Galvanometric Recorders

Galvanometric Recorders Principle:

These recorders operate on the "deflection principle" that is, when current passes through the coil of the galvanometer, it shows deflection (θ). The deflection 'θ' is a measure of current passing through it. This current is proportional to the quality' being measured.

Construction:

A coil of the wire is wounded on a rectangular aluminium frame. It is mounted in the air space between the poles of a permanent magnet as shown in figure. In these recorders, the writing arm is a pen or an 'ink filled stylus', which is attached to the coil.

Hardened steel pivots attached to the coil fit into jewelled bearings so that the coil rotates with a minimum of friction. Springs attached to the coil frame return the pen and coil always to a fixed reference point. The drive motor consists of a synchronous motor and a gear box, to move the paper or chart with a constant speed. A constant speed is the basic requirement of the 'drive motor' because the recorded events are time correlated.

Working:

When current flows through the coil, a magnetic field is developed. This magnetic field interacts with the magnetic field of the permanent magnet. It causes the coil to deflect or to change its angular position. The direction of the rotation depends upon the direction of flow of current in the coil. The better the amplitude of the incoming signal, the better is the deflection that is, the scale of pen deflection is proportional to the current runs through the coil. When the coil deflects, the pen presses against the paper and writes on it.

In the pen type of galvanometer recorder the wiring may be done with ink flowing through the pen. The method is very simple but has the following disadvantages.

(i) the recording is curved, (curvilinear) which is difficult to study.

(ii) the ink tends to dry up inside the pen.

In the new types of recorders, these two disadvantages are eliminated by using 'heated stylus' instead of ink pen. The paper used with such a recorder has a special coating which turns black at points where a heated stylus touches the paper with the heated stylus recorder, rectilinear recording is obtained and there is problem of ink clogging.

Principle of a Direct Writing Galvanometer Recorder

Advantages or uses of galvanometric recorders:

(a) These recorders are generally used in clinical laboratories, because of their low cost.

(b) They will give the permanent record of the quantity being measured.

Disadvantages of galvanometric recorders:

(a) Frequency response is very low. The overall frequency response of these recorders is limited to 0-45 Hz.

(b) These recorders are not useful for recording fast variations (current or voltage or power), because, the moving parts of the galvanometer have more weight and large moment of inertia and friction of the writing arm over paper. They introduce a damping effect. Hence, the galvanometer can not move very fast.

(c) Since the pen of the galvanometer passes against the paper there are frictional forces opposing the movement of the pen. In addition, the pen together with the galvanometer coil has considerable inertia. As a result considerable torque is required to move the pen. Therefore, the current through the galvanometer coil must be high.

(d) It has low input impedance and a limited sensitivity.