Temperature data loggers are used in furnace construction, heat treatment, metal and plastics processing as well as in industrial drying and firing processes. They do not merely display an individual temperature value, but document the complete temperature profile of a process over time.
At temperatures up to 1,000 °C, however, selecting a logger with a sufficiently large measuring range is not enough. The thermocouple type, probe design, sheath material, connecting cable, positioning and calibration influence the result just as much as the actual recording instrument.
It is particularly important to distinguish between the process temperature being measured and the permissible ambient temperature of the logger. While a connected thermocouple may measure 900 or 1,000 °C, the logger housing itself may often only be operated within a range of approximately −20 to +70 °C.
This article explains how high-temperature measuring points using thermocouples and data loggers should be planned, how meaningful temperature profiles can be produced and which typical errors occur when measuring furnaces and heat-treatment systems.
Table of contents
- What a high-temperature data logger actually measures
- Distinguishing process temperature from the logger environment
- Correctly selecting thermocouple type K, J or N
- Considering the measuring ranges of the logger and probe separately
- Probe design, diameter and measuring junction
- Sheath and thermowell materials for high temperatures
- Thermocouple cable, compensating cable and connectors
- Cold-junction compensation and environmental influences
- Correctly positioning probes inside the furnace
- Evaluating temperature profiles and uniformity
- Defining the measuring interval and storage capacity
- Realistically assessing the overall measurement uncertainty
- Calibrating the logger and thermocouples
- Protecting cables against heat and mechanical damage
- Typical errors in high-temperature recordings
- Practical example: Temperature profile of a heat-treatment furnace
- Which measuring instruments / products are suitable?
- Conclusion
- Frequently asked questions about temperature data loggers up to 1000 °C
What a high-temperature data logger actually measures
A thermocouple generates a very small thermoelectric voltage that depends on the temperature difference between the measuring junction and the reference junction. The data logger measures this voltage, performs cold-junction compensation and converts the result into a temperature using the characteristic curve of the selected thermocouple type.
The complete measuring chain therefore consists of:
- thermocouple measuring junction
- probe sheath or thermowell
- thermocouple or compensating cable
- connectors and transition points
- logger input and cold-junction compensation
- software for recording and evaluation
The logger only displays the value produced by this complete measuring chain. A high-quality recording instrument cannot compensate for an unsuitable, aged or incorrectly installed probe.
When measuring a furnace, it must also be clarified which temperature is actually to be assessed:
- air or gas temperature inside the furnace chamber
- surface temperature of the workpiece
- core temperature of a component
- temperature of a furnace segment or heating zone
- inlet and outlet temperature in continuous processes
These temperatures can differ significantly during heating. Measuring the furnace air therefore does not automatically prove that the workpiece core has reached the required treatment temperature.
Distinguishing process temperature from the logger environment
The specification “measuring range up to 1,000 °C” refers to the thermocouple input and not to the permissible temperature of the logger housing.
For stationary furnace measurements, the logger is therefore normally installed outside the hot area. Only the probes and suitable connecting cables are routed into the furnace through a dedicated feedthrough.
The following temperature zones must be considered separately:
| Area | Typical exposure | Selection criterion |
|---|---|---|
| Measuring tip inside the furnace | Up to 1,000 °C or higher | Thermocouple type, sheath and thermowell |
| Cable close to the furnace wall | High radiant and ambient temperature | Glass-fibre, ceramic or metal sheathing |
| Cable outside the hot zone | Moderate ambient temperature | Suitable thermocouple or compensating cable |
| Data logger | Limited instrument temperature | Observe the manufacturer’s operating range |
If the logger is positioned too close to the furnace door, a hot housing or above an exhaust opening, its permissible ambient temperature may be exceeded. This can cause measuring deviations, reduced battery life or instrument failure.
A thermal barrier can temporarily reduce the thermal load on the logger. However, its permissible operating temperature and protection duration must match the process. A simple protective container designed for moderate ambient temperatures does not automatically make a logger suitable for travelling through a furnace at 1,000 °C.
Correctly selecting thermocouple type K, J or N
The thermocouple type defines the combination of metals used and therefore its characteristic curve, operating temperature, ageing behaviour and sensitivity.
| Thermocouple | Typical strength | Points to consider |
|---|---|---|
| Type K | Wide temperature range and universal application | Consider drift and atmosphere at high temperatures |
| Type J | Good sensitivity and frequently installed in older industrial systems | Generally not the first choice for oxidising atmospheres and temperatures around 1,000 °C |
| Type N | Improved high-temperature stability compared with many type-K versions | The logger or input must explicitly support type N |
| Type T | Good properties at low and medium temperatures | Not suitable for furnace processes up to 1,000 °C |
Type K is frequently used for measurements up to 1,000 °C. Whether the probe can withstand this temperature permanently, however, depends on the wire diameter, sheath material, atmosphere and mechanical design.
