Field of invention
The present invention relates to a calibration system for temperature sensor and more particularly it relates to a calibration system for temperature sensor and method thereof for continuous and accurate calibration of temperature sensors at lower-wider temperature range of -196°C to -30°C.

Background of invention
The term “temperature” defines an intensity of heat present in a substance or object, especially as expressed according to a comparative scale and shown by a temperature measuring instruments. The temperature is the most significant and important parameter, and therefore the precision temperature measurement is essential in electrical power, automotive & material processing industries, and widely in all aspects of aviation, aerospace, weapons, ships and civil research in production and testing. Since long time, various kinds of temperature measuring instruments/devices are being utilizing for temperature measurement i.e. thermocouples, thermistors, resistance temperature detector (RTD), pyrometer, Longmuir probes (for electron temperature of plasma), platinum resistance thermometers (PRT), sheath-protected resistance thermometer (SPRT), infrared and other common temperature sensors.

The sensor calibration is a method of improving the sensor performance by removing structural errors in sensor outputs. The structural errors are differences between a sensor’s expected output and its measured output, which shows up consistently every time a new measurement is taken. The temperature sensor calibrations are used in instruments which are designed for measuring of temperature. Occasionally, an existing conventional temperature sensor might fail during calibration even though the temperature sensor seemed to be functioning properly prior to sending it for calibration.

With the development of science and technology, various kinds of thermometers and sensors for temperature measurement are increasing, and the temperature range that needs to be measured and calibrated is also getting wider and wider. The temperature measuring and detecting devices have been greatly evolved since last few years for improving the accuracy. To be accurate, all temperature sensors must be calibrated against a known standard. During calibration, only a short-term stability is to be checked, however the long-term stability must be monitored and determined.

The calibration system plays an important role in calibrating of temperature sensors in calibration industries and laboratories for the purpose of testing and calibration. Occasionally, the temperature sensors might fail during calibration, even though the temperature sensors seemed to be functioning properly prior to sending it in for calibration. The conventional temperature sensors might fail during calibration due to self-heating in thermistors and PRT, low insulation resistance and leakage currents, broken or intermittent lead wires, contamination, hysteresis and non-repeatability, inhomogeneity etc.

To calibrate a temperature sensor, a reference temperature with accuracy better than the temperature sensor to be calibrated is desired. The output of the temperature sensor can then be compared to the reference temperature for calibration purposes. The calibration schemes are generally either thermal calibration schemes or electrical calibration schemes. For example, one thermal calibration scheme includes, using a temperature bath or chamber to produce a reference temperature, and a high accuracy thermometer to measure the reference temperature. The measured reference temperature is compared to the output of the temperature sensor to be calibrated. However, it is very difficult to accurately control and measure the temperature of the reference chamber environment in thermal calibration scheme. Additionally, it takes a relatively longer time (on the order of minutes or tens of minutes, for example) for thermal contact and stabilization. Thereby, this increases the cost of the test-stages of the manufacturing process of each part.

One such temperature calibration system of US Patent No. 3699800 utilizes three wells, each operating over a limited temperature range, to provide an instrument suitable for industrial use. This instrument incorporates a ramp type proportional control circuit for reducing the hunting of the actual temperature about the desired temperature. This instrument work satisfactorily only for one set of conditions of an ambient temperature and line voltage, but variations in an ambient temperature and/or supply voltage introduced errors in the system. One more such temperature calibration device is disclosed in US Patent No. 3738174 which is an improvement of earlier device of US Patent 3699800 and had a very fast thermal response so that temperature hunting was not a problem. However, the practically useable range of the instrument is limited and the commercial embodiment has an operating range of 100°F to 600°F.

Conventionally, the prior art temperature calibration instruments calibrate the temperature sensors in narrow ranges i.e. (-50°C to -150°C), (-80°C to - 180°C) and (-80°C to -180°C) etc. Further, the existing conventional temperature calibration devices essentially require additional instruments to cover the lower-wider temperature range (-196°C to -30°C) for calibration of the temperature sensors. In addition, said existing temperature calibration devices are bearing with the problem of non-uniform temperature distribution and controlling thereof becomes more difficult.

