Thermocouple data saturation phenomenon appears irregular output slow release time: 2019 - 07 - 19 reading times: zero

Thermocouple in the process of use will appear some problems, the following will help you analyze thermocouple in a slow output saturation phenomenon. The board collected thermocouple back to weak signal, because the patch is located in the cryogenic vacuum chamber, Copper constantan) Stick wall thermocouple compensation wire need through a vacuum connector, and then to carry on the cold junction compensation, and after 50 m twisted-pair cable transmission to acquisition boards, collection of amplification. Transmission process, the single-ended shielded twisted-pair cable grounding, pick up location is located in the thermocouple. The problem is that the collected thermocouple data saturation phenomenon appears irregular output slow ( Is the collection temperature display in maximum) , appear, will enter the card cable is disconnected, the multimeter measure thermocouple two pins, and then go back, can normal collected again. Thermocouple saturated at a slower speed. Analysis at the beginning, doubt vacuum chamber in vacuum discharge, then recharge the parasitic capacitance of the cable, with discharge number increase, the charge accumulated to a certain extent makes the thermocouple output voltage increase. Problem is that the thermocouple end impedance is small, if there is a capacitor charging, can provide completely discharge channel, and saturation phenomenon will not occur. How can appear the capacitance and cables at both ends? Another doubt is thermocouple signal in transmission to the front of the card, the transmission cable after many transfer ( Vacuum flange adapter, transfer cold junction compensation, compensation wire transfer to the shielded cable) , shielding is not complete, but will not appear this kind of saturation problem. Now want to add a separate module for processing, don't know how the effect will be. The back is a differential signal, isolation effect is questionable. First appear this kind of problem, we'll look at, after many times of transmission cable transfer, easy to produce the phenomenon of poor contact, due to the thermocouple output millivolt signal is small, poor contact will cause the input end of the acquisition card open, showing the maximum range. After you remove the cable, when using a multimeter to measure, equal to the thermocouple loop increases a set of power, now has more than more than 1 ma current flows through the thermocouple circuit, poor contact part of the contact resistance due to the cause of the current, will rapidly decreases, and so, at this point is connected cable to the acquisition card, temperature measurement circuit will return to normal! If you don't measure after just pulling cable, access acquisition card again, certainly not back to normal. And will return to normal after measuring the resistance must be! This is me the basis of analysis. The concrete solution is, all welding connection cable junction; Can't welding parts, such as vacuum flange connection best tin plating processing, prevent the oxidation of copper wires lead to poor contact. The vacuum vessel inside connection must be welded connection! Otherwise, the fault will continue to happen! Believe that through the above introduction can we also know what is going on, this requests us in use process to understand the characteristics of the thermocouple. Can effectively avoid the thermocouple data slow saturation phenomenon.