Type J can be evaluated by the testo 176T4 up to +750 °C. For higher furnace temperatures, a type-K probe is therefore required when this logger is used.
Type N can be an interesting alternative for demanding high-temperature processes. However, the testo 176T4 does not have a direct type-N input. If type N is to be used, a compatible data logger, transmitter or multichannel data-acquisition system is required.
For temperatures significantly above 1,000 °C, noble-metal thermocouples such as types S, R or B may be required. These also need an evaluation system specifically designed for them.
Considering the measuring ranges of the logger and probe separately
The usable measuring range is always limited by the component with the lowest permissible temperature.
In a type-K measuring chain, the following limits may apply, for example:
- logger input: up to +1,000 °C
- thermocouple wire: depending on diameter and version
- metal sheath: depending on the alloy and atmosphere
- insulation material: depending on temperature and mechanical loading
- connecting cable: potentially significantly below +1,000 °C
- connector: frequently suitable only for considerably lower temperatures
A probe must therefore not be selected solely according to the printed thermocouple designation. A type-K surface probe with a plastic handle and PVC cable, for example, may have a considerably lower permissible range than a mineral-insulated sheathed thermocouple of the same type.
Short-term and continuous temperature limits must also be distinguished. A probe may be suitable for a brief measurement at 1,000 °C without being capable of withstanding this temperature continuously for many hours or repeated furnace cycles.
Probe design, diameter and measuring junction
The mechanical probe design influences response time, service life and measuring deviation.
Sheathed thermocouple
In a sheathed thermocouple, the thermocouple wires are embedded in highly compacted mineral insulation inside a metal sheath. This design is robust, flexible and available in small diameters.
A small diameter responds more quickly but offers less mechanical and thermal reserve. A larger diameter is more robust but responds more slowly.
Thermocouple with thermowell
A thermowell protects the thermocouple against mechanical loading, aggressive furnace atmospheres and direct contact with the process. At the same time, it increases the thermal mass and therefore the response time.
For durable stationary furnace measuring points, a suitable thermowell is often more appropriate than an unprotected probe. For rapid profile measurements on thin workpieces, however, a finer sheathed thermocouple may be more suitable.
Insulated or grounded measuring junction
A measuring junction connected to the sheath generally responds more quickly. However, it has an electrical connection to the metallic probe body, which can result in ground loops and potential differences.
An insulated measuring junction provides better galvanic isolation but may respond slightly more slowly. For multichannel measurements on different metallic plant components, this version can reduce interference.
Sheath and thermowell materials for high temperatures
At 800 to 1,000 °C, the maximum temperature of the material is not the only decisive factor. The furnace atmosphere, oxygen content, exhaust gases, sulphur, carbon and temperature changes have a significant influence on service life.
Typical questions include:
- Is the atmosphere oxidising, reducing or inert?
- Are sulphur-containing or halogen-containing components present?
- Can the probe come into contact with melts, dust or ash?
- Are there rapid heating and cooling cycles?
- Do flow, vibration or workpiece movements act on the probe?
A material that withstands a high temperature in clean air may corrode or become brittle considerably faster in a specific process atmosphere.
The compatibility of the material with the medium and atmosphere must therefore be assessed using the specific process conditions. General statements based only on the maximum temperature are not sufficient.
Thermocouple cable, compensating cable and connectors
An ordinary copper cable must not be used indiscriminately between the thermocouple and the logger. Additional junctions of different metals generate their own thermoelectric voltages and can cause temperature-dependent measuring errors.
The following are used:
- Thermocouple cable: made from the same or thermally equivalent materials as the thermocouple
- Compensating cable: made from more economical materials with a characteristic curve suitable for a limited temperature range
The cable and connector must match the thermocouple type. Type-K cables, for example, must not be combined with connectors or extensions intended for type J.
The polarity must also remain correct. Reversed polarity can cause the displayed temperature to fall during heating or result in an implausible measured value.
The colour coding depends on the applicable standard. In existing older systems, the polarity should therefore not be assumed solely from the cable colour, but must be verified clearly.
Cold-junction compensation and environmental influences
A thermocouple does not measure an absolute temperature, but a temperature difference. The logger therefore requires the temperature at its input terminals as a reference. This function is known as cold-junction compensation.