Although the conventional calibration systems of temperature sensors, hereinbefore described, are advantageous in terms of lower cost, however the disadvantages include a relatively lower accuracy at moderate temperatures and they are susceptible to inhomogeneity. In the prior arts, the resistance temperature sensors are calibrated by the conventional calibration method of fixed point verification and formula indexing. The drawbacks and disadvantages associated with the said conventional calibration method include a large deviation of the calibration results from the actual value and low reliability which can not meet the needs of model tasks.

Hence, it becomes imperative to solve the problems and difficulties of the existing conventional calibration systems of temperature sensors. For that, it is imperatively needed to provide a bettered calibration system for temperature measuring devices (temperature sensors) which can overcome the existing drawbacks and difficulties of the conventional calibration systems that allows the temperature calibration in wider range of temperature from (-30°C) to (- 196°C) and solves the problem of non-uniform temperature distribution in temperature calibration system.

Object of invention
The main object of the present invention is to provide a calibration system for temperature sensor and method thereof.

Another object of the present invention is to provide a calibration system for temperature sensor and method thereof which can calibrate the temperature sensors at lower-wider temperature range (i.e. -196°C to -30°C).

Still another object of the present invention is to provide a calibration system for temperature sensor and method thereof which can be used as a comparator calibrator for calibration of RTD, SPRT, thermocouples and other temperature probes at very low temperatures.

Yet another object of the present invention is to provide a calibration system for temperature sensor and method thereof that is having comparatively moderate capital cost and lower operational cost.

Yet another object of the present invention is to provide a calibration system for temperature sensor and method thereof which is portable and imparts ease of operation.

Yet another object of the present invention is to provide a calibration system for temperature sensor and method thereof which can overcome the drawbacks and shortcomings of the conventional calibration system for temperature sensors.
Summary of invention
A calibration system for temperature sensor and method thereof comprises insulated container having removable top cover being filled with liquid nitrogen, electrically and thermally conductive calibration block is wrapped with silicon rubber heater having thermostat and covered with outer block being coaxially disposed within container, controller chamber having PID controller and precision RTD being electrically connected to calibration block through RTD cable, and solid state relay being configured for varying power to silicon rubber heater which is electrically connected to controller through heater cable. The calibration block includes longitudinal insertion holes along inner partial length of calibration block, one said hole for insertion of master calibration sensor and holes for separately single insertion of reference temperature sensors. Said calibration system provides ease of portability and calibration in range of (-196°C) to (-30°C).

Brief description of drawings
Fig. 1 is a schematic cross-sectional view of a calibration system for temperature sensor according to the present invention.

Fig. 2 depicts a schematic perspective view of calibration block wrapped with a silicon rubber heater and covered by an outer block according to the present invention.

Fig. 3 is a detailed perspective view of a comparative calibration block having longitudinal insertion holes according to the present invention.

Fig. 4 depicts a pictorial perspective view of insulated container having removable top cover being removed for filling of liquid nitrogen into the container according to the present invention.

Fig. 5 is a pictorial perspective view of controller chamber having PID controller according to the present invention.

Fig. 6 illustrates a pictorial perspective view of insulated container, being filled liquid nitrogen, in which the calibration block covered by an outer block has been immersed from the top of the container that causing fumes to be come out from the container according to the present invention.

Fig. 7 illustrates a pictorial perspective view of insulated container being closed with the removable top cover on which the insulation wool has been provided to prevent the cooling losses according to the present invention.

Fig. 8 illustrates a pictorial perspective view of controller chamber of the calibration system depicting the connection ports for connecting of heater cable and RTD cable according to the present invention.

Fig. 9 depicts a pictorial view of a calibration system for temperature sensor according to the present invention.

Fig. 10 is a graphical representation of experimental readings of temperature of master calibration sensor (along vertical axis) with respect to timeframe of 5 minutes (along horizontal axis) that representing stability at set-point temperature of -196°C according to the present invention.

Fig. 11 is a graphical representation of experimental readings of temperature of master calibration sensor (along vertical axis) with respect to timeframe of 5 minutes (along horizontal axis) that representing stability at set-point temperature of -100°C according to the present invention.

