Study on calibration of surface temperature source

Surface temperature source is widely used to provide adjustable, stable and uniform temperature fields to calibrate surface thermometers. Parameters such as temperature deviation, temperature uniformity, and temperature fluctuation can directly decide the metrological performance of the instrument. At present, there is no relevant calibration specification for surface temperature sources in China. This article presents a novel calibration method for surface temperature sources by determining corresponding environmental conditions, calibration standards and calculation equations. The calibration results show that the calibration method is scientific and practical for the metrological performance evaluation of surface temperature sources.


Introduction
The surface temperature source mainly consists of a hot plate, a temperature control device, a measurement standard and other supporting measurement instruments [1][2][3][4] .The hot plate whose temperature is controlled by the heating device and measurement standard buried under the hot plate provides an adjustable, stable and uniform temperature field.Surface temperature source is widely used to provide an adjustable, stable and uniform temperature field to calibrate surface thermometers [5-8]   .There are mainly two kinds of surface temperature sources according to the accuracy of the instruments, which are standard surface temperature source and surface temperature source for working.However, there is no calibration specification for the surface temperature source in China at present and its metrological performance cannot be ensured.At present, most surface temperature sources are calibrated based on JJF1409-2013 Calibration Specification for the Surface Thermometers [9]   , whose appendix D provides the measurement method of temperature stability and uniformity for surface temperature source, but the measurement method of temperature deviation is missing; HJ 1133-2020 Ambient Air and waste gas -Determination of arsenic, selenium, bismuth, antimony in ambient particle matter -Atomic fluorescence spectrometry [10] only stipulates the temperature control precision of surface temperature source, but the calibration method of temperature precision is not specified; JJF1101-2019 Calibration Specification of Environment Testing Equipment for Temperature and Humidity Parameters [11] is mainly applied to the temperature calibration of confined space, but with limitations when applied to surface temperature source, whose temperature field is open in the air.Therefore, to comprehensively improve the accuracy of metrological performance evaluation of surface temperature sources to ensure the reliable working performance of surface temperature sources, there is an urgent need to develop an effective calibration method for surface temperature sources.This paper studies the influential factors on essential technical parameters of surface temperature sources such as "temperature deviation", "temperature uniformity" and "temperature fluctuation" to present a suitable and scientific calibration method for the temperature source, which meets the metering requirements of the equipment in practical use.

Environmental conditions
During the whole process of the measurement, the temperature should be (10-28)℃ with fluctuation of no more than 0.5℃.Humidity should be no more than 80% RH.The power supply should be with good grounding.Its AC voltage should be (220 ± 22) V or (380 ± 38) V. Its frequency should be within the range of (50 ± 1) Hz.The surface temperature source should be firmly installed.There should be no electromagnetic interference or mechanical vibration around it to affect its normal operation.No explosive gases or corrosive liquids are allowed to be present.

Metrological standards
Multi-channel surface temperature measuring instrument equipped with the same type of surface temperature thermometer with a weight of 100 g is used to calibrate the surface temperature source.The number of temperature thermometers should meet the requirement of calibration point arrangement of the calibration method presented in this article.The measurement range of the instrument should be no narrower than (0-400)℃.The definition should be better than 0.01℃.The maximum permissive error is ±0.5℃.The whole set of the multi-channel surface temperature measuring instrument should be used with the corrected value participating in the calculation.The temperature sensors of the multi-channel surface temperature are installed to measure the instrument.According to Figure 1, the surface temperature source is set to the calibration temperature, starting the operation.After the surface temperature source reaches a stable state, the indication value of each temperature sensor at a time interval of 1 minute is recorded for 10 minutes.11 indication values of each sensor are recorded.

Check before calibration
The surface temperature source should have a nameplate indicating the name, model, serial number, manufacturer and production date of the instrument.The surface of the temperature source should be clean, flat without oil stains, obvious scratches and unevenness, etc. and free from surface oxidation which may affect the measurement accuracy of the instrument.

Temperature deviation calibration
The temperature deviation of the surface temperature source is calculated based on the measurement results of each temperature sensor of the multi-channel surface temperature measuring instrument.Temperature deviation includes upper deviation ∆ and lower deviation ∆ , which are calculated according to Equations ( 1) and ( 2): where  is the maximum of measurement results of all temperature sensors,  is the minimum of measurement results of all temperature sensors and ts is the set temperature of the surface temperature source.

Temperature uniformity calibration
The temperature uniformity of the surface temperature source is calculated based on the measurement results of each temperature sensor of the multi-channel surface temperature measuring instrument.Temperature uniformity ∆ is the arithmetic mean of the 11 differences between the maximum and the minimum of the temperature measurement values of all temperature sensors recorded during one measurement every 1 minute when the surface temperature source is in a stable state, as demonstrated in Equation (3): Where  is the maximum of measurement results of all temperature sensors during measurement i (i = 1, 2,...,11), while  is the minimum.

