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Axial and Radial uniformity

Temperature calibration provides a mean of quantifying uncertainties in temperature measurement in order to optimise sensor and/or system accuracies. Uncertainties result from

various factors including:

• a) Sensor tolerances which are usually specified according to published standards and manufacturers specifications.
• b) Instrumentation (measurement) inaccuracies, again specified in manufacturers specifications.
• c) Drift in the characteristics of the sensor due to temperature cycling and ageing.
• d) Possible thermal effects resulting from the installation, for example thermal voltages

created at interconnection junctions

A combination of such factors will constitute overall system uncertainty. Calibration procedures can be applied to sensors and instruments separately or in combination. Calibration can be performed to approved recognised standards (National and International) or may simply constitute checking procedures on an “in-house” basis. Temperature calibration has many facets, it can be carried out thermally in the case of probes or electrically (simulated) in the case of instruments and it can be performed directly with certified equipment or indirectly with traceable standards.

Thermal (temperature) calibration is achieved by elevating (or depressing) the temperature sensor to a known, controlled temperature and measuring the corresponding change in its associated electrical parameter (voltage or resistance). The accurately measured parameter is compared with that of a certified reference probe; the absolute difference represents a calibration error. This is a comparison process. If the sensor is connected to a measuring instrument, the sensor and instrument combination can be effectively calibrated by this technique. Absolute temperatures are provided by fixed point apparatus and comparison measurements are not used in that case.

This series of whitepaper is an attempt to bring the technical awareness to the termperature calibration engineer. Please help us upgrade this with your suggestions at temperature sensor Technology technical@tempsensindia.com

1. Axial uniformity

The vertical gradient in the insert is termed “axial uniformity Due to dissimilarities in construction and length of temperature sensing elements, one must consider the axial uniformity The top and bottom of a dry-well lose heat at different rates than at the center. This occurs because the bottom end is better insulated from ambient effects than is the top end.

The result is a temperature gradient axially along the well. The design of the dry-well compensates for this gradient by attempting to distribute heat to varying optimal degrees along the length of the block. This is very difficult to do, however, because axial temperature uniformities vary at different temperatures, creating ever-changing profiles of needed heat distribution. A thermometer’s reading in a dry-well is the average value of the sensed temperatures along the sensor in the block of the dry-well. PRT sensors have varying lengths and may be located at slightly different positions within their sheaths. Comparing different types of sensors(for example short, sensitive thermocouples or thermistors to long PRT sensors) can create a significant axial location difference making these comparisons particularly susceptible to axial gradients. Therefore, axial temperature non-uniformity of a dry-well calibrator can be a significant contributor to calibration error

How to calculate Axial Uniformity

The Axial temperature uniformity of a dry well block calibrator with single zone is measured using two calibrated sensor T1 & T2.at any constant temperature T put both sensor in Axially aligned position and note down deviation of both sensor As the one sensor T1 moved up from the bottom about 10 mm and found that measured tempT1 vary. In same way note down the deviation of both sensor (T1-T2) on different position of T1

How to improve Axial Uniformity

Using Pad By Applying Pad on upper and lower part of block improvement in Heat uniformity in block has been found

Using three zone furnace In order to reduce calibration errors and improve the performance of field-usable calibrators, a new type of calibrator with Three-zone control was developed .Many new technologies are applied in the Three-zone furnace , and overall performance is improved dramatically over dry-wells. The biggest improvement comes from the excellent axial temperature uniformities across entire temperature range. This improvement comes from technology that automatically adjusts the temperature at the top zone to minimize the differential temperature between the two zones at any temperature setting.

2. Radial Uniformity

The thermal gradient from one well to another is referred to as radial uniformity. Measurement errors caused from imperfect radial uniformity are attributed to Distance between wells and heaters The thermal properties of the insert material Uneven heat distribution caused by non symmetrical loading Two thermometers designed for small stem conduction were placed in each of the holes.

Measurements were recorded and then the probes were moved between the two pockets and repeat measurements made. The temperature, _ t, was calculated to remove the small offsets between the two probe Radial Temperature Homogeneity 50ºC = 0.055ºC

Generally error introduced by radial temperature uniformity is only a small part of the total calibration error. Best results are found when using a comparison insert with a reference probe of the same diameter as the UUT and measuring directly across from the UUT. Because poor axial temperature uniformity in a dry-well calibrator can cause large errors when it is used for calibration, one method to reduce the effect of axial temperature non uniformity is to build a dry-well calibrator with an adjustable axial temperature gradient.

A dual-zone dry-well calibrator, the Metrology Well Calibrator

Conclusion

Axial uniformity is the major component for the cause of error in a calibration furnace. So, it should be taken care that the temperature sensors are at the same point of depth in the furnace. Also it should be noted that the RTDs if calibrated with a thermocouple in a dry block will have a large difference in results, so special care has to be taken. Using of proper pad at top and bottom will reduce drastically this error.Radial uniformity would always be present. It depends on the proper design of the dry block furnace so that, this is kept to the minimum extent.

External Links

• Temperature Sensors Technology of INDIA
• Calibration Services