Fig. 12 is a graphical representation of experimental readings of temperature of master calibration sensor (along vertical axis) with respect to timeframe of 5 minutes (along horizontal axis) that representing stability at set-point temperature of -30°C according to the present invention.

Detailed description of invention
The nature of the invention and the manner in which it works is clearly described in the complete specification. The invention has various embodiments and they are clearly described in the following pages of the complete specification. Before explaining the present invention, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation.

Before explaining the present invention, it is to be noted that the term “PID Controller” is a proportional integral derivative controller which continuously calculates an error value {e(t)} as a difference between the desired set-point value (SP) and a measured variable present value (PV) and applies a correction based on proportional, integral and derivative terms. Further, it is to be understood that the term “RTD” refers to a resistance temperature detector which contains an electrical resistance source, the resistance of which changes depending on temperature of the element itself.

Referring to Fig.1, a calibration system for temperature sensor according to the present invention comprises an insulated container (1) being filled with liquid nitrogen (N2) and having a removable top cover (2), an electrically and thermally conductive comparative calibration block (3) being coaxially disposed within said insulated container (1) and is wrapped with a silicon rubber heater (4) on outer cylindrical surface thereof that defining a heating furnace; a controller chamber (5) having a self-tuned PID controller (6) includes a precision PT-100 RTD as a temperature sensing element and being electrically connected to the calibration block (3) through an RTD cable (7); and an electrically driven fan (not shown in figure) for cooling the internal circuits of the controller chamber (5). The silicon rubber heater (4) wrapped onto the calibration block (2) is electrically connected to the PID controller (6) through a heater cable (8). As shown in Fig. 2, the silicon rubber heater (4) includes a thermostat (11) to detect and regulate the temperature and the comparative calibration block (3) wrapped with the silicon rubber heater (4) is covered by an outer block (12).

Said liquid nitrogen offers very low temperature range from -196°C to -30°C for calibration of the temperature sensors like thermocouples, thermistors, resistance temperature detector (RTD), pyrometer, Longmuir probes (for electron temperature of plasma), platinum resistance thermometers (PRT), sheath-protected resistance thermometer (SPRT), infrared and other common temperature sensors. Further, said comparative calibration block (3) and an outer block (12) is preferably, but not limited to, made from aluminum material.

Further, said silicon rubber heater (4) having excellent source is for uniform heat especially on challenging shapes and applications with flexibility or vibration. It plays an important role in temperature controlling, because of fast cooling and heating. It has also an advantage of better calibration result in any harsh environmental condition. Said silicon heater (4) is wrapped at the outer side of the calibration block (3) and insulated with rubber sheet. It has an advantage of fine controlling, which helps in using the present calibration system in lower wide range from (-196°C) to (-30°C).

Before explaining the detailed invention, it is to be understood that the term “silicon rubber heater”, hereinbefore described, refers to a silicon heater having insulated with a rubber sheet. Further, the term “set-point temperature”, hereinafter described, refers to a temperature to be set in self-tuned PID controller for calibration in the range of -196°C to -30°C.

Further referring to Fig.1, in present embodiment, said insulated container (1), comparative calibration block (3) and outer block (12) are cylindrical in shape. However, it is to be understood that it may be in any desired shape. The comparative calibration block (3) that is covered by an outer block (12) act as a heat exchanger while heating through the silicon rubber heater (4) as per set-point temperature of the PID controller (6). Said heater cable (8) and the RTD cable (7) are electrically conductive cable being covered with insulation material. The silicon rubber heater (4) wrapped onto the comparative calibration block (3) performs the function of heating of comparative calibration block (3) that is disposed within the insulated container (1). In addition, insulation wool (10) is provided onto the top cover (2) of the insulated container (1), as shown in Fig. 7, to prevent cooling losses.

Turning now onto Fig. 3, the comparative calibration block (3) includes multiple longitudinal insertion holes (H1, H2, H3, H4) along an inner partial length of the calibration block (3). Among them, at least one longitudinal hole (H1) for allowing an insertion of a master calibration sensor and remaining longitudinal holes (H2, H3, H4) for separately single insertion of reference temperature sensors (RTD, SPRT, thermocouples and other temperature probes) to be calibrated.

