Cold-junction-aware converter
Convert Type J Temperature and Millivolts
Uses the nominal Type J reference function and one canonical °C/mV calculation path.
At the default 100°C hot junction and 0°C reference junction, the nominal measured output is 5.268916 mV (displayed as 5.269 mV).
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Nominal reference values
This conversion is not a probe accuracy, calibration, service-life, sheath-temperature, atmosphere-suitability, or project-approval result. Do not extrapolate beyond the stated function range.
Calculated checkpoints
Type J Thermocouple Reference Table
Limited nominal Type J ITS-90 checkpoints relative to a 0°C reference junction.
Search a temperature or EMF value. Use the converter above for intermediate temperatures or a nonzero reference junction.
No displayed checkpoint matches this search or range. Clear the filters or use the converter for an exact value within −210°C to 1200°C.
Scroll horizontally to view additional columns and vertically to view additional rows. Column headers remain visible while scrolling.
| Temperature | Nominal Type J output at 0°C reference | Derived and scope | |||
|---|---|---|---|---|---|
| Function segment | |||||
| −210 | −346 | −8.095 | −8095 | 19.1 | −210 to 760°C segment |
| −200 | −328 | −7.890 | −7890 | 21.9 | −210 to 760°C segment |
| −150 | −238 | −6.500 | −6500 | 33.1 | −210 to 760°C segment |
| −100 | −148 | −4.633 | −4633 | 41.1 | −210 to 760°C segment |
| −50 | −58 | −2.431 | −2431 | 46.6 | −210 to 760°C segment |
| −40 | −40 | −1.961 | −1961 | 47.5 | −210 to 760°C segment |
| 0 | 32 | 0.000 | 0 | 50.4 | −210 to 760°C segment |
| 25 | 77 | 1.277 | 1277 | 51.8 | −210 to 760°C segment |
| 50 | 122 | 2.585 | 2585 | 52.8 | −210 to 760°C segment |
| 100 | 212 | 5.269 | 5269 | 54.4 | −210 to 760°C segment |
| 150 | 302 | 8.010 | 8010 | 55.2 | −210 to 760°C segment |
| 200 | 392 | 10.779 | 10779 | 55.5 | −210 to 760°C segment |
| 250 | 482 | 13.555 | 13555 | 55.5 | −210 to 760°C segment |
| 300 | 572 | 16.327 | 16327 | 55.4 | −210 to 760°C segment |
| 350 | 662 | 19.090 | 19090 | 55.2 | −210 to 760°C segment |
| 400 | 752 | 21.848 | 21848 | 55.2 | −210 to 760°C segment |
| 450 | 842 | 24.610 | 24610 | 55.4 | −210 to 760°C segment |
| 500 | 932 | 27.393 | 27393 | 56.0 | −210 to 760°C segment |
| 550 | 1022 | 30.216 | 30216 | 57.0 | −210 to 760°C segment |
| 600 | 1112 | 33.102 | 33102 | 58.5 | −210 to 760°C segment |
| 650 | 1202 | 36.071 | 36071 | 60.3 | −210 to 760°C segment |
| 700 | 1292 | 39.132 | 39132 | 62.2 | −210 to 760°C segment |
| 750 | 1382 | 42.281 | 42281 | 63.7 | −210 to 760°C segment |
| 760 | 1400 | 42.919 | 42919 | 63.9 | 760°C segment boundary |
| 800 | 1472 | 45.494 | 45494 | 64.6 | 760 to 1200°C segment |
| 850 | 1562 | 48.715 | 48715 | 64.0 | 760 to 1200°C segment |
| 900 | 1652 | 51.877 | 51877 | 62.4 | 760 to 1200°C segment |
| 950 | 1742 | 54.956 | 54956 | 60.7 | 760 to 1200°C segment |
| 1000 | 1832 | 57.953 | 57953 | 59.3 | 760 to 1200°C segment |
| 1050 | 1922 | 60.890 | 60890 | 58.3 | 760 to 1200°C segment |
| 1100 | 2012 | 63.792 | 63792 | 57.8 | 760 to 1200°C segment |
| 1150 | 2102 | 66.679 | 66679 | 57.6 | 760 to 1200°C segment |
| 1200 | 2192 | 69.553 | 69553 | 57.2 | 760 to 1200°C segment |
This is a limited, independently calculated checkpoint set—not the complete NIST or ASTM table and not a probe tolerance or rating table. EMF values are nominal relative to 0°C.
How to Read and Use the Type J Chart
- Confirm that the sensor, extension wire, connector, and instrument are configured for Type J.
- For a true 0°C reference junction, read the row directly or enter the hot-junction temperature in the converter.
- For an ambient reference junction, enter its measured temperature. The converter subtracts its nominal Type J EMF from the hot-junction EMF.
- For reverse conversion, enter the measured differential EMF and the actual reference-junction temperature.
To reverse the measurement, first calculate E0 = Emeasured + EJ(Treference), then solve the bounded Type J reference function.
Why the reference junction changes the reading
A thermocouple instrument measures a voltage difference, not the hot-junction voltage alone. A 25°C terminal block already corresponds to about 1.277 mV on the Type J reference function, so that contribution must be restored before converting the measured voltage back to temperature. Electronic cold-junction compensation performs this correction using a separate terminal-temperature sensor.
Reference Range, Tolerance Range, and Probe Rating
- Reference function
- −210°C to 1200°C defines the nominal mathematical temperature–EMF relationship used here.
