How must the inserts be designed in order to get the best results?
The inserts should be drilled in accordance with the diameter of the test items. In general, the bore diameter, taking the thermal expansion of the test item into account, should be selected to be as small as possible. Air gap between the thermometer and the bore will act as resistances to heat transfer and will significantly affect the heat transfer through heat conduction. As a consequence, the test bore diameter should be a maximum of 0.5mm larger than the external diameter of the thermometer.
What does radial gradient mean?
Temperature difference between the individual bores of an insert
What does stability mean?
The temperature difference between the minimum and the maximum for fluctuating temperature, taken over 30 minutes.
What is an axial gradient?
Temperature difference/temperature gradient from the bottom – maximum immersion depth – to the top - surface of the calibrator.
What is the difference between a dry well calibrator and a micro calibration bath?
Dry well calibrators/micro calibration baths consist of a thermally insulated metal block which is heated, and for instruments which work with Peltier elements, which can also be cooled. The reference, which the calibrator controls, is mounted directly into the metal block. The working range of commercially available temperature calibrators, using Pt resistance thermometers as a standard, extends from approximately -45 °C to 650 °C. Calibrators that work with Peltier elements are typically used from -35 °C to 165 °C, and those that are fitted with resistance heating from 35 °C to 700 °C. In addition, there are high-temperature dry well calibrators which can be used, depending on the model, up to 1300 °C. They work with precious-metal thermocouples as standards and control thermometers. In these cases, the measurement uncertainties are higher than in calibrators which use a resistance thermometer as a standard. The working range of micro calibration baths is (unlike the dry block calibrators) strictly limited, due to the use of liquids (usually silicone oil) instead of inserts. In order to be able to calibrate using these liquids, they must be sufficiently viscous at ambient temperature. This requirement then limits the upper end of the temperature range to approximately 250°C. They do have the advantage of homogeneous mixing of the liquid due to a magnetic stirrer at the bottom, meaning no axial nor radial gradients are observed.
What is the minimum immersion depth with dry well calibrators?
The minimum immersion depth for dry well calibrators is 70mm. This is driven by the axial gradient, since warm air rises from the bottom up. Should this minimum immersion depth of 70mm not be reached, an external reference thermometer must be used for calibration. This is then placed on the same level as the object to be calibrated and the results are compared with each other.
The other alternative is to use a micro calibration bath, since with the thorough mixing of the liquid, there is no requirement for a minimum immersion depth.
Rule of thumb for recommended immersion depth:
1. Diameter of the test item x 15
2. for a known sensitive length: sensitive length + Diameter of the test item x 10
What material are the inserts for a dry well calibrator made from?
The material for the inserts depends on the temperature range of the dry well calibrator. The selected material should have a temperature range far remove from the melting point.
e.g. for a temperature range of
-35…165°C Material = aluminium
40…650°C Material = brass
200…1100°C Material = Inconel