Discussion On Calibration Method Of Digital Thermometer

1. Experimental subjects and standard equipment

The experimental object for this article is a digital thermometer with a resolution of 0.01 ℃ and above. The calibration experiment is carried out using the comparative method. The standard equipment used for calibration is shown in Table 1.

2. Experimental content

Classification and experimental analysis of temperature sensors, there are the following situations:

1. A digital thermometer with a waterproof sensor and a length that meets the insertion depth requirement (insertion depth> 10cm).

The sensor can be directly inserted into the thermostat for calibration.

2. Digital thermometer with waterproof sensor length ≤ 5cm.

In order to meet the requirements of insertion depth, the sensor needs to be calibrated in a glass test tube. According to the different temperature zones, it is divided into two cases in Table 2 for processing:

3. Digital thermometer with non-waterproof sensor.

The calibration of this type of digital thermometer is divided into two cases in Table 3 according to the temperature range.

4.3 Comparison of calibration methods

It can be judged from Fig. 1 that the calibration method of direct contact with the heat source has the fastest heat conduction speed of the sensor and the shortest time required to reach a constant temperature; the calibration method of installing an air test tube has the slowest heat conduction speed of the sensor, and the time required for heating and constant temperature longest.

Three, other matters needing attention

1. Digital thermometers that use platinum resistance as temperature sensors generally operate in a temperature range of -200℃～+850℃. Some sensors are shorter and need to be connected with lead wires. Usually, a temperature-resistant adhesive is used to connect the sensor and lead.

For example, solder connection: the melting point of pure tin is 231℃, and the melting point of solder will be lower. Usually the upper limit temperature of soldering insurance is 170℃. Therefore, it is necessary to evaluate the temperature resistance strength of the lead of the device under test before calibration to avoid excessive temperature resistance. The upper limit damages the sensor.

2. For sensors with a small diameter (≤2mm), in order to ensure the sensor insertion depth requirement during the calibration process, no matter whether the sensor is waterproof or not, it needs to be inserted into the test tube for calibration to avoid floating in the constant temperature bath due to the light weight of the sensor , Measurement deviation caused by not meeting the insertion depth requirement.

3. In order to ensure the stability of the calibration state of the instrument, it needs to be placed in a constant temperature laboratory environment before the test to warm up. Digital thermometers powered by AC power should be warmed up for more than 15 minutes before calibration (or as required by the manufacturer); digital thermometers with external zero adjustment function can be adjusted after warming up, and it is not allowed to interrupt the test and adjust again during the calibration process.

4. The power supply mode of digital thermometer is divided into AC power supply and DC power supply. DC-powered digital thermometers usually increase the deviation of the calibration results when the display voltage is low. Therefore, the instrument needs to be turned on and checked before calibration to ensure that the device under test has sufficient power.

Four, summary

The digital thermometer has many advantages in temperature measurement, and its application fields have covered a large range of industry, agriculture, and medical treatment. Since there is no corresponding national verification regulations or calibration specifications for digital thermometers as a guide for the traceability and transmission of the value, the content of this article is only based on my repeated experimental operations for many years and the analysis and statistics of a large number of experimental data, and refer to the relevant calibration Implementation rules and related operating specifications.

With the demand and development of the market, it is particularly urgent to issue a more systematic and rigorous work guidance basis. It is hoped that the calibration work system of digital thermometers will be more complete, and the measurement institutions can achieve more accurate, efficient and convenient testing services.