Fluid Displacement Comparator

what is fluid displacement comparator & how does it works?

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The fluid displacement comparator is a linear measurement device and it is based on the fluid displacement principle.

This article is about fluid displacement comparator along with their construction (parts), working principles, applications, advantages, and disadvantages.

Fluid displacement comparator is a type of comparator which works on the basic principle of fluid displacement and capillary action.

Let’s see the construction of the fluid displacement comparator.

Construction (or) parts of fluid displacement comparator:

Fluid Displacement Comparator
Fluid Displacement Comparator

This comparator is constructed using the following parts,

  • Diaphragm
  • Fluid Chamber
  • Plunger
  • Capillary tube
  • Scale
  • Pointer

Fluid chamber

The Fluid chamber acts as a pool, which contains fluid in the large cross-sectional area. The fluid in the chamber is low viscous in nature.


The diaphragm is fitted at the bottom of the fluid chamber. It is made up of a flexible material. It acts as a pressure transmission element in this comparator.


The plunger is a pressure sensing element in this comparator. One end of the plunger is attached with the diaphragm and the other end is set free. This free end detects the change in pressure.

Capillary tube

It is a glass tube with a fine bore where capillary action takes place. It is fitted along with the fluid chamber.


A measuring scale is fixed parallel to the capillary tube. There are two limit fingers (such as high-limit finger and low-limit finger) in the measuring scale.


The pointer on the measuring scale is set at the free height fluid. Due to temperature changes, the fluid expands in the same way as in the thermometer, hence the pointer is required to indicate the free height of the fluid in the capillary tube.

The above-stated parts are used to construct a fluid displacement comparator. Now let’s see its working principle.

Working principle of fluid displacement comparator:

  • Fluid displacement and capillary action are the basic principles used in the fluid displacement comparator.
  • The surface of the workpiece to be measured is placed under the measuring plunger.
  • The movement of the plunger occurs due to the small difference on the surface of the workpiece.
  • Due to the movement of the plunger, the diaphragm placed at the bottom of the fluid chamber start to deflect. This is because the pressure is transmitted to the diaphragm by the plunger movement.
  • When the diaphragm deflects, a small amount of fluid is displaced from the fluid chamber to the capillary tube.
  • The level of fluid rises at a certain point limit above its free height.
  • The rise in the fluid is directly measured by a scale fixed parallel to the capillary tube. The limit pointers (or) limit fingers may be set relative to this to indicate the high and low limits of the fluid in the capillary tube.

Magnification of fluid displacement comparator:

The magnification of this comparator is obtained by the ratio of the cross sectional area of the chamber to the tube cross sectional area.$$ \displaystyle \text{Magnification = }\frac{{{{D}^{2}}}}{{{{d}^{2}}}}$$

Applications of fluid displacement comparator:

This comparator has limited applications when compared to other comparators. The certain applications of this comparator are stated below.

  • It is used for laboratory testing purposes.
  • It is also used as an inspection tool.

Advantages of fluid displacement comparator:

The certain advantages of this comparator are stated below.

  • It is based on a simple working principle.
  • Easy to operate.
  • It is portable.
  • It is cheap in cost.
  • The entire design setup is simple.

Disadvantages of fluid displacement comparator:

The certain disadvantages of this comparator are stated below.

  • This comparator is not suitable for all temperature conditions. Due to the temperature difference, the fluid in the capillary tube may expand.
  • There is certain inactivity in the comparator due to the characteristics of the diaphragm and the fluid properties.
  • The deflection of the diaphragm per unit measuring force varies regularly; hence the resistance offered by the diaphragm is proportional to the measuring force.
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