The reference temperature sensors to be calibrated that being separately disposed into the insertion holes (H2, H3 and H4) are hereinafter referred to as “units under calibration”.

Referring back to Fig. 1, the controller chamber (5) having PID controller (6) includes a precision RTD as a controlling sensor being configured for controlling the temperature of silicon rubber heater (4) and in-house temperature of the liquid container (1); a solid state relay (9) being configured for varying power to the silicon rubber heater (4) to maintain a requisite temperature according to the set-point temperature of the PID controller (6). An up-down key is also provided in the controller chamber (5) for setting and changing of set-point temperature in the controller (6) for calibration of the temperature sensors in the range of -196°C to -30°C. Said PID controller (6), solid state relay (9) and a communication port are to be interconnected into the controller chamber (5). The controller chamber (5) and all components thereof are powered by an input supply voltage/power (e.g. 110 VAC).

Now turning onto Fig.1 to Fig.9, the installation of calibration system for temperature sensors (i.e. RTD, SPRT, thermocouples and other temperature probes) according to the present invention includes the following steps:
1) designing of electrically and thermally conductive comparative calibration block (3) by forming multiple longitudinal insertion holes (H1, H2, H3, H4) along an inner partial length of the calibration block (3) as shown in Fig.3, at least one said longitudinal hole (H1) for insertion of master calibration sensor and the longitudinal holes (H2, H3, H4) for insertion of reference temperature sensors to be calibrated;
2) wrapping of said calibration block (3) obtained in step (1) by silicon rubber heater (4) having an insulation of rubber sheet defining a heating furnace;
3) covering of said calibration block (3) obtained in step (2) by an outer block (12);
4) filling of liquid nitrogen (N2) into the insulated container (1) after removing of the top cover (2), as shown in Fig. 4;
5) designing and developing of controller chamber (5), as shown in Fig. 5, by electrically and signally interconnecting of components i.e. self-tuned PID controller (6) having precision RTD, solid state relay (9), power supply and communication port into a single chamber;
6) coaxially immersing of said calibration block (3) covered by an outer block (12), that is obtained in step (3), into the insulated container (1) of step (4), as shown in Fig. 6, that causing fumes to be comes out from the container (1);
7) fitting of top cover (2) at the top of the nitrogen container (1) through the RTD cable (7) and heater cable (8); and providing an insulation wool (10), as shown in Fig. 7, onto the top cover (2) to prevent cooling losses;
8) electrically connecting the calibration block (3) obtained in step (6) to the precision RTD of the controller (6) through the RTD cable (7), and electrically connecting the silicon rubber heater (4) to the PID controller (6) through the heater cable (8) as shown in Fig. 8-9.

After installation of calibration system for temperature sensors as illustrated in Fig.1 to Fig.9; the calibration process according to the present invention comprises the following steps:
a) connecting of controller chamber (5) at communication port to the power supply source and providing an input supply power/voltage to the self-tuned controller (6);
b) setting of set-point temperature between -196°C to -30°C in the self tuned PID controller (6) by using an up-down key (not shown in figure) for calibration of the reference temperature sensors (in temperature range of -196°C to -30°C) that are disposed into the insertion holes (H2, H3 and H4);
c) after setting of set-point temperature in step (b), automatically gradually increasing the temperature of the master calibration sensor (disposed into the longitudinal insertion hole H1) until it reaches to the set-point temperature;
d) allowing of temperature stabilization of the master calibration sensor (disposed into insertion hole H1) and the units under calibration (disposed into the insertion holes H2, H3, H4) until the present value (PV) gets equal to the set value (SV) of the set-point temperature;
e) after temperature stabilization in step (d), recording of readings of the master calibration sensor and the units under calibration, when the master calibration sensor and the units under calibration are thermally stable at or near the set-point temperature;
f) comparing the readings of the master calibration sensor with the readings of the units under calibration obtained in step (e), and thereafter finding out the error by comparison method.

The present invention is illustrated more in details in the following experimental examples. The example describes and demonstrates the embodiments within the scope of the present invention. This example is given solely for the purpose of illustration and is not to be construed as limitations of the present invention, as many variations thereof are possible without departing from spirit and scope.

Example 1:
In the first experiment, the calibration of heating furnace by using master calibration sensor was carried out at set-point temperature of -196°C, in which the temperature of -196°C was set in the self-tuned PID controller (6). Thereafter, the master calibration sensor and heating furnace were gradually thermally stabilized until the temperature of master calibration sensor was reached to or near the set-point temperature. Then, the readings of temperature of the master calibration sensor were recorded at regular interval of 5 minutes that have been illustrated in below Table 1. To check the stability of heating furnace, said readings of the master calibration sensor were compared with the temperature of the heating furnace and error was found out by the comparison method. Graphical representation of said readings of temperature of master calibration sensor with respect to timeframe of 5 minutes at set-point temperature of -196°C is shown in Fig. 10.
Table 1
Sr. No. Time
(Min) Set-Point Temperature/or
Temperature of Heating Furnace (°C) Temperature of master calibration sensor
(°C)
1 5 -196 -196.42
2 10 -196 -196.4
3 15 -196 -196.37
4 20 -196 -196.32
5 25 -196 -196.28
6 30 -196 -196.27
7 35 -196 -196.26
8 40 -196 -196.26
9 45 -196 -196.24
10 50 -196 -196.23
11 55 -196 -196.22
12 60 -196 -196.22
13 65 -196 -196.22
14 70 -196 -196.22
Example 2:
In the second experiment, the calibration of heating furnace by using master calibration sensor was carried out at set-point temperature of -100°C, in which the temperature of -100°C was set in the self-tuned PID controller (6). Thereafter, the master calibration sensor and heating furnace were gradually thermally stabilized until the temperature of master calibration sensor reached to or near the set-point temperature. Then, the readings of temperature of the master calibration sensor were recorded at regular interval of 5 minutes that have been illustrated in below Table 2. To check the stability of heating furnace, said readings of the master calibration sensor were compared with the temperature of the heating furnace and error was found out by the comparison method. Graphical representation of said readings of temperature of master calibration sensor with respect to timeframe of 5 minutes at set-point temperature of -100°C is shown in Fig. 11.

Table 2
Sr. No. Time
(Min) Set-Point Temperature/or
Temperature of Heating Furnace (°C) Temperature of Master Calibration Sensor
(°C)
1 5 -100 -100.36
2 10 -100 -100.34
3 15 -100 -100.32
4 20 -100 -100.28
5 25 -100 -100.22
6 30 -100 -100.19
7 35 -100 -100.18
8 40 -100 -100.17
9 45 -100 -100.17
10 50 -100 -100.17
11 55 -100 -100.16
12 60 -100 -100.16
13 65 -100 -100.16
14 70 -100 -100.16
Example 3:
In the second experiment, the calibration of heating furnace by using master calibration sensor was carried out at set-point temperature of -30°C, in which the temperature of -30°C was set in the self-tuned PID controller (6). Thereafter, the master calibration sensor and heating furnace were gradually thermally stabilized until the temperature of master calibration sensor reached to or near the set-point temperature. Then, the readings of temperature of the master calibration sensor were recorded at regular interval of 5 minutes that have been illustrated in below Table 3. To check the stability of heating furnace, said readings of the master calibration sensor were compared with the temperature of the heating furnace and error was found out by the comparison method. Graphical representation of said readings of temperature of master calibration sensor with respect to timeframe of 5 minutes at set-point temperature of -30°C is shown in Fig. 12.
Table 3
Sr. No. Time
(Min) Set-Point Temperature/or
Temperature of Heating Furnace (°C) Temperature of Master Calibration Sensor
(°C)
1 5 -30 -30.35
2 10 -30 -30.31
3 15 -30 -30.29
4 20 -30 -30.25
5 25 -30 -30.22
6 30 -30 -30.2
7 35 -30 -30.18
8 40 -30 -30.18
9 45 -30 -30.16
10 50 -30 -30.16
11 55 -30 -30.15
12 60 -30 -30.15
13 65 -30 -30.15
14 70 -30 -30.15
The calibration system for temperature sensors of the present invention provides ease of portability and operational characteristics having moderate cost. Further, said calibration system also provides an accurate and continuous calibration of temperature sensors in lower wide temperature range (-196°C to -30°C). The design of removable calibration block of the present invention allows the user to easily changeover the calibration block. Further, the silicon rubber heater employed in the calibration system of the present invention is flexible that can be fixed around circular surface of the calibration block, which helps in maintaining uniform temperature distribution of the comparison calibration block.

