Types of thermometer sensors
The inflated temperature system is designed to display or record the temperature at a distance from the measuring point. The inflated temperature system is essentially a compression gauge connected by a small cross-section tube to a bubble that acts as a temperature sensor.
The whole system is tightened in terms of gas seal and filled with a suitable gas or liquid under pressure and filling. As the temperature changes, the pressure from the trapped fluid also changes and is displayed by the Bardon tube.
There are different types of filling systems, each with its own characteristics and benefits. The Scientific Equipment Manufacturers Association has divided the filled thermometers into four main classes based on the filling materials.
In most industrial applications, the use of mercury-filled systems is obsolete due to health risks. In addition to the use of liquid and liquid filled systems, they have lost their position for two reasons.
1- The necessary cost to compensate for the effects of the capillary environment
2- Existence of error due to the difference in height between the bubble and the reading section
Currently, most filling temperature systems use gas and steam, but both have their limitations. The gas-filled type has large dimensions for operation in the desired temperature range
And steam-filled systems are also limited due to nonlinearity and error due to height difference. In general, filled systems are better compared to non-metal elements and weaker compared to electric thermometers.
As mentioned earlier, filled thermometers are divided into four classes based on the filling material:
1- Liquid filled systems:
These systems are completely filled with a liquid and work based on the expansion of the liquid as a result of the increase in temperature. The filler fluid is usually a bare hydrocarbon such as auxin, which has an expansion coefficient of six times that of mercury, allowing smaller bubbles to be used.
Other liquids are also used as fillers. The necessary criterion is that the pressure inside the system should be higher than the vapor pressure of the fluid to prevent the formation of bubble vapors and also the solidification of the fluid.
The minimum working temperature is usually determined based on the freezing point of the fluid, which is between -75 and -210. The maximum temperature is determined based on the point at which the fluid is no longer stable, and is usually 315 degrees.
The minimum temperature range is a function of bubble size and the maximum is a function of linearity. With large bubbles, the measured temperature range can be reduced from 12 to 25 degrees. While the maximum temperature range is 167 degrees due to its non-linearity.
The maximum temperature that a bubble can reach without damaging it is known as the outside of the system. Out of range is usually expressed as a percentage of the total range. In the case of fluid-filled systems, the out-of-range rate is 100%
2- Steam filled systems:
The capillary pressure and bubble element in a system has a filler fluid in both liquid and vapor forms. The situation between the two must occur in the bubble. So that it moves slowly with increasing temperature and its pressure does not change. The intra-system pressure is a function of the vapor pressure of the filler fluid at operating temperature.
The filler fluid used includes methyl chloride, sulfur dioxide, butane, propane, hexane, ether methyl, ethyl chloride, ethyl ether, ethyl alcohol, and chlorobenzene. Each of these fluids has a different pressure-temperature characteristic.
In general, the minimum temperature that can be used for steam-filled systems is about -40 degrees and the maximum is about 315 degrees Celsius. The maximum temperature is limited by the critical point of the filler fluid, and the minimum temperature limit is due to the decrease in sensitivity at low temperatures.
The nonlinear nature of steam-filled temperature systems is a disadvantage, but if the upper end of the range requires high sensitivity, nonlinearity will be an advantage.
The response speed of steam-filled systems is generally in the range of 1 to 10 seconds. This speed is faster than systems filled with liquid and mercury and is about the same as gas-filled systems. The allowable limit is out of range in steam-filled systems because the steam pressure increases exponentially with increasing temperature.
Class IIA systems: In a temperature system filled with a liquid that is in equilibrium with the vapor, the liquid is always placed at the cold end and the vapor at the hot end. In Class IIA temperature systems, the bubbles are filled with steam, filled with capillaries and equipped with liquid reading.
This type of system can only be used in applications where the bubble temperature is always higher than the ambient temperature of the measuring equipment. As the process temperature increases, more liquid evaporates, which increases the vapor pressure in the bubble and in the measuring tube of the equipment.
Class IIB systems: In Class IIB systems, all the filler fluid is in a bubble and the rest of the system is filled with steam. This type of temperature system can only be used in applications where the bubble temperature is always lower than the ambient temperature of the capillary or measuring equipment.
If this situation occurs creatively, the liquid will start to boil in the bubble and condense again in the Bardon tube. This fluid transfer cannot be completed due to insufficient fluid to fill the Bardon and capillary tubes. The read temperature will not be real when this transfer occurs.
Class IIB systems use the smallest bubble since the bubble should not act as a thermal expansion chamber.
IIC Class System: This system is capable of operating as both Class IIA (right) and Class IIB (left); The bubble is large enough. To accept all the filler fluid of Bardon and capillary tubes. The required bubble in this case is larger than the previous two types of temperature systems.
This type of design is used in applications where the process temperature is higher or lower than the ambient temperature. This system can not be used in cases where it is possible to pass the ambient temperature because after the liquid / steam interface is changed, some time is needed to transfer the filler liquid to the cold end of the temperature system and establish it.
Class IID system: In Class IID system, there are two filler fluids: one that is volatile and always in the bubble, and the other that is non-volatile and fills the capillary and part of the bubble with the Bardon tube.
The role of the non-volatile liquid is only to transfer the vapor pressure into the volatile liquid / vapor composition that is trapped in the bubble. In this type of design, the process temperature can include anything beyond the ambient temperature. This system requires an even larger bubble than the IIT class.
3- Gas-filled systems:
The basis of the operation of gas-filled systems is that the pressure of a completely trapped gas in a constant volume is proportional to its absolute temperature. Nitrogen is a useful filler fluid for Class III systems because it is inert and inexpensive.
At temperatures above 427 degrees Nitrogen reacts with the bubble constituents and at very low temperatures it acts less as a complete gas. In such cases helium must be used. Different working ranges can be achieved by selecting the appropriate filler fluid.
In general, the bubble should be considered as large as possible to reduce the temperature effects on the capillaries.
Class III systems are primarily used to measure high and low temperatures. At low temperatures these systems are limited to -268 degrees by the critical temperature of the filling gas and at high temperatures due to temperature restrictions due to the material of the bubble constituents is limited to 667 degrees.
The rate of response of gas-filled systems is usually good, since the maximum temperature in Class III systems is limited only by the allowable pressure and temperature of the bubble. They have out-of-range protection of 150% to 300%.
The gas-filled bubble is suitable for measuring a wide range of ambient temperatures, such as dryers and ovens.
4- Mercury filled systems:
Because the mercury is liquid, the V-class system is similar to the I-class system. These two classes differ in their unique properties of mercury and their importance as a temperature measuring fluid. Mercury has a fast response and high accuracy, and working pressures are relatively high.
Mercury-filled systems can measure temperatures between freezing and boiling points of mercury, ie -40 degrees to 649 degrees. The response rate of mercury-filled systems is faster than that of liquid-filled systems, but slower than that of gas- or steam-filled systems.
Class V systems have at least 100% protection outside the range 0
* Simple operation
* Hard and durable
* No need for nutrition
* Easy to maintain
* Good sensitivity and accuracy
* Inherently explosion-proof
* Bulky bubbles
* Low response speed
* For large ranges only
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