complicated than it might seam.
The pressure drop is not static.
I.e. if given enough time there would be no pressure drop.
Conversely, if the patient is breathing rapidly there would be greater pressure drop.

Also, pressure drop is lessened if the tubing diameter is larger.

I would attempt to have the pressure sensor as close as possible to the mouth, maybe in the mouth.
Of course, this requires longer I2C wires but greatly reduced pressure reading errors.

Duane J

4x5n

Since your dealing with a medical device it's important to consider the regulatory standards and legal implications of what you're considering. Is this something that needs approval before actually using?

If accuracy is important then the sensor needs to be as close to what's being measured. The more tubing in between the more errors and chance for problems. Even a small leak can cause a large error.