1. Introduction
Features common vacuum furnace, the temperature measured at a given degree of vacuum. As expected thermocouple accurate and reliable, long service life, we must better explore the factors that affect the accuracy of temperature measurement under vacuum: the vacuum, emissivity, thermal conductivity and thermocouple insertion depth. Therefore vacuum furnace for thermocouple choice is particularly important.
2, Thermocouple for vacuum furnace
2.1, thermocouple classification
International Organization for Standardization (ASTME344-08) thermocouple divided into three categories.
① standardized metal thermocouple; ② standardized precious metal thermocouples; ③ refractory metal thermocouples, a refractory metal thermocouple is defined as: Thermocouple unipolar material melting point higher than 1935 ℃, and thermocouple poles must be refractory metal or alloy.
2.2 Standardized metal thermocouple
Standardization inexpensive metal thermocouple, T, J, E, K and N type. When the vacuum furnace operating temperature below 1260 ℃, the choice of inexpensive metal sheathed thermocouple, K-type thermocouple temperature due to the poor stability, we recommend using the N-type sheathed thermocouples at 1300 ℃, can be a partial substitute for S-type thermocouple.
1) Ni-Cr-Si - Ni-Si-Mg thermocouple (N type) features
① high temperature anti-oxidation ability, good long-term stability;
② in the range of 250 ~ 500 ℃ good short-term stability of thermal cycles, and suppressing magnetic transition, no significant change in the thermal electromotive force;
③ in the range of 400 ~ 1300 ℃, the linearity of the N-type thermocouple thermoelectric properties better than K-type.
2) N-type thermocouple replace S-type thermocouple
Alternative S-type thermocouple with N-type thermocouple to measure the temperature of the heating element of the multi-purpose furnace (1180 ℃). Since 2008, five years,
Our company offers a total of 496 Ipsen N-type thermocouple replace S-type thermocouples, the total savings of 1.74 million yuan for the company. In 2012, for example, the company uses the N-type thermocouple 49, S-type thermocouples only four, indicating replaced with N-type thermocouple S thermocouple has exceeded 90 percent, the economic benefits is extremely significant.
3) The sheathed thermocouple selection
Rugged thermocouple and armored assembled into two categories. From saving metal materials, ease of use and reduce heat loss considerations, as far as possible sheathed thermocouple. Sheathed thermocouple material has the following main categories:
① armor materials in high-temperature region
In the temperature region of armor material is mainly 304,321 or 1Cr18Ni9Ti, while the high temperature area 310S, Inconel600, NiCr alloy. Experimental results show that: in 1000 ~ 1200 ℃ temperature range, with different casing materials, drift sizes, its rule is: NiCr <Inconel600 <310S drift ratio is 0.75:1:5.
② new high temperature sheathed thermocouple material Nicrobell
This alloy is added on the basis of Nicrosil chemical properties of magnesium and niobium, improved oxidation resistance and high-temperature strength of the alloy, with alloy Nicrobell new sheathed thermocouple. Fengdong AG for more than 1200 ℃ temperature zone, and achieved good results.
2.3, standardization precious metal thermocouples
Standardization precious metal thermocouple S, R and B type. In the range of 1260 ~ 1750 ℃, the choice of precious metal thermocouples.
