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Pressure sensor selection
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Unlike other parts of the system there is no doc with pressure sensor requirements. Below are several questions generated by threads here:
What is the required sensor pressure range for measurement and survival? Is it required to be a differential (gauge) sensor? Will it be exposed to cleaning liquids? Is the device mounted inside the pressurized area? ???? Bob? KD8CGH |
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On Fri, Mar 27, 2020 at 02:15 AM, KD8CGH wrote:
Is it required to be a differential (gauge) sensor?Or can we reliability use two sensors, one for ambient and one for the airway pressure. The big issue may be using a sensor or sensors that are available in the quantities needed. That could override the other choices.? If my estimates are correct, based on what I could learn on the web, a storm front could come through and change the pressure by as much a 0.5 PSI to maybe even 1 PSI.? A way to continually keep track of the ambient air pressure would be needed. Not just calibration on start-up. For the people that know more about these systems than myself, could a single sensor solution be devised that relied in measuring the pressures throughout the breathing cycle and through some clever math be able to accurately calculate the ambient pressure. Or is there a magic spot in the breathing cycle that the pressure would be reliably at ambient pressure. Will these calculation differ for a patient totally relying on the ventilator and one breathing on their own but assisted by the ventilator.? And please, are my assumptions about the change of pressure due to weather changes or how much pressure error would be safe for a person on the ventilator correct? As I learn more I'll know with more certainty and can answer more of these questions myself. But there are people with much more real world knowledge on this, and making sure if this is real concern is important. And not spin the design into a needless direction.? Tom, wb6b |
开云体育In order to make flow measurements, which may be an FDA requirements and is a British requirement, you have to have a differential pressure measurement that is quite sensitiveOn Mar 27, 2020, at 13:20, Tom, wb6b <wb6b@...> wrote:
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开云体育I agree with you that specifications are not well completed, I have pointed this out repeatedly.?There are two different pressure measurements that have to be made. One is to measure the airway pressure ?for safety purposes of detecting disconnects and obstructions, as well as the potential for future making it a pressure control ventilator. ?Currently it is a timed cycleD which is extremely primitive ? ?This system needs to measure pressures between -20 cm of water and +60 cm of water, with the resolution of 2 cm of water.? Second measurement is needed to calculate instantaneous flow, and that has to be a sensitive differential pressure. Another person on the site is working on the calculations for that. On Mar 27, 2020, at 13:20, Tom, wb6b <wb6b@...> wrote:
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K9HZ
开云体育So that translates to roughly -0.3 to 0.9 Psi, with 0.03 Psi resolution.? The sensors so far discussed till easily meet that. ? ? Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? ? Moderator – North American QRO Group at Groups.IO. ? email:? bill@... ? ? From: [email protected] [mailto:[email protected]] On Behalf Of Gordon Gibby
Sent: Friday, March 27, 2020 12:29 PM To: [email protected] Subject: Re: [VentilatorDevelopers] Pressure sensor selection ? I agree with you that specifications are not well completed, I have pointed this out repeatedly.? ? There are two different pressure measurements that have to be made. One is to measure the airway pressure ?for safety purposes of detecting disconnects and obstructions, as well as the potential for future making it a pressure control ventilator. ?Currently it is a timed cycleD which is extremely primitive ? ?This system needs to measure pressures between -20 cm of water and +60 cm of water, with the resolution of 2 cm of water.? ? Second measurement is needed to calculate instantaneous flow, and that has to be a sensitive differential pressure. Another person on the site is working on the calculations for that.
