What is the failure cause of thermocouple _ instrument
Thermocouple temperature sensor is a common, in use process will often because of the influence of various interference factors in the course of using the thermocouple in fault and error.
Cause thermocouple failure have five points: 1, the connection problem: many thermocouple intersection measurement error caused by the accident.
Please keep in mind that any can lead to two different metal border crossing.
If you need to increase the length of your thermocouple compensation wire, you must use the correct type of thermocouple extension cord (
Such as k type thermocouple)
The use of any other types of line, will introduce a thermocouple junction.
Use any connector must make the right thermocouple materials, must abide by and the correct polarity.
2, lead resistance, in order to minimize the hot tap and improve response time, thermocouple is made up of a thin metal wire (
Platinum types of cases, the cost is also a consideration)
This may lead to thermocouple with high resistance, it can make noise sensitive, could also lead to the mistake, because the input impedance of a measuring instrument.
A typical 32 awg wires (
25 mm in diameter)
At the junction of the thermocouple, there will be a resistance of about 15 ohm/meter.
082 m Ω input impedance, so will produce this cable 12 meters error is less than 0.
If you need to use long fine wires or cables, thermocouple, it is worth keeping thermocouple wire short, and then with the thermocouple extension cord (
This is much more thick, so it is relatively low resistance)
, running between the thermocouple and measuring instrument.
It is always a good preventive measures, to your thermocouple resistance measurements before use.
Usually is the cause of atmospheric particulate matter spread to the metal in the extreme working temperature.
Another reason is that the impurity diffusion and chemicals to the thermocouple wire insulation.
If at high temperature operation, check the probe specification of insulating layer.
3, noise: from the output of the thermocouple is a small signal, so it is easy to generate electrical noise picked up.
Most of the measuring instrument to refuse any common-mode noise signal is the same for the two wires, so that noise can through the cable twisting together, to help ensure that the two wires picked up the same noise signal is small.
If the work in a very noisy environments (
Such as a large motor near)
, it is worth considering using shielding extension cord.
If you suspect noise pick off all suspicious devices, to see if the change of reading.
4, the common-mode voltage: although the thermocouple signal is very small, often there is a greater voltage input of a measuring instrument.
The voltage may be the cause of inductance picked up (
A problem, when the test and the temperature of the transformer windings)
The corner of, or 'ground'.
A typical example is the 'ground' the junction temperature of hot water pipe insulation with a thermocouple.
If you have any bad grounding connection a few volts may exist between the pipe and geodetic instruments.
These signals to normal mode (
In the two thermocouple wires is the same)
, so not cause problems that most of the instruments they won't be too big.
Use the same cable noise overview of prevention measures, can reduce common-mode voltage, can also through the use of insulating thermocouple.
5, hot tap: all thermocouples have certain quality.
Heating quality, need energy, and thus affect you try to measure temperature.
Thinking of the liquid in a test tube temperature measurement example: there are two potential problems.
1, is the heat will travel to the thermocouple wires and dissipate into the atmosphere, thus reducing the wire around the temperature of the liquid.
Similar problems may occur, if not fully immersed in a liquid, due to a cold environment air temperature thermocouple wires, heat conduction, could lead to a border of thermocouple, in different temperature, the liquid itself.
With a thinner wire thermocouples may be helpful, because it can lead to a steep temperature gradient along the border between liquid and the surrounding air thermocouple compensation wire.
If you use a thin wire of the thermocouple, pay the lead resistance must be considered.
Thermocouple with thin wire connected to the use of thicker thermocouple extension cord, often provides a good compromise.