Temperature Measurement

Converting Thermocouple Millivolts to Temperature: Methods and Examples

• Converting the small voltage produced by thermocouples into temperature readings is crucial for their application in temperature measurement.
• This process relies on reference tables specific to each thermocouple type, correlating millivolt readings to known standard temperatures.
• Different types of thermocouples exhibit unique voltage-temperature characteristics due to the thermoelectric effect.
• This effect produces a voltage in response to a temperature difference between two dissimilar metals in the thermocouple.
• Understanding how this voltage varies with temperature is fundamental to accurately converting millivolt readings to temperature values.

To convert millivolt signals to temperature:

1. Identify the appropriate reference table for the thermocouple type being used (e.g., J, S, T, etc.).
2. Locate the millivolt reading within the table and find the corresponding temperature value.
3. Note that the accuracy of the temperature determination is limited to the increments on the reference table scale. For greater precision, interpolation can be employed between two table values.

Interpolation considers the proportionate difference between two adjacent table values. It involves calculating the temperature corresponding to a measured voltage based on the formula:

Where:

• TM is the measured temperature.
• VM is the measured voltage.
• VH and VL are the higher and lower voltages read from the table.
• TH and TL are the corresponding higher and lower temperatures read from the table.

Utilizing this formula, along with values from the reference table, enables a more accurate determination of temperature through interpolation.

For specific thermocouple types, reference tables provide the necessary data for conversion. For instance, the thermoelectric voltage in millivolts for a type J thermocouple can be found in the respective reference table.

Let’s say we have a type J thermocouple and we measured a voltage of VM=10.399mV.

To convert this millivolt reading to temperature, we’ll use interpolation with the following reference table excerpt:

Using the provided values and the interpolation formula:

where:

• TM is the measured temperature.
• VM is the measured voltage (10.399 mV)
• VL=10.224 mV (lower voltage from the table).
• VH =10.501 mV (higher voltage from the table).
• TL=190 °C (temperature corresponding to lower voltage).
• TH =195 °C (temperature corresponding to higher voltage).

Plugging in the values in to this formula:

TM=190+{(10.399−10.224​)/(10.501−10.224)]×(195−190)

TM=190+(0.175/0.277​)×5

TM=190+(0.632)×5

TM=190+3.16

TM=193.16°C

To calculate the temperature from voltage when the reference junction temperature is greater than zero, follow these steps:

• Identify the appropriate reference table for the thermocouple type being used (e.g., J, S, T, etc.).
• Find the millivolt reading for the reference junction in the body of the selected reference table.
• Note the corresponding temperature value.
• Add the millivolt reading obtained from the reference junction to the measured millivolt reading obtained by the instrument.
• This corrected millivolt reading now represents the total millivolt reading to be used for temperature conversion.
• Utilize the thermocouple reference table along with the corrected millivolt reading to determine the measured temperature.
• Follow the interpolation or direct lookup process outlined in the reference table to obtain the corresponding temperature value.

For Example The measure voltage is 5.138mV to correct the measured voltage for the reference junction at 26°C, we need to add the reference voltage to the measured voltage:

Vcorrected=Vmeasured+Vreference

where:

• Vcorrected is the corrected measured voltage.
• Vmeasured is the measured voltage (5.138 mV).
• Vreference is the reference voltage at 26°C (1.329 mV).

Vcorrected =5.138mV+1.329mV

Vcorrected=6.467mV

So, the corrected measured voltage is 6.467 mV

Using the provided values and the interpolation formula:

where:

• TM is the measured temperature.
• VM is the measured voltage (6.467 mV).
• VL=6.360 mV (lower voltage from the table).
• VH=6.525 mV (higher voltage from the table).
• TL=120 °C (temperature corresponding to lower voltage).
• TH=123 °C (temperature corresponding to higher voltage).

So.

TM=120+[(6.467−6.360)/(6.525−6.360)]×(123−120)​

TM=120+(0.107/0.165)×3

TM=120+(0.648)×3

TM=120+1.944

TM= 121.944

So, for a measured voltage of 6.467 mV using a type J thermocouple with a zero reference junction temperature, the corresponding temperature is approximately

To convert thermocouple millivolts to temperature, you need to identify the appropriate reference table for your thermocouple type, locate the millivolt reading within the table, and find the corresponding temperature value. For greater precision, interpolation can be used between two table values.