Bimetal thermometer
These thermometers use the following two principles:
- All metals change in size, which expands or contracts as the temperature changes.
- The rate at which this expansion or contraction occurs depends on the coefficient of expansion temperature of the metal, and this coefficient of expansion temperature varies for different metals. Therefore, differences in thermal expansion rates are used to generate deviations proportional to temperature changes.
The thermometer consists of two bimetallic strips. It is made of two thin metals that have different coefficients of expansion. These two metal strips are attached to the rope, so that the relative movement between them stops.
An increase in temperature will lead to a deviation of the free end of the bar as shown in the diagram. This is a linear deviation and may be related to temperature changes.
The radius of curvature of the bimetallic strip, which was initially flat, is determined using the following equation.
R = t {3 (1 + m) ² + (1 + mn) [m² + 1 / mn]} / 6 (άh-άl) (T2-T1) (1 + m) ²
Everywhere
R = radius of curvature at temperature T2.
T = total thickness of bimetallic strip = (t1 + t2)
m = t1 / t2 = Low metal thickness / Higher metal thickness.
ll = Low expansion coefficient of the metal
α = higher expansion coefficient of metal expansion.
T1 = initial temperature.
T2 = Temperature.
The following are important characteristics that should be chosen for bi-metal thermometers.
1. Expansion coefficient
- Modulus of elasticity.
3. Elastic limit after cold rolling.
4. Electrical conductivity.
- Plasticity.
- Metallurgical ability.
Common forms of bimetal thermometer sensors are listed.
1. Spiral type.
2. Spiral type.
3. Type of concealer.
4. Bed type
Materials used in bimetal thermometers:
- Nickel iron alloy with chromium and manganese
- Brass
- Inver (nickel and iron alloy)
applications
1. Bimetal thermometer is used in control devices.
2. Bimetal thermometer is used in air conditioning thermostats.
3. Bimetal thermometer is used for process applications such as refineries, oil burners, analyzers, etc.
Advantages of using bimetal thermometers
- They are simple, durable and inexpensive.
- Their accuracy is between + or – 2% to 5% of the scale.
- They can withstand up to 50% of the range.
- They can be used where the glass thermometer is used frequently.
Restrictions on the use of bi-metal thermometers
- They are not recommended for temperatures above 400 degrees Celsius.
- With regular use, bimetallic metal may deform permanently, which in turn causes errors
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