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On Fri, Mar 27, 2020 at 10:29 AM, Gordon Gibby wrote:
One is to measure the airway pressure for safety purposesVery good. That makes things a little less critical than if this sensor was the main parameter controlling the ventilator. Yet, without an ambient reference the ventilator may falsely trigger an overpressure state if a weather front move away, or a closed room has a overpowered air condition or heating system and it switches on.? These Bosch BME280 pressure sensors seem to have both remarkable relative resolution and absolute calibration. In a world where gauge or differential sensors were available in the tens of thousands in stock, that would be the only way to go. But if he Bosch absolute pressure sensors are all that is available, then using two, one ambient and one measuring the airway pressure, to emulate a differential sensor, might work if the sensors can be counted not to drift over time and temperature beyond a safe value.? The flow meter discussed in another thread may be able to use this, also. Unless the sensors are stable to a remarkable level, some points in the breathing cycle would hopefully have quiescent values that could be frequently checked to keep the individual sensors matched to each other. These sensors do offer hope that they are very stable and accurate. Tom, wb6b |
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K9HZ
开云体育Differential sensors for flow are almost mandatory, because a slight drift in one of two absolute sensors is a disaster. ? If I were designing this with unlimited budget, I would use a hot-wire anemometer technique… where the heat removal from a wire adjusts it’s resistance and that is directly related to its heat capacity and velocity (in a fixed area, area times speed is flow).? I’m sure all of the medical devices that are ultra-low flow gasses like this use this technique or some sort of peristaltic/ positive displacement pump which meters the flow. ? ? Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Moderator – North American QRO Group at Groups.IO. ? email:? bill@... ? ? From: [email protected] [mailto:[email protected]] On Behalf Of Tom, wb6b ? On Fri, Mar 27, 2020 at 10:29 AM, Gordon Gibby wrote:
Very good. That makes things a little less critical than if this sensor was the main parameter controlling the ventilator. Yet, without an ambient reference the ventilator may falsely trigger an overpressure state if a weather front move away, or a closed room has a overpowered air condition or heating system and it switches on.? |
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On Fri, Mar 27, 2020 at 06:37 PM, K9HZ wrote:
hot-wire anemometerI believe that is what my car uses for the Mass Flow sensor. Would it be realistic to do something like a heated and non heated thermistor pair. That may be heading down an engineering effort beyond what would be practical for our group. Tom, wb6b |
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K9HZ
开云体育Yes a hot-wire anemometer is typically what cars use for the mass sensor… and you know what that thing costs for your car. ? Thermocouples (metal composite) have too much thermal mass to respond quickly to changes in temperature caused by a fluid (air).? The ratio of the thermocouples heat capacity to that of the air makes it almost impossible to use for flow indication (the time constant to get to steady state is quite long unless the fluid flow is almost infinite velocity). ? ? Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton – J68HZ Soufriere, St. Lucia W.I. Rent it: Like us on Facebook! ? Moderator – North American QRO Group at Groups.IO. ? email:? bill@... ? ? From: [email protected] [mailto:[email protected]] On Behalf Of Tom, wb6b ? On Fri, Mar 27, 2020 at 06:37 PM, K9HZ wrote:
I believe that is what my car uses for the Mass Flow sensor. Would it be realistic to do something like a heated and non heated thermistor pair. That may be heading down an engineering effort beyond what would be practical for our group. |
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On Fri, Mar 27, 2020 at 06:54 PM, K9HZ wrote:
Thermocouples (metal composite) have too much thermal massMaybe Jack's suggestion that someone from the university contact the sensor manufactures is the best hope for getting large quantities of the proper sensors. There is bound to be a solution with the number of people involved here following various leads and ideas. Tom, wb6b |
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Hi,
Doing a search on quantity in stock here are some interesting sensors, that are available in quantity at reasonable price points. One has an exposed back, the others are gauge type. It the body of the sensor was placed on one side of the flow meter constriction and the port ported to the other side, possibility that could suffice as the differential sensor. The gauge type would be better for the pressure safety sensor.? The down side of these sensors is they are analog. The?MP3V5004GP (0.57 PSI) sensor looks like it can supply an analog voltage direct to the microprocessor A/D convertor. The rest will need an op-amp amplifier to bring the signal to a level that can be converted by the microprocessor A/D. Don't know if we want to go there in a crunch project.? The whetstone bridge type sensors should be able to handle positive and negative pressures, but will need to read the data sheets in more detail to see if that is a limitation. These sensors are stocked in large quantities so manufactures must like these for some reason, so hopefully that means will continue to be easy to obtain over time.? There are likely more sensors if we keep looking.? 1000 pcs prices: MP3V5004GP - $7.58 MPXM2053GS - $5.019 MPX2300DT1 - $4.26 NPC-100T - $5.29 (there is also a NPC-120) ---------- ----------- |
Bob Benedict, KD8CGH