I’m looking for a pressure transducer to attach to a side port of some intravenous tubing and measure the liquid pressure in the tubing. The flow rate and pressures are both quite low (imagine pushing some water out of a small syringe against not much resistance).
I’m familiar with RPi and micro python so it would be great if it is compatible with them.
Any thoughts? Thanks in advance!
Hi Andrew
That would be about nine tenths of nothing. You could get some idea of what to expect by measuring the height from the liquid surface and measuring point ten calculate it.
I haven’t done any conversions as I have had no need but I remember that 1 foot of water is 62.25 pounds per square foot or 0.4323 pounds per square inch.
Cheers Bob
Hey Andrew,
Welcome Back!
This would be a crazy awesome project but with full admission you have me stumped as to how it could be done effectively. I think the closest I can think of getting this done was to attach something surrounding the tubing that would put slight pressure on the tube to measure any change of pressure inside of it. The hard part about this one is potentially being non invasive with the measuring device, which makes things difficult as flow sensors and the like are generally going to be in contact somewhere along the IV line and it just wouldn’t be safe.
Hopefully someone has done a similar project and may be able to jump in and give some idea of how this could be done effectively, but I hope you are able to as I would love to see the solution here, its an awesome project idea!
Cheers,
Blayden
Hi Andrew,
We don’t have any water pressure sensors designed to measure IV pressure. Since they are used in medical applications I would think any sensors designed to work with an IV would have undergone some pretty specific testing and be pricey.
Can you let us know a bit more about your project? Will this be a medical application or something else which the intravenous tubing has been repurposed for?
Hi Andrew
I believe your first hurdle would be to find something that will measure these very low pressures. This would be something like the low pressure in domestic gas appliances which I think require a bit of specialist gear to measure, or like the difference in pressure across a filter. Sometimes measured in inches of water (USA) or mm of mercury like blood pressure.
Having succeeded thus far what are you going to do with this information. Any form of electronics will require this info to be in digital form.
I think the pressure will be too low for Blaydens method
to work.
That bit would not be too hard. If you insert a “T” piece into the IV line and arrange it vertically with the end sealed off the IV liquid should not enter. The end would be sealed off with the pressure sensing device. The pressure at the “T” is transferred to the air in this bit of tube and measured at the end. The rule of hydraulics applies here that states
Quote:
When pressure is applied to a fluid it is transmitted equally and undiminished throughout that fluid.
End quote.
The fluid in this case would be the air in the piece of vertical tubing.
Cheers Bob
Thanks for your inputs so far. A bit more information: the device would need to sample the pressures over time (maybe, 2 secs) in order create a graph of pressures over that time. The purpose is to create a signature which will hopefully give information about downstream obstruction in a part of the body (for example, it may suddenly unblock after reaching a certain pressure). I would imagine driving the fluid into the tubing at a constant rate, perhaps with a syringe driver, or perhaps with something bespoke so it could also be controlled by the microcontroller. I like Bob’s idea of maybe using a sensor which doesn’t necessarily come into contact with the liquid but which has the pressure transmitted to it. Does this kind of thing exist in the makerverse?
Could I get something like this to work? I could glue it into place, or use an O ring. It’s I2C, so I’m guessing it would work with RPi Pico? (I have limited experience with sensors)
Hi Andrew
Is this IV feed gravity or is it being artificially pressurised. I am assuming gravity but your last post suggests pressurised.
I think that device you linked has a range between 300 and 1200 mBar. If you are pumping stuff into a human blood system 300mBar might be a bit high, But that is the sort of thing that might work if they have one in a suitable range.
Cheers Bob
Pressurised by a pump, perhaps a syringe pump, but expecting quite low pressures, because the resistance is very low.
Hi Andrew
My maths might be out but I make 300mBar 225 mmHG which is probably a bit high blood pressure (mine is usually 120 - 130).
You are going to have to ascertain your expected pressures and there may be a similar sensor or even one of those in a more suitable range which would be ideal sealed into a piece of plastic tubing.
Cheers Bob
There are Differential Pressure sensors out there e.g. Jaycar ZD1904 (0-10 kPa), but that’s possibly not sensitive enough. You can also buy the ones that are used in portable Blood Pressure monitors, or even a cheap BPM to experiment with the parts (Pump, Sensor and a discharge valve).
Since these sensors are differential, you may need to apply a low vacuum (or at least something less than atmospheric pressure) to the port that’s not connected to the intravenous tube.
When you get to the actual medical application, have a look at brands like Honeywell.TruStability™ HSC Pressure Sensors | Honeywell.
Hi Andrew,
Honeywell have pressure sensors that have a full scale range of 2.5" H2O, ranging up to 150psi.
They have I2C, SPI or analog interfaces and are through hole mount. They are designed for medical instruments. They have a small barb type input port to suit medical type silicone tubing.
Cheers Phil
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Thanks, guys. I have sourced a sensor used for blood pressure monitoring, which works in the appropriate pressure range, and is conveniently attached to intravenous tubing already. It is attached to a medical monitor by a telephone jack plug with four wires - red, black, green and yellow. The voltage across red and black when attached to the monitor is 2.1V. I have hooked it up to a pico according to these instructions to see if it is I2C, but the code returns ‘no i2c device’. (I used a bench top power supply to get the 2.1V). I wasn’t sure which wire would be SDA and which SDC so I tried both ways. Any advice on how I can find out which protocol the sensor uses?
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If you suspect that the connection is digital then the device you need is a logic analyzer.
It provides a visual representation of a digital signal, like an oscilloscope. The software includes analysis for multiple protocols. With medical instrumentation there is always the risk that the protocol is proprietary, but even if that’s the case you might be able to decode it with this device.
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Thanks I think I have one of them lying around but have never had a reason to use it. Your answer got me thinking, though, as to whether the transducer is digital or not.
When powered with 2.1V, the voltage and resistance across the other two wires seems to vary in a consistent manner with pressure. From about 130mV to 100mV, and from about 4.8kOhms to 3.9kOhms across the pressure range in which I’m interested. These values vary when the power supply voltage to the transducer is varied. So am I right in assuming this is an analogue sensor? And that my next steps would be to get myself an ADC in order to hook it up to a RPi pico? I’ve not used an ADC before - all guidance welcome.
If you can find that logic analyser it will indicate whether the device is analog or digital - if it’s analog then you won’t read any intelligible output.
It is quite possible that the analog reading is referenced to the supply. ADC modules are available. This one is marked as compatible with the Pi and includes a precision reference:
