spiral type pressure gauge.

Spiral type Bourdon Tube

The spiral Bourdon tube makes a few windings in one plane around the fixed shaft of the pointer.

When the tube is being uncoiled by the process pressure, the free end will have a larger displacement compared to the C-shaped tube. The more windings, the larger the displacement will be.

A transmission mechanism is therefore no longer necessary. When the number of windings are correctly determined for a selected measuring range, a fixed connection between the free end of the Bourdon tube and the pointer is sufficient for a full deflection on the scale.

Using a fixed link avoids transmission losses due to friction or backlash in the transmission mechanism. This increases the accuracy and sensitivity of the pressure gauge. Also, recalibration is no longer necessary.

Backlash is mainly caused by wear on the teeth of the gears. Vibrations and pulsations are making it even worse. A pressure gauge with backlash on the gears will always indicate a pressure which is too low or too high. Subjected to pulsations, the backlash will cause the pointer to rotate further to the left and right than would normally be the case without backlash. Having a fixed link is making the spiral Bourdon tube pressure gauge very resistant to extreme vibration or pulsation.

For low pressure ranges, the spiral is made of a flat oval tube, while round tube is being used for the high pressure ranges.

The same low pressures can be measured with the spiral Bourdon tube as with the C-shaped Bourdon tube but the cost for a spiral pressure gauge is higher.

The use of spiral Bourdon tubes is consequently more likely in the high pressure ranges which cannot be measured with the C-shaped tube because the wall thickness of the tube would be too large so that the tip movement is too small to have sufficient accuracy. The suitable ranges up to 30,000 kPa.

WORKING:

When pressure is applied, the elliptical tube (Bourdon tube) tries to acquire a circular cross section; as a result, stress is developed and the tube tries to straighten up. Thus the free end of the tube moves up, depending on magnitude of pressure. A deflecting and indicating mechanism is attached to the free end that rotates the pointer and indicates the Pressure reading. The materials used are commonly Phosphor Bronze, Brass and Beryllium Copper. For a  2″ overall diameter of the C-tube the useful travel of the free end is approximately 1/8″ . Though the C-type tubes are most common, other shapes of tubes, such as helical, twisted or spiral tubes are also in use.

As the fluid pressure enters the bourdon tube, it tries to be reformed and because of a free tip available, this action causes the tip to travel in free space and the tube unwinds. The simultaneous actions of bending and tension due to the internal pressure make a non-linear movement of the free tip. This travel is suitable guided and amplified for the measurement of the internal pressure. But the main requirement of the device is that whenever the same pressure is applied, the movement of the tip should be the same and on withdrawal of the pressure the tip should return to the initial point.

ADVANTAGES:

*Construction is simple and cost is low. 

* It has excellent sensitivity. 

* It has high accuracy.

DISADVANTAGES:

1. They are subjected to hysteresis
2. They are sensitive to shocks and vibrations
3. Amplification is a must as the displacement of the free end of the bourdon tube is low
4. It cannot be used for precision measurement

Link for the videos :