Page 88 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

J – Type Thermocouple
Positive wire — Iron Negative wire — Constantan It can be used up to 1000oC. It is a very common type of
thermocouple. Its stability is high.
K — Type Thermocouple
Positive wire — Chromel Negative wire — Alumel It can be used up to 1200oC. It is a widely used type of
thermocouple. It is a cheaper type as compared to other types.

S – Type Thermocouple
Positive wire — Platinum 10% Rohodium Negative wire — Platinum It can be used up to 1400oC. It has very high
precision thus used for very high accuracy requirements.

The current will flow from + ve marked lead to the – ve marked lead. In thermocouples, negative lead is generally
a red colored wire. The color of positive lead will be according to its type.
Advantages:
They follow the temperature changes with a small time-lag.So it can be used in the applications where very rapid
changes in temperature take places.It responds to those changes very quickly.These are very convenient for
measuring the temperature at one particular point in any apparatus or setup.

Disadvantages:
They have very low accuracy. So they cannot be used for very high precision measurement.They have to protect
against contamination to ensure long life.They are placed at a very large distance from the measuring device.
This increases errors in the result.
Strain Gauge/Load Cell
Strain gauges and load cells Load cells are blocks of metal that have been machined in such a way that specific
areas are put under high strain when weight is applied to them. These areas have strain gauges attached to
them with a high strength adhesive. The materials used to manufacture load cells are Aluminum, Steel Alloy,
and Stainless steel. Strain gauges themselves consist of thin foil elements that are glued to the load cell body.
Strain gauges are cunningly shaped so that even very small movements or “stretching” of the gauge results in
comparatively large changes in resistance. The relationship between strain and change in resistance is almost
perfectly linear. Accuracies between ±0.01% and ±0.02% are not uncommon for a high accuracy load cell.

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CITS : E & H - Electronics Mechanic - Lesson 43 - 47