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Wednesday, 9 January 2019

Potentiometric Recorder

The disadvantage of the galvanometer recorder can be eliminated by using an amplifier between the test terminals and the moving coil meter. However this reduces the accuracy of the record. In a potentiometric recorder the accuracy is improved by a process of comparison of input signal with a reference voltage. The reference voltage will be supplied from an internal source of the recorder itself. When the input signal is given it is applied in series with the reference signal such that the difference of the input signal and the reference signal will produce an error. If the input is lower than the reference voltage the error can be taken as negative. If the input is more than the reference voltage the error can be taken as positive. Only in one condition when the input equals the reference voltage the error will be zero.

The error signal will be given to the amplifier. The amplifier output will control a motor. The speed and direction of rotation of the motor depend on the output of the amplifier which is the error signal. Self balance is obtained by sliding the slider of the potentiometer to get a null output. Reversible motors are used in the D.C. system. In A.C. system a two phase motor is used.

(a) Constructional Details of Self Balancing Recorder (Block Diagram): 

Potentiometric recorder block diagram
The self balancing type recorder is also called the potentiometric recorder. T he constructional details are shown in figure. From the block diagram shown, it can be seen that the input signal is applied to the amplifier in series with the part of the potentiometer. The potentiometer is supplied with a reference voltage derived from an internal power supply. The output of the internal power supply is made stable.

The field coil of the motor is energized by the output of the amplifier. The output of the amplifier is the error signal. The variable arm/slider/wiper of the potentiometer is connected to the armature of the motor. The wiper carries a pen. The paper feed mechanism moves the paper with a constant speed.

(b) Potentiometric Recorder Working: 

It can be seen from the block diagram that the input signal and the part of the voltage across the potentiometer are in series. The difference between these voltages is the error signal. This error signal is available at the input terminals of the amplifier. The field coil of the motor is connected to the output of the amplifier. The construction of the motor is such that, it turns in a direction that rotates the wiper of the potentiometer to reduce the error The balance is obtained by the current through the armature of the motor flowing in one direction or the other depending on whether the input is higher or lower than the reference voltage. When the error reduces to zero the motor slows down and stops. At the instant of zero error the motor stops giving null balance.

As- the wiper of the potentiometer is driven by the motor's armature and as a pen is arranged over the wiper, the pen executes motion in the direction of movement of the wiper. As the armature moves in either direction depending on the error the pen moving in synchronized direction records the waveform. The paper feed motor will be synchronized with power line frequency. Capillary action of a recorder is defined as the process of establishing flow of ink from the reservoir through the tubing and into the hallow of the pen.

(c) Range: 

The bandwidth is around 0.8 Hz
The sensitivity is 4 mV/mm with an error of less than ± 0.25%

(d) Applications:

Mostly used to record process temperature
They are also used in the control of process temperature.

(e) Variations in Design:

Instruments that record variations of one variable under measurement are termed Single Point Recorders. There are multipoint recorders that record the waveform of many inputs. The maximum number of channels can be 24 and can extend up to 36. Colored recording in six colors is possible. The frequency ranges from 0 Hz to 5 kHz in improved models.

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