JohnFrim Loc: Somewhere in the Great White North.

Only from having to climb the ladder to read the water column at the higher pressures... :D

jerryc41 Loc: Catskill Mts of NY

:D

Would you use antifreeze in the winter?

JohnFrim Loc: Somewhere in the Great White North.

In my car... yes, in the rad and the windshield washer, all year.
In my veins... yes, daily; alcohol works well, even in the summer.
In my BP calibrator... seeing how accurate the transducer of my Omron BP monitor is after almost 2 years I would only check it infrequently, and indoors... so no, just plain water (and that way I don't have to adjust the conversion factor to account for a change in specific gravity).

Did I answer the question(s)?

MtnMan Loc: ID

Yes.

I went to my heart Dr. Monday and she came back after the EKG pressing the questions "Are you tired?" and "Do you find yourself getting winded?" Answering no to both I found that the reason she was asking is that my pulse on the EKG was very low (48) and my BP was low (110/58).

I'd been on a better diet and no alcohol the last three months so figure that is the cause. After some trending of my home meter I'll reduce one of the BP meds I've been on to see if that adjusts things (as per her suggestion).

But the main point for here is that I've been taking readings with the home meter to get a baseline. I take three readings each time. They vary by moe than plus or minus 10. Pulse also varies by a similar amount. This should be with the same input actual BP and pulse because they are taken witin a few minutes and with no movement or environment change. So this represents the repeatability accuracy of the device. You cannot improve on that with calibration.

Calibration can improve precision; i.e. the closeness of the average reading to the actual value. A test like the one described can measure that. But it appears to me that effect i likely small compared to the repeatability.

I know also actual BP varies during the course of the day and with recent exercise and with environment.

Bottom line: detailed calibration of the device probably isn't necessary. It is more important to take multiple measurements each time. But if you do try to calibrate take several readings at each point to quantify the repeatability.

MtnMan Loc: ID

Minimum three readings is good advice. You might also take them alternating arms. That can help in two ways: a systematic difference of using on one arm vs. the other and a random effect due to affixing the instrument a little differently each time. You'll get a better idea of the overall variability.

n3eg Loc: West coast USA

I pay attention to silly details. I can tell my wife's BP by watching the needle on the pump up meter, even if the doctor doesn't tell her. I can also tell my BP by watching a digital meter and paying attention to the feeling in my arm with the cuff on it.

I can be late to an appointment and scramble to get to the office and still be 120/80. I was 120/80 after an argument with my wife. The only thing that makes mine go up is pain. It doesn't even go up when someone tells me my micro four thirds camera isn't a "real" camera - so Sharpshooter can post away and not worry.

JohnFrim Loc: Somewhere in the Great White North.

My BP monitor is set to take 3 readings 1 minute apart and calculate/display the average. If I want the individual readings I have to manually scroll the display. I record the average as my BP record.


Not when using a live human as the source of the "test signal". Your BP and HR will NOT be the same.


I don't agree with that statement. When you take several readings on your arm even in close succession you are not checking the repeatability accuracy of the device, but rather the repeatability of your BP over time. Your BP and HR will vary over even short intervals of time, so what you are seeing is your body's variability, not the variability of the device.


Repeatability of the transducer is excellent. I ran calibration tests over several days and consistently got results such as the data sample I showed in my original post. Trust me, I would not be posting a DIY technique that deals with calibration of a human health monitoring device unless I was certain of what I was doing.

For any of you still following this post, remember that my schematic of the setup is just that -- a schematic. There are several fine details that need attention in using the device and I describe those in my construction/usage document. For example, during calibration the BP monitor pump runs continuously while the pressure reading remains stable. This means that there is a pressure difference between the pressure at the transducer and the pressure at the bottom of the regulator air tube. A larger diameter regulator tube makes for a smaller "resistor" in the system and thus a lower delta-P; and placing the manometer "T-off" close to the BP monitor gets the manometer closer to the actual internal transducer. My data shows the monitor reading consistently slightly higher than the manometer by about 1 mm Hg, which I believe could be due to the small resistance between the monitor transducer and the manometer take-off point. But I would hardly consider a 1 mm Hg error a problem.

Another critical subtlety -- you have to ensure that the manometer column has no air bubbles. These can arise when the pressure drops and the water runs back into the reservoir. Some water "drops" remain stuck to the walls of the narrow tubing creating multiple "slugs" of water that have to be completely cleared before doing another test.


When I use my calibrator there is no human in the system. The BP monitor is simply raising the pressure in the system until it reaches the regulation pressure as determined by the depth of regulator tube in the ABS pipe. The monitor pump remains on and the reading remains steady for quite some time. I am actually able to get 3 or 4 readings before the device displays "Error", thinking there is something wrong with how the "cuff" is placed because it has not detected any pulses. I simply shut the device off and start it up again for the next set of 3 or 4 readings.

So in my opinion, accurate calibration of the pressure transducer in the BP monitor can be done quite easily at home to ensure that ANY displayed pressure is, in fact, correct. This is the starting point for any truth/accuracy in home BP measurement. Beyond that, the relationship between BP and the results displayed by the monitor is dependent on the algorithm in the processor of the machine. Finally, reproducibility of BP measurements on a human arm is subject to cuff placement, body movement, time variation of BP during and between readings, etc, etc.

Sorry for being long winded here, but I think it is important to clarify some of these details.

MtnMan Loc: ID

My point was that the details are not relevant due to the overall variability. It is most important to take multiple measurements. The variabilty information is as important as the average...possibly more so.

One reading is always inaccurate.

JohnFrim Loc: Somewhere in the Great White North.

I don't question that BP variability is important, and so is the average. But if your transducer is off by 10 mm Hg what good are ANY of the readings?

MtnMan Loc: ID

Yes, that is a reason to check the calibration.

Checking against a skilled person with a meter and stethoscope will likely get you close enough. If you don't have one home (wife is nurse) you can take your meter along on your next Dr. visit.

Alas some of the nurses and aides now use the electronic cuffs. I have had a few spurious readings that way. When I ask them to do it over with a stethescope they get a more believable reading.

Anyhow if they don't tell you the reading you should ask and if it seems off ask them to repeat it.

I questioned the pulse reading of 48 on my visit on Monday. She claimed it came from the EKG and had to be right. I haven't gotten anything like that since (about 56). My nurse/wife told me the finger pulse readings can be off if not mounted properly or if the subject moves during measurment. I'm thinking that must be how they took the pulse reading...but don't recall.

BTW I have found the sit down units in pharmacies to often be way off. Once again you should take at least three readings. Most of them only let you use one arm.

JohnFrim Loc: Somewhere in the Great White North.

So, do you suffer white coat syndrome with your wife... or does she take it off before checking your BP? :-)

MtnMan Loc: ID

Actually she has one of the worst cases of white coat syndrome I have seen. Her upper number is up to 40 points higher in the Dr.'s office.

If I have one it appears to be negative. I lay back for the EKG and if they aren't perky about it might just take a nap. They do perk me up with the cold electrodes, though. I accused them of keeping them in the refrigerator with the stethoscopes.

denoferth Loc: Portsmouth, NH

[quote=JohnFrim]The nurse taking your readings must be VERY attractive!!!

You beat me to it.

:thumbup: :thumbup:

JohnFrim Loc: Somewhere in the Great White North.

Isn't that where they also keep the proctoscopes? They really should warm them up as well, maybe on themselves... er, maybe not. Forget I said that...

Speaking of cold and medical procedures, many years ago my colleagues conducted a study comparing mammography ("invasive" in the sense of X-rays) with thermography (thermal emission). To increase the "contrast" between warm and cool tissue the subjects had to sit topless in a cool environment with their hands raised over their heads for about 20 minutes. It was definitely nippy for them... and this is not a made-up story; I have the 35 mm slides to prove it (see how this gets back to photography?).

MtnMan Loc: ID

Probably perky, too.

:oops: