[silence]
(Dr. Wagner) Hi, this is Dr. Dale Wagner, I’m an exercise physiologist at Utah State
University. We’re here in the lab this morning to take a look at um, uh, a body composition
measurement using a densitometry method, and we’re going to use hydrostatic weighing.
So we have two densitometry methods—one is the Bod Pod and the other is this one—our
hydrostatic weighing tank. Uh, with hydrostatic weighing there are two important
measurements that we need to get in order to do densitometry. We need to get body
mass, which is easy enough to measure just with a scale, and then we need to get body
volume, which is the whole point of putting someone in the tank and measuring them
underwater. Um, the hydrostatic weighing method works off of a principle known as
Archimedes’ principle. And Archimedes’ principle basically states that if we put an
object in water, in a tank or in a tub, a certain amount of water is going to be displaced.
The amount of water that’s displaced is equivalent to the volume of whatever we put
in, and so that’s how, um, that’s how the underwater weighing procedure works.
So, we have our client, and she’s all ready to go. We’ve already measured her, uh, her
mass. The other information that we need to know beforehand, we need to know what’s
called the tare weight that’s everything other than the client, and so in this case, that
would be our chair. Our chair is fairly heavy, so we need to have a baseline measurement
of what our chair is. And on here you can see that if you can, uh, view the scale, you’ll see
that it’s a 5, once the needle goes around one time it’s 5 kilograms. So you can see
that the needle has already gone around twice. So we’re at a little over 10 kilograms
here, we’re 10.6, 10.6 kilograms, um, that’s our, that’s our tare weight for the, for the
chair. One other thing that we need to know is the temperature of the water. We have a
little thermometer in here, and so we just measure the temperature of the water. That’s
important because as the temperature of the water changes, the density of the water
changes, and that makes it easier or more difficult for the subject to float, and so all that
gets taken into consideration when we put this information into equations. We need to
know the tare weight, the density of the water, and of course the subject’s weight.
Okay, um, Jen’s ready to go, so we’ll have her get into the tank.
Be careful.
K, and she’s just going to have a seat on the chair, and the water level’s right at her neck
so she doesn’t have to bend too far, and the other important thing there’s two things that
really makes a person float that would be fat and air. K, and so it’s important that we try to
get the subject to blow out all their air under the water. This is very difficult. It’s an
unnatural maneuver to do because naturally when you go underwater you want to hold
your breath and so we want you to do the exact opposite. So when you go underwater I
want you to blow out as much as you possibly can. Once you think you have all
your air out, try to push out just a little bit more. K?
(Jen) Okay.   (Dr. Wagner) Try to be as still as you can.
The more that you move around under the tank, the more difficult it makes it to read the,
the needle on the scale.
(Jen) K.
(Dr. Wagner) Once you feel like you can’t stay down there anymore that you need
some air, just come back up. I’m not gonna tap the wall or anything it’s all up to you. K?
(Jen) K.
(Dr. Wagner) So whenever you’re ready.
(Jen) K, so I go under and blow it all out.
(Dr. Wagner) Yep.  (Jen) K.
(Jen) K. [laughing] Oh no.
[bubbling]
(Dr. Wagner) Blow, blow, blow.
[bubbling]
I can still see bubbles, so we know she’s still blowing...
...and right now she’s finished.
(Dr. Wagner) How was that?   (Jen) It was good!
(Dr. Wagner) K. And we’re going to try a few more trials ‘cause usually what happens is
people learn that they can blow out, they, they learn that they have more air than what
they think and they can blow out more than what they think they can.
(Jen) K.
[bubbling]
[silence]
(Dr. Wagner) Aaand, once I don’t see any more bubbles, then I take a quick look at the
scale, and you can see there’s a certain technique involved here. Um, it’s, it’s much
more difficult for the technician to do underwater weighing than it is to do the Bod
Pod because you have to, uh, look at several different things at one time. I’m looking at the
client to see when I don’t see any more air bubbles, and then as soon as I don’t see any
more air bubbles I have to take, uh, a quick look at the scale to see where the needle has
stopped. So there’s a little bit of a technique involved that’s a little bit more difficult for the
technician, much more difficult for the client. This is a difficult procedure to blow out all
your air under, under the water.
And you can see that the scale stopped right around here.
So on her that would be about 12.6 kilograms, and obviously she weighs more
than 12.6 kilograms on land, so you can see what the, you know, it’s a huge difference
between your land mass and your underwater mass.