- Manufacturing tolerance
- The BIPM guide lists IEC Type J Class 1 and Class 2 tolerance coverage from −40°C to 750°C. A tolerance is not the same as the reference-function range.
- Actual probe rating
- Wire diameter, sheath, insulation, atmosphere, exposure time, construction, and manufacturer limits determine usable service.
Do not treat 1200°C as a universal rating
ASTM’s public scope also distinguishes reference tables from recommended upper-temperature limits. Check the exact probe and installation before service near any limit.
Reference function is not measurement accuracy
| Error source | What to check |
|---|---|
| Wire tolerance and drift | Calibration history, batch, exposure temperature, and time. |
| Cold-junction sensor | Terminal temperature, gradients, and compensation accuracy. |
| Instrument and wiring | Type selection, polarity, input accuracy, extension wire, connectors, grounding, and noise. |
| Installation | Immersion, heat conduction, thermal contact, sheath condition, and gradients across damaged wire. |
| Inhomogeneity | Bends, strain, corrosion, contamination, oxidation, and prior high-temperature exposure. |
Worked Type J Thermocouple Examples
100°C with a 0°C reference
Assumption: nominal Type J behavior. EJ(100°C) = 5.268916 mV and EJ(0°C) = 0 mV, so the measured output is 5.268916 mV. The table displays 5.269 mV.
A real probe can differ because of wire tolerance and the complete measurement chain.
100°C with a 25°C reference
EJ(100°C) − EJ(25°C) = 5.268916 − 1.277288 = 3.991628 mV. Reading 3.991628 mV as if the reference were 0°C would be incorrect.
Reverse 3.991628 mV at 25°C
Add the reference contribution: 3.991628 + 1.277288 = 5.268916 mV. Bounded reverse conversion returns approximately 100.0°C.
The compensated EMF must remain within the published Type J inverse range.
Type J Materials, Service Limits, and Common Mistakes
- Positive leg
- Iron
- Negative leg
- Copper-nickel
- Typical strength
- Established industrial use and tolerance of reducing atmospheres when the probe construction is suitable.
- Important weakness
- The iron leg can rust in moist atmospheres, and irreversible errors can increase above about 500°C.
The BIPM guide notes magnetic transformation near 760°C and an iron crystal transformation near 910°C. It also warns that older Type J definitions differed: pairing modern sensors with old instrumentation can create significant error. Generic “constantan” formulations used in Types E, J, and T should not be assumed interchangeable.
Common Type J mistakes
- Selecting Type K or another thermocouple curve in the instrument.
- Reversing polarity or using incompatible extension wire and connectors.
- Confusing mV and µV, creating a factor-of-1000 error.
- Assuming the instrument terminals are at 0°C.
- Extrapolating beyond the reference function or treating it as a calibration certificate.
- Ignoring iron corrosion, bends, drift, and the probe’s high-temperature history.
Source, Scope, and Data-Rights Notes
Calculated from BIPM Type J ITS-90 functions; verified against NIST SRD 60 Version 3.0; source checked August 9, 2026.
The limited table is calculated locally from the published BIPM coefficients. NIST is used as an independent reference check, and ASTM is cited only for its public scope. No complete NIST or ASTM table is reproduced or offered as a download.
- BIPM — Guide on Secondary Thermometry: Thermocouple Thermometry, Part 12025-12-19; Type J materials, ranges, tolerances, service guidance, and Appendix A coefficients. BIPM material is reused under CC BY 4.0 with attribution and without implied endorsement.
- NIST Temperature Scale Database, SRD 60Version 3.0; used to verify Type J reference-function scope, coefficient segments, inverse ranges, and endpoints. The protected compilation is not reproduced.
- ASTM E230/E230M-23a public scopeUsed only for the public distinction among reference tables, tolerances, inverse functions, and recommended upper-temperature limits. Proprietary tables are excluded.
- BIPM terms and conditionsConfirms the CC BY 4.0 reuse basis for BIPM website material, subject to attribution and separately identified third-party rights.
Dataset and Source-Check Details
- Reference condition
- Nominal Type J EMF relative to 0°C; ITS-90 temperature scale
- Calculated fields
- °F, mV, µV, local sensitivity, cold-junction compensation, and limited checkpoints
- Internal precision
- Full double precision; display rounded to 0.001 mV, nearest µV, 0.1°C/°F, and 0.1 µV/°C
- Excluded material
- Complete standards tables, full 1°C dataset, tolerances as a calculator, color codes, and product ratings
- Download decision
- No CSV; copy is limited to the current calculated result or displayed page values
- Publisher
- Turn2Engineering
J Type Thermocouple Chart FAQs
Approximately 5.269 mV nominal relative to a 0°C reference junction. A warmer reference junction reduces the measured differential voltage.
Only after adding the nominal EMF for the actual reference-junction temperature. The resulting equivalent 0°C-reference EMF must remain within the published Type J range.
The chart is nominal. Wire tolerance, cold-junction error, connectors, gradients, corrosion, drift, noise, instrument error, and calibration history all affect a real reading.
Not automatically. The 1200°C value is a reference-function endpoint; actual use depends on tolerance coverage, iron-leg durability, sheath construction, atmosphere, wire size, and manufacturer limits.
No. They use different thermoelements and temperature–EMF functions, so the instrument, connectors, extension wire, and configured thermocouple type must match.
Use the Nominal Chart with the Real Measurement Chain
Use the table for quick 0°C-reference checkpoints and the converter for intermediate values or cold-junction compensation. For field decisions, verify thermocouple type, polarity, reference-junction measurement, probe construction, environment, instrument accuracy, and calibration history.