Why deep diver take oxygen mixed with helium instead of pure oxygen while going deep in sea for respiration?

Answer:

To breathe underwater a diver must be supplied air at a pressure equal to that of the water surrounding the diver.

However there is an upper limit of oxygen pressure above which the oxygen becomes biochemically toxic.

Therefore, it is necessary to include something to dilute the oxygen in a diver's breathing gas. To satisfy normal breathing requirements at high pressures, it is necessary to supply between 0.2 and 1.5 atmospheres of oxygen with the rest of the balance made up by a non-toxic diluent (such as nitrogen).

Air of course is a suitable breathing mixture for a diver based primarily on nitrogen as the oxygen diluent.Air is in fact the preferred breathing mixture for all dives to depths of less than about 150-200 fsw (feet of sea water).

However, even at 150 fsw, when breathing air most divers feel the effects of nitrogen narcosis.

Beyond this depth helium is preferred as the diluent and is in fact particularly well suited to the depth range immediately beyond air diving (e.g., 150-250 fsw). Helium does not cause narcosis at these pressures, is relatively inexpensive and is readily available. Moreover helium has a low density and is, therefore, easy to breathe at such pressures.

There are problems with helium though, problems that are seriously amplified as diving depths approach those of the outer continental shelves and beyond.

First there is the problem of communication. Everyone knows what breathing helium will do to your voice. Due principally, it is believed, to changes in the speed of sound in the gas medium, this effect is a sensitive function of depth. Helium speech at sea level is distorted, in a way that seems funny to both the listener and the speaker, but it is completely intelligible. At 200 fsw speech with Helium is still reasonably understandable.

However, as depths increase to the range between 400 and 600 fsw the situation becomes more serious, and to someone trying to get a job done helium speech is no longer considered funny. Speech in this range is totally lost on an untrained ear, though anticipated statements can be understood by a listener familiar with the voice and the situation.

So often, however, a sudden change in the topic of conversation throws everyone off, and it is necessary for the diver to speak slowly, repeat himself and to try to say things a different way. It can be done but it is slow and consequently expensive.

The other problem is that Helium is about 4 times as good at conducting heat as nitrogen. Which when at the chilly depth of 500 fsw means that you are going to get VERY cold.

Reading a Capacitor's Value

Reading a Capacitor's Value

• Capacitor codes are similar to resistor codes. But with capacitors, the numbers are usually printed on the component, so you don't have to remember which colors stand for which numbers.
• Usually, a capacitor's nominal value in picofarads (pF) is indicated by three numbers on the capacitor's body.
  • The first two numbers represent the first and second digits of the nominal value. The third number represents the number of zeroes following those first two digits.
  • Example: If a capacitor's numeric code is 472, then the nominal value is 4700 pF, which is the same as 4.7 nF.

    
    

    Capacitor Tolerance Codes

    • Often the numeric code on a capacitor will be followed by a letter that indicates the capacitor's tolerance rating. Here are the tolerance letters that you'll see most often:
      

      Letter	Tolerance Rating
      F	±1%
      G	±2%
      J	±5%
      K	±10%
      M	±20%

Resistor Color Code

Resistor Color Code

• Usually, a resistor's value in ohms is indicated by several colored bands on the resistor's body.
• Each integer from 0 to 9 is represented by a color. The table below shows which colors represent these integers.

Integer	Color
0	Black
1	Brown
2	Red
3	Orange
4	Yellow
5	Green
6	Blue
7	Violet
8	Gray
9	White

• Two other colors (silver and gold) are also part of the resistor color code, but they have special meanings. Instead of simply representing numbers, these two colors represent tolerances ratings. In particular, gold represents a 5% tolerance, and silver represents a 10% tolerance. We often write this as ±5% or ±10%. (The expression ±5% is read as "plus or minus five percent." Similarly, ±10% is read as "plus or minus ten percent.") Finally, a tolerance of ±20% is represented by no colored band at all. 

Tolerance	Color
±5%	Gold
±10%	Silver
±20%	None

Reading a Resistor's Value

• Now that you know what the individual colored bands stand for, let's see how to put them all together to find a resistor's value.
• Usually, a resistor's value in ohms is indicated by four colored bands on the resistor's body.
• The first three bands of the color code give the resistor's nominal value.
  • The first two colored bands represent the first and second digits of the nominal value. The third band represents the number of zeroes following those first two digits.
  • Example: If the first three bands are yellow-violet-red, then the nominal value is 4700 ohms, or 4.7 kilohms. That's because yellow stands for 4, violet stands for 7, and red stands for 2. (So the red band tells you to add two zeroes after the 4 and the 7.)
• The fourth band (or tolerance band), gives the percentage variation from the nominal value that the actual resistance may have.
  • Example: If the four bands are yellow-violet-red-gold, then we saw above that the nominal value is 4700 Ω, or 4.7 kΩ. But you would not expect the resistor's actual value to be exactly 4700 Ω. It might actually be a bit higher or lower. The gold band is the manufacturer's way of assuring you that the actual value lies within 5% of the nominal value.