Know-how for Temperature Measurement Temperature

When measuring temperatures at multiple points with a single measuring instrument, noise may appear in the output due to the potential difference between each measuring points. Noise can be eliminated by short-circuiting the negative terminal of each channel’s thermocouple to the same potential.
(When the potentials of the measuring points are different and short-circuiting produces a large flow of current, it would be very dangerous. Please check the insulation of the thermocouples, as well as the potential and charge of the measuring points in advance before short-circuiting.)

Insulating Thermocouples

Thermocouples are made of metal and are conductive materials. There are several ways to insulate the tip of the thermocouple to prevent static electricity.
For example, a sheathed thermocouple is generally insulated on the external metal and the thermocouple. When using a thermocouple with the tip exposed, simply insulate it with polyimide tape or the like before making contact with it to increase its resistance to static electricity, ensuring safe use.

Negative Terminal Commonization

When a single measuring instrument is used to measure temperatures at multiple points, static electricity stored at each measuring point may be captured by the instrument to destroy the instrument itself. To prevent static electricity between measuring points from being captured by the instrument, the negative terminal connecting the thermocouples can be short-circuited to increase static electricity resistance.

The measurement point is the tip of the thermocouple (where the positive and negative wires are connected).
The thicker the tip of the thermocouple, the longer it takes to warm up, resulting in a slower response (i.e. larger thermal time constant).

Differences in Measurement Results due to Differences in Response

• If the thermocouple is thin, the thermal time constant at the tip will be small, exhibiting high followability of temperature change. However, it faces disadvantages such that it is susceptible to breaks, vulnerable to noise, and may have high resistance value.
• If the thermocouple is thick, the thermal time constant at the tip becomes larger, worsening the followability to temperature change. Although there are disadvantages such as stiff wiring, slow response, and heaviness, the thermocouple is durable, making it possible to obtain stable measurement results since instantaneous temperature fluctuations are not visible.

In the case of temperature measurement using a thermocouple, as shown in the figure on the right, the tip of the thermocouple is in contact with both the object and the air.
When the object is heated, the temperature of the object and the temperature of the air are different.
The temperature at the tip of the thermocouple that becomes the measurement value is determined by the ease of transmitting the heat in each direction.
Ease of transmitting the heat is determined by thermal resistance and thermal capacity.

In general, thermal conductivity in solids and liquids >> thermal conductivity in gases; however, the amount of error varies depending on the thickness and shape of the thermocouple.
The following are some countermeasures against this issue:

• Use a thermocouple for surface temperature
• Use a thermocouple with low thermal capacity (thin)
• Embed the thermocouple
• Cover the thermocouple with tape or adhesive

There are two types of thermal conductivity to the air: thermal conduction by convection and radiation heat by electromagnetic waves.

When covering the tip of the thermocouple, it is recommended to use one with an emission level close to that of the object to be measured. (It is sufficient to use either metal or resin.)

When heat transfer to air is enhanced by creating strong airflow or by other means, thermal conductivity in solids or liquids may be smaller than the thermal conductivity in gases.
A typical example is cooling using fans and heat sinks.

A thermocouple is composed of two different metals. When these two metals are connected, the temperature difference between their two ends generates a thermoelectromotive force.
The temperature of the contact point can be measured using a thermocouple and a thermometer.
The configuration of the measuring instrument is shown in the figure on the right.

Temperature difference between the two ends of the thermocouple = Temperature 1 – Temperature 2

The voltage generated by the above formula is read by the measuring instrument.
At the same time, the temperature of the thermometer inside the instrument is also read and calculated to estimate the temperature of Temperature 1.

Temperature reading = Temperature 1 – Temperature 2 + Temperature 3

If Temperatures 2 and 3 are equal, Temperature 1 can be measured accurately, but the difference in temperatures will be the measurement error.
We recommend installing the measuring instrument in a location with a constant temperature environment.

The temperature measurement of a thermocouple is based on the sum of the temperature difference between the two ends of the thermocouple and the temperature of the measuring instrument.
The temperature of the measuring instrument refers to the state where there is no temperature difference between the two ends of the thermocouple. The temperature inside the measuring instrument can be output as a measurement value by short-circuiting the thermocouple with a conductor instead of a thermocouple.
When wired as shown in the figure on the right, ch1 outputs the temperature measured by the thermocouple and ch2 outputs the temperature of the measuring instrument.

Heat-resistant cases have the effect of raising the thermal resistance and thermal time constant of the measurement unit.
When using a heat-resistant case in a location where the ambient temperature is higher than the temperature range specified in the specifications of the measurement unit, it becomes important to know how long the case can be used in a high-temperature environment.

Optional Accessories: Heat-resistant Case

The specifications of heat-resistant cases may show the results of use at an example ambient temperature, but if the temperature conditions are different from those at the time of use, the figures of the results cannot be used as they are.

The parameters related to the temperature of the heat-resistant case are thermal resistance and heat capacity.
If the characteristics of the heat-resistant case do not change due to heat, the temperature difference will change proportionally.
(The characteristics is subject to change if convection conditions, such as wind, change.)

The following calculation shows the predicted internal temperature rise when used in a 200ºC thermostatic chamber using the results of the internal temperature rise when used in a 100ºC thermostatic chamber:

In air, the temperature is stabilized by heat transfer via the surrounding air.
In a vacuum, heat transfer through the air is eliminated, and heat is transferred by radiation/absorption or by contact with a material.

The thermocouple wiring absorbs and conducts the temperature of the tip of the thermocouple to be measured or of an object that comes into contact with the thermocouple when it is being pulled around. Then, the heat stored in the thermocouple heats or cools the terminal block of the measuring instrument.

There are no issues if the temperature of the thermocouple is equal to that of the measuring instrument, however, if a temperature difference occurs, the temperature inside the measuring instrument will not be uniform, causing a measurement error.

Measurement Know-How: Principles of Thermocouple Measurement

The temperature difference between the thermocouple and the measuring instrument can be reduced by forcibly determining the temperature of the thermocouple by placing the thermocouple’s sheath in contact with the housing of the measuring instrument or the ground on which it is installed, immediately before connecting it to the measuring instrument.

When measuring temperatures using thermocouples in a vacuum, it is recommended that the sheath of the thermocouple be thermally connected to the housing of the measuring instrument to reduce temperature differences between the terminal block and the thermometer inside the measuring unit. This way, temperature difference between the thermocouple and the measuring instrument may be prevented.