For it to operate reliably, the logger should:
- be operated within its permissible temperature range
- not be heated directly by furnace radiation
- not be exposed to a strong stream of hot air
- be allowed sufficient time to acclimatise after a temperature change
- not rest partly on a hot surface
Significant temperature gradients around the connection area can impair cold-junction compensation. A critical example is a logger whose underside is heated by a hot furnace wall while its upper side remains in cooler room air.
Thermocouple connectors should also be positioned in an area with a stable temperature wherever possible. A heated connector outside the actual measuring zone can create additional errors.
Correctly positioning probes inside the furnace
The position of the thermocouples determines which conclusions can be drawn from the recorded data.
For furnace profiling, the following measuring points may be useful, for example:
- near the furnace door
- in the centre of the usable chamber
- in the rear section of the furnace
- close to heating elements
- on a typical workpiece
- inside the core of a solid component
The probe should not unintentionally rest directly against a heating element, furnace wall or air outlet. Otherwise, a local surface or radiant temperature is measured that is not representative of the complete process.
For surface measurements, reproducible contact must be established. A loosely positioned thermocouple may be influenced more strongly by the surrounding hot air than by the actual workpiece surface.
For core-temperature measurements, the insertion depth, bore diameter and heat dissipation through the probe must be considered. An excessively large gap between the probe and bore slows the response and can produce an incorrect temperature profile.
Evaluating temperature profiles and uniformity
A multichannel logger enables several positions to be recorded simultaneously. This makes it possible to identify not only maximum temperatures, but also time differences.
A typical temperature profile includes:
- initial temperature
- heating rate
- time at which the target temperature is reached
- temperature differences between the channels
- soak time within the permissible process window
- overshoot after heating
- cooling rate
During heat treatment, it is often not sufficient for one individual probe to reach the target value briefly. It may be decisive that all relevant workpiece areas remain within an approved temperature band for a defined minimum period.
The recording can reveal problems such as:
- uneven heat distribution
- heating that is too fast or too slow
- insufficient soak time
- excessive overshoot
- delayed core heating
- different behaviour with different loads
Defining the measuring interval and storage capacity
The measuring interval must match the dynamics of the process. An interval that is too long can fail to capture brief temperature peaks or rapid transitions.
| Process | Typical approach for the measuring interval |
|---|---|
| Very rapid heating process | Approximately 1 to 5 seconds |
| Normal batch furnace | Approximately 5 to 30 seconds |
| Slow long-term treatment | Approximately 30 seconds to several minutes |
| Long-term monitoring of a stable process only | Several minutes, provided brief events can be excluded |
A short measuring interval increases the time resolution but also generates more data. Before starting the measurement, the process duration, number of channels, measuring interval and available memory should therefore be considered together.
With four channels and a measuring interval of one second, 14,400 individual values are already generated within one hour. Clear software evaluation is therefore just as important as the pure storage capacity.
Realistically assessing the overall measurement uncertainty
A resolution of 0.1 °C does not mean that a measurement at 1,000 °C is accurate to 0.1 °C. The display resolution only describes the smallest displayed step.
The overall measuring deviation includes, among other factors:
- accuracy of the logger input
- tolerance class of the thermocouple
- ageing and drift of the thermocouple
- cold-junction compensation error
- influence of extension cables and connectors
- positioning and contact errors
- temperature gradients within the process
- dynamics and response time of the probe
At high temperatures, the probe tolerance can account for the largest proportion of the uncertainty. Repeated furnace cycles can additionally change the thermocouple characteristic curve.
For quality-relevant heat treatment, the logger accuracy alone should therefore not be considered. The uncertainty of the complete measuring chain under actual installation conditions is decisive.
Calibrating the logger and thermocouples
Calibration should reproduce the measuring chain actually used as closely as possible. If only the logger is tested electrically, deviations of the thermocouple and connecting cable remain unaccounted for.
Three basic levels can be distinguished:
Electrical testing of the logger
A thermocouple simulator supplies defined thermoelectric voltages or temperatures. This enables the input, thermocouple characteristic curve and display to be checked.
Calibration of the probe
The thermocouple probe is compared with a traceable reference thermometer in a suitable furnace or temperature bath.
Calibration of the complete measuring chain
The probe, cable, connectors and logger are tested together. This version takes most electrical components of the measuring chain used later into account.
The calibration points should match the process. For a heat-treatment process at 850 °C, calibration exclusively at 0 and 100 °C is only of limited significance.
Following mechanical damage, severe overheