Choosing pressure sensor to meet accuracy requirements for Blood Pressure Measurement

[groitman]

Hello,

I'm working on the design of a Non-Invasive Blood Pressure monitor. I'm choosing a pressure sensor to meet the specifications needed to comply with standard IEC 60601-2-30, which applies to this kind of devices. I see that there are many application notes where NXP suggests that different pressure sensors are adecuate for this application. For example, Application Note AN4328, where pressure sensor MP3V5050 is used.

The above mentioned international standard requires that the device has a manometer mode to measure the air pressure in the patient's cuff, and that the accuracy of the device must meet the following rquirement:

"201.12.1.102 Limits of the error of the manometer from environmental conditions

Over the temperature range of 10°C to 40°C and the relative humidity range of 15% to 85% (non-condensing), the maximum error for the measurement of the CUFF pressure at any point of the nominal measurement range shall be less than or equal to +/-3mmHg (+/-0.4kPa) or 2% of the reading, whichever is greater."

The standard also requires that the manometer system must measure up to 230mmHg (201.12.1.103 and 201.12.1.106). This sets the limit of the error to +/-3mmHg when the measured pressure is in the range 0mmHg to 150mmHg, and 2% of the reading in the range 150mmHg to 230mmHg.

All pressure sensors specify Accuracy in percent relative to FSS, and not percent relative to the reading. So, for example, for MP3V5050GP accuracy is +/-2.5%FSS over the compensated temperature range 0°C to 85°C (no references to relative humidity). FSS is 50kPa = 375mmHg, so accuracy is 9.375mmHg, which is greater than the limits in all the measurement range.

1) Am I correct in the meaning of the sensor specification?

2) Is there a way to choose MP3V5050GP and comply with the limits in the error required in the standard?

3) We can make an offset and gain adjustment in production process at room temperature (25°C), and also we could do an Auto-Zero procedure before each measurement. Could the Accuracy of +/-2.5% be reduced by some of these adjustments? If so, how can we calculate the new accuracy after the adjustments?

I've already read Application Note AN1636 where Auto-Zero is explained for another sensor. The application note assumes that the only error sources are Offset Calibration and Temperature Coefficient of Offset and ends up with error bounds that only account for ADC resolution, but doesn't take into account other error sources (pressure and thermal hystheresis, non-linearity, thermal effect on span).

Thanks,

Gustavo

[reyes]

Hi Gustavo,

It is important to let you know that unfortunately, the pressure sensor MP3V5050GP is not a medical grade or medical qualified Pressure sensor, so a determination must be made if this impact is acceptable for your application.

Said that, I would try to answer to your questions:

1. Correct, accuracy of MP3V5050GP is +/-2.5%VFSS (VFSS=2.7V) and it have a pressure range of 50kPa. So the accuracy in pressure is +/-50*2.5%=+/-1.25kPa (9.375 mmHg). All outputs within this error band is allowed.

2 and 3. Unfortunately it is not possible to improve the accuracy value for the NXP pressure sensors.

Have a great day,
Jose

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[sebastien_risle]

Hello,

I don't understand this answer. I agree with the conclusion on the accuracy, and I agree that it does not meet the requirement for medical grade.

So why MP3V5050 is everywhere when NXP is talking about blood pressure measurement ? (first page of https://www.nxp.com/docs/en/fact-sheet/MEDPRESSENSFS.pdf )

Why "blood pressure measurement" and "medical application" are in the "application section on the datasheet ?

Is there any alternative with a sensor which is really "medical grade" ?

Thank you to clarify this point