Experimental verification of calibration method
5 typical types of surface temperature sources are selected as the calibration subjects for the verification experiment.Type PLBMY-400 and type 3125 are standard surface temperature sources, while type RC-1AB, type TP300 and type DB-EFS are surface temperature sources for working.The calibration results are demonstrated in Table 1.
Table 1.Calibration results of 5 typical types of surface temperature source.As we can see from Table 1, the maximum temperature deviation of type PLBMY-400 and type 3125 standard surface temperature source is no more than ± 1.0℃, which can meet the requirement of JJF1409-2013 Calibration Specification for the Surface Thermometers of no more than ± 2.0℃.The maximum temperature uniformity of type PLBMY-400 and type 3125 standard surface temperature source is no more than 0.5℃, which can meet the requirement of JJF1409-2013 Calibration Specification for the Surface Thermometers of no more than 0.5℃ within the temperature range of (room temperature -100)℃, no more than 1.0℃ within the temperature range of (100-300)℃ and no more than 1.5℃ within the temperature range of (300-400)℃.The maximum temperature fluctuation of type PLBMY-400 and type 3125 standard surface temperature source within the temperature range of (room temperature-100)℃ is no more than ± 0.4℃, which can meet the requirement of JJF1409-2013 Calibration Specification for the Surface Thermometers of no more than ± 0.4℃.The maximum temperature fluctuation of type PLBMY-400 and type 3125 standard surface temperature source within the temperature range of (100-300)℃ is no more than ± 0.5℃, which can meet the requirement of JJF1409-2013 Calibration Specification for the Surface Thermometers of no more than ± 0.6℃.The maximum temperature fluctuation of type PLBMY-400 and type 3125 standard surface temperature source within the temperature range of (300-400)℃ is no more than ± 0.4℃, which can meet the requirement of JJF1409-2013 Calibration Specification for the Surface Thermometers of no more than ± 1.0℃.The maximum temperature deviation of type RC-1AB, type TP300 and type DB-EFS surface temperature source is no more than ± 4.3℃, which can meet the requirement of HJ 1133-2020 Ambient Air and waste gas -Determination of arsenic, selenium, bismuth, antimony in ambient particle matter -Atomic fluorescence spectrometry of no more than ± 5.0℃.The maximum temperature uniformity of type RC-1AB, type TP300 and type DB-EFS surface temperature source is no more than 1.9℃, which can meet the requirement of JJF1101-2019 Calibration Specification of Environment Testing Equipment for Temperature and Humidity Parameters of no more than 2.0℃.The maximum temperature fluctuation of type RC-1AB, type TP300 and type DB-EFS surface temperature source is no more than ± 1.5℃, which can meet the requirement of operation instructions of manufacturers.The feasibility and applicability of the calibration method have been proven.

Discussion and analysis
There are several factors that can influence the precision of the calibration result of the surface temperature source.

The weight of the surface temperature thermometer of the multi-channel surface temperature measuring instrument
The weight of the surface thermometer represents the pressure applied to the hot plate of the surface temperature source.The temperature measurement results of the same surface thermometer with different extra weights are presented in Table 2.As we can see from Table 2, the selection of a suitable weight for the surface thermometer is crucial for the calibration results of the surface temperature source.

Environmental condition
There should be no obvious air convection in the room where the surface temperature source is placed.If there is air conditioning in the room, it is necessary to add baffles around the work area of the surface temperature source to avoid the influence of air convection on the calibration accuracy of the surface temperature source.

The necessity of preheating
The surface temperature thermometers of the multi-channel surface temperature are placed to measure instrument at the edge of the surface temperature source to preheat the thermometers before calibration, which can reduce the influence of surface temperature thermometers on the already stabilized temperature field of the surface temperature source to improve the reliability of calibration results.

Conclusion
This article presents a novel method for surface temperature sources to calibrate temperature deviation, temperature fluctuation and temperature uniformity of the instrument by determining corresponding environmental conditions, calibration standards and calculation equations.The calibration results show that the calibration method is scientific and practical for the metrological performance evaluation of surface temperature sources.The follow-up study will develop a specific calibration device for surface temperature sources of suitable size, high accuracy and good portability.

3 .
Calibration method 50℃, 100℃, 200℃, 300℃ and 400℃ are selected as the temperature calibration points.The shape of the hot plate of the surface temperature source is usually in two forms: square and circular, as shown in Figure1.Measurement point O is the geometric center point of the working area of the surface temperature source.The other measurement points A, B, C, and D are evenly distributed within the effective working area of the surface temperature source at a distance of 1/10 from the boundary of the effective working area.

Figure 1 .
Figure 1.Plan sketch of the hot plate of the surface temperature source.

Table 2 .
The temperature measurement results of the same surface thermometer with different extra weights.