Types of Thermal Calibration
There happen to be chiefly 3 types of thermal calibration methods, which are as follows:
Fixed Point Method of Thermal Calibration
Fixed points happen to be the highly accurate devices, which are available to determine the temperature’s scale. The devices that are fixed-point in nature tend to utilize completely pure materials in sealed, inert environments; these tend to be generally delicate in nature, so, must be handled with utmost care. This method i.e. the fixed point method is basically an absolute methodology that is utilized for realizing the ITS-90 (which is the International-Temperature-Scale). The thermometer tends to be calibrated by measuring a series of temperature points that are fixed: for example melting or freezing point, triple point and vapor pressure point. This particular method consists in placing the thermometer in a cell with a fixed point, which provides an environment that is isothermal in nature. A fixed point cell is basically a flask that is almost filled completely with pure material & that is protected by a shell. The cell is placed in an apparatus, which provides fine temperature control as well as sufficient cell immersion for producing a uniform temperature in that particular measuring zone. The apparatus should have cooling that is fixed point & regulated heat. This can be a furnace; usually a vertical tube one with adequate insulation, or a liquid bath one. Multiple set points are selected. These must be as few as possible, which is consistent with creating a fine interpolation formula. Such methodology provides a very accurate calibration, and is being used only for the most high-quality thermometers. However, this is hard to implement due to the intricacy of the equipment, and the very many precautions needed to be taken for making fixed points successful. Fixed points are realized with the help of many pure substances. But, many don’t provide the necessary stability & reproducibility, and some others may also need an intricate procedure & special lab facilities.
Comparison Method of Thermal Calibration
In this type of method, usually the following are needed:
- A stable source for temperature
- A calibrated master sensor
- A calibrated meter
The selectable calibration medium largely depends on the range of temperature as described below:
- For low temperature‘s calibration (that is usually below minus 180 degree Celsius) a copper block that is vacuum-jacketed.
- A controlled mixed liquid bath above minus ninety degree Celsius and up to 300 degree Celsius is the most common calibration method. Freon/alcohol could also be utilized from minus ninety degree Celsius, water at room temperature, oil up to 300 minus ninety degree Celsius.
- Salt-bath/liquid bath/dry-block can be used at temperatures up to 600 degree Celsius.
- Furnaces are more suitable for much higher temperatures. There are many models. Some have low thermal mass & fast response; while others may have high thermal mass & slow response.
For minimizing or reducing the temperature gradients in the bath/furnace, a metal equalizing block with thermowells is placed to receive both the standard as well as the thermometers, which are to be calibrated. This particular methodology makes it possible to cover a wide temperature range in a short time, point-by-point/continuously, & for calibrating several thermometers at the same time. However, the comparative method is less accurate than the fixed one.
Electrical Method of Thermal Calibration – Simulators & Sources
Indicators & controllers tend to be calibrated using injection signals that usually simulate thermocouples, thermistors or resistance thermometers. The simulators tend to give quite fast & comfortable methods to calibrate the instrument at several points.
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