Broken CPAP - Internal parts

[-SWS]

I don't know if we could just change out the sensor, and if the chip would automatically re-adjust / re-calibrate the pressure.

Dunno. But I'd be surprised if Respironics didn't write firmware code to compensate for ordinary sensor drift. Zero-sum inspiratory & expiratory flow reversals should make that a fairly easy task.

I would say it can't. In order to calibrate the flow and pressure sensors, it would have to send in a series of known flows and pressures (see above Service Report where this is done). Without additional flow and pressure sensors, it can never be sure what the flow and pressure is at any given time.

Further, "IMHO" it can't even zero. It would have to be sure there is no flow or pressure, and that scenario can never be assumed to be true in the field. The procedure would, out of necessity, have to be done at start up, and since plenty of people put the mask on first, then turn on the machine, there is never a true zero state.

That said, if there were such a routine, then there should be an indicator if it fails, and if we look at the available Error Codes for flow and pressure:

Pressure Sensor Errors

E-70 ERR_PRESSURE_SENSOR_ABSENT
E-72 ERR_PSENS_UNABLE_TO_OBTAIN_BUS
E-73 ERR_SENSOR_PRESS_OFFSET_STOP

Flow Sensor Errors

E-80 ERR_UNABLE_TO_INIT_FLOW_SENSOR
E-81 ERR__FLOW_SENSOR_TABLE
E-82 ERR_FLOW_SENSOR_OFFSET
E-83 ERR_FSENS_UNABLE_TO_OBTAIN_BUS
E-84 ERR_FLOW_SENSOR_STOP
E-85 ERR_FLOW_SENSOR_OCCLUDED
E-86 ERR_FLOW_SENSOR_ABSENT
E-87 ERR_FLOW_SENSOR_BUS

there are those OFFSET things, but it cannot necessarily be assumed that where you're offsetting from is actually zero.

That said2, in a review of Respironics technology, it seems that Flow Offset only comes into play when the patient is actually "doing something":

"FlowOffset" corresponds to a final value of patient flow during inspiration responsive to the final value being positive, otherwise FlowOffset is set to zero.

That said3, that certainly does not mean that once one exceeds zero flow and/or pressure, that those delivered parameters are accurate.

Well, I think you just described ideal diagnostic & calibration procedures.

By contrast, I'm referring to a practical and on-the-fly sensor-drift compensation routine, which I think is algorithmically possible. Given that sensor drift is inevitable over time, I suspect the manufacturers might reference zero-flow on-the-fly, by noting where inspiratory and expiratory volumes break even. Sensor drift and orifice flow would each be fixed functions, with non-fixed leaks as the routine's potential spoiler. While that type of zero-flow referencing would be less desirable than bench calibration, I think not compensating for inevitable sensor drift may be even less desirable. Then again, that thought begs the question of whether the typical sensor-drift range stands to impair xPAP efficacy.

[Sludge]

I suspect the manufacturers might reference zero-flow on-the-fly, by noting where inspiratory and expiratory volumes break even.

Certainly agree on this part (although one cannot know the precise flow rate unless the machine is calibrated as above). However, keep in mind that the machine does not see "zero" per se. The flow is continuous, so what it's really seeing is flow rates that vary around the continuous flow (i.e., 35 LPM @ 10 cmH2O) and sticks a zero reference in there. Gain on these things is automatic to create a readable waveform.

The problem comes with pressure.

[-SWS]

The problem comes with pressure.

Generally, auto-calibration of pressure transducers is an attainable objective. Below is an example specific to CPAP:
http://www.google.it/patents/US6237592

The above method assumes atmospheric pressure as transducer reference before the blower spins. Resmed and Respironics no longer offer pressure-offset adjustments in their clinician's menu---or as part of the clinical setup routine. My hunch is they now use auto-calibration for field adjustments instead....

That said, if there were such a routine, then there should be an indicator if it fails, and if we look at the available Error Codes for flow and pressure:

Those are all critical errors requiring special handling or notification. Each of those errors are potential CPAP show-stoppers. By contrast, sensor-drift compensation in the above patent is treated as ordinary and expected maintenance, rather than being isolated as a critical error condition.

[Sludge]

That said, if there were such a routine, then there should be an indicator if it fails, and if we look at the available Error Codes for flow and pressure:

Those are all critical errors requiring special handling or notification. Each of those errors are potential CPAP show-stoppers.

In the aforementioned codes, E-81, 82, 85, 86 and 87 are non-fatal errors. The others are varying degrees of "show-stoppers".

[-SWS]

In the aforementioned codes, E-81, 82, 85, 86 and 87 are non-fatal errors. The others are varying degrees of "show-stoppers".

Well, IMHO they're all critical errors, with some being fatal. I'm guessing the designer(s) decided that ordinary sensor-drift maintenance didn't belong on that list of critical errors.

If, on the other hand, detected offset values far exceed typical sensor drift values, then I think we can expect auto-calibration to be terminated. That termination scenario would probably be worth flagging with an error code like E-73.

[Lukie]

Go to Craigs List New Hampshire. I saw your machine barely used for sale. Check it out.

[-SWS]

That said2, in a review of Respironics technology, it seems that Flow Offset only comes into play when the patient is actually "doing something"

That makes sense if they're doing this to reference zero-flow:

I suspect the manufacturers might reference zero-flow on-the-fly, by noting where inspiratory and expiratory volumes break even.

Certainly agree on this part...

[Sludge]

That said2, in a review of Respironics technology, it seems that Flow Offset only comes into play when the patient is actually "doing something"

That makes sense if they're doing this to reference zero-flow:

I suspect the manufacturers might reference zero-flow on-the-fly, by noting where inspiratory and expiratory volumes break even.

Certainly agree on this part...

OK, but "referencing" where the zero is and actually zeroing are 2 different things.

It appears that discussion is occurring about a number of points:

• Zeroing
• Establishing an offset after zeroing
• Calibrating
• Doing above for both the pressure and flow sensors
• Whether these procedures are done in the field or on the workbench
• Are those procedures done statically (machine on, blower not running) or dynamically
• What Error Codes (if any) are generated if one/some/all of the above go awry
• What specific machines are these routines present in

While that above referenced patent is interesting and offers insight into the discussion at hand, we must remember that it's 2001 and from ResMed.

Similarly, Respironics technology has changed significantly, even recently. For instance, there used to be a separate Sensor PCA 1O44501 in M-Series, and Auto-Nulling was referenced in the manual. However, that board is now gone (as is M-Series - I'm just sayin').

Is sensor technology to the point nowadays where constant zeroing is more trouble than it's worth?

That said, if we refer to SystemOne Performance Testing, Auto-Nulling is present (see items 24 and 28). However, I would continue to offer that unmonitored Auto-Null will create a heap o'problems, and this procedure is only done in bench testing (admittedly this is my opinion, but there appears to be some major changes between M-Series and PRS1, and they sure didn't come about because M was working so well).

[Sludge]

OTOH, we could just try Craig's List...