Touch type heart rate monitors? How do they work?

[NorthJ]

Google has failed me. :-(

Can some one tell me how the touch/contact style heart rate monitors work? Usally on gym equipment there are two metal contacts you place your hands on. There are also the watch type that you place two fingers one each on different contact points.

I'm assuming/guessing your creating some kind of loop and your capacitance changes up/down with every heart beat?

2nd question would be if I took the watch style and just pressed it into say my fore arm or my neck with both contactors touching the skin would it still work? or is there some reason it has to be through your fingers?

Thanks a bunch!!
Joe

 

[MacGyverS2000]

Heartbeats require an electrical impulse... think along those lines.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[DRWeig]

Go to the US patent office web site and search for patent number 4406290.

You can see ALL about one method.

Good on ya,

Goober Dave

 

[CharlieGill]

The finger tip type like the nurse in the doctor's office puts on your finger is optical in nature. The blood capillaries expand with each pulse so if you shine a led through the finger and filter for red on the detector you will see a pulse with each heart beat.
Charlie

Charlie Gill
--

http://www.calibrator.com--

 

[roydm]

I'm with CharlieGill pn this one, I worked at a hospital for a couple of years, we had several that operated optically. They can also detect the Oxygen content of the blood in a similar way. If you remember your high school biology blood changes from red to blue with Oxygen (or v.v)
Roy

 

[DRWeig]

Very true, Charlie and Roy --

I was a biomed type guy for a part of my career. The opticals are much more reliable than what they put on the gym equipment.

The metal plates you touch on a treadmill or stair-stepper or whatever are just electrical contacts, though. The device has to pick up the extremely-low electrical impulses that trigger heartbeats. It's looking for the peaks that you see on an EKG. It's tough to do with just a couple of sweaty hands on metal plates...

Good on ya,

Goober Dave

 

[IRstuff]

see:

http://en.wikipedia.org/wiki/Pulse_oximeter

The typical optical oximeter actually uses two LEDs, one is red and the other is near-infrared. The oxygen saturation is calculated from ratio of absorbances of the two LEDs' emissions.

TTFN

FAQ731-376

 

[itsmoked]

I suspect the cheap pulse monitors are NOT detecting biological electrical impulses. Those electrical signals are completely swamped by any large muscle signals. Ever get an EKG? If you move a muscle they freak out.

They are instead looking at the resistance. They're just ohmmeters. Your skin contact resistance is fairly high. On the order of 5k Ohms. But, like any area resistor as you increase the area of contact, or the pressure, the connection resistance is going to change. If your ohmmeter is completely biased to see changes in resistance, it can easily see the periodic changes in your grip, or touch, caused by the systolic pulses.

Grab the leads of an ohmmeter in your fingers and look at the reading. It wants to wander all over constantly. All you have to do is watch that wander, (much faster than your ohmmeter normally does), for periodic changes - they're caused by your heartbeat.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DRWeig]

Hadn't thought of it that way, Keith. I do know that the el cheapo monitors are really iffy, result-wise.

Good insight! I'm not quite interested enough to take my NordicTrak apart yet, but if I have too much idle time one weekend....

Good on ya,

Goober Dave

 

[IRstuff]

Huh, you'd think they'd be able to tell you your body fat content as well, then. I think the body fat testers also look at resistance.

TTFN

FAQ731-376

 

[geekEE]

The gym equipment that I've seen has two pads for each hand, so I assume they're doing a 4-wire measurement to reduce resistance changes when your grip is tighter or looser.

I think it's probably not measuring body fat content because those measurements are not valid if you are exercising and sweaty.

Glenn