The invention has been explained in relation to specific embodiment. It is inferred that the foregoing description is only illustrative of the present invention and it is not intended that the invention be limited or restrictive thereto. Many other specific embodiments of the present invention will be apparent to one skilled in the art from the foregoing disclosure. All substitution, alterations and modification of the present invention which come within the scope of the following claims are to which the present invention is readily susceptible without departing from the spirit of the invention. The scope of the invention should therefore be determined not with reference to the above description but should be determined with reference to appended claims along with full scope of equivalents to which such claims are entitled.

We claim,

1. A calibration system for temperature sensor comprising:

an insulated container (1) having a removable top cover (2) being filled with liquid nitrogen (N2), an insulation wool (10) being provided onto said top cover (2) to prevent cooling losses;
an electrically and thermally conductive comparative calibration block (3) being coaxially disposed within said insulated container (1);
a controller chamber (5) having a self-tuned PID controller (6) includes a precision PT-100 RTD being electrically connected to the comparison calibration block (3) through an RTD cable (7), and a solid state relay (9);
an electrically driven fan for cooling of internal circuits of the controller chamber (5),

characterized in that,

said comparative calibration block (3) is wrapped with a silicon rubber heater (4) to define a heating furnace onto the cylindrical surface of the calibration block (3); said heating furnace is covered by an outer block (12);
said silicon rubber heater (4) having a thermostat (11) is electrically connected to the PID controller (6) through a heater cable (8), and being configured for heating of the comparative calibration block (3) as per set-point temperature being sent in PID controller (6);
said comparative calibration block (3) having longitudinal insertion holes (H1, H2, H3, H4) along an inner partial length of the calibration block (3), one said longitudinal hole (H1) for insertion of a master calibration sensor and longitudinal holes (H2, H3, H4) for separately single insertion of reference temperature sensors to be calibrated;
said self-tuned PID controller (6) is configured for digitally inputting an alterable set-point temperature in the range from (-196°C) to (-30°C) and said solid state relay (9) is configured for varying power to the silicon rubber heater (4) to maintain a requisite temperature of the heating furnace.

2. The calibration system for temperature sensor as claimed in claim 1 wherein said comparative calibration block (3) and outer block (12) is made from aluminum.

3. The calibration system for temperature sensor as claimed in claim 1 wherein said heater cable (8) and the RTD cable (7) are electrically conductive cable being covered by an insulating material.

4. The calibration system for temperature sensor as claimed in claim 1 wherein said reference temperature sensors includes an RTD, SPRT, thermocouples and temperature probes.

5. A method for calibration of temperature sensor comprising following steps:

a) connecting of controller chamber (5) at communication port to the power supply source and providing an input power/voltage to the self tuned PID controller (6);
b) setting of set-point temperature between -196°C to -30°C in the self tuned PID controller (6) using an up-down key for calibration of the reference temperature sensors (disposed in the insertion holes (H2, H3, H4) in the range of -196°C to -30°C;
c) increasing the temperature of the master calibration sensor (disposed into the insertion hole H1) until it reaches to the set-point temperature;
d) allowing of temperature stabilization of the master calibration sensor (disposed into the insertion hole H1) and the units under calibration (disposed into the insertion holes H2, H3, H4) until the present value (PV) gets equal to the set value (SV) of the set-point temperature;
e) recording of readings of the master calibration sensor and readings of the units under calibration, when the master calibration sensor and the units under calibration are thermally stable at or near the set-point temperature;
f) comparing the readings of the master calibration sensor with the readings of the units under calibration obtained in step (e), and thereafter finding out the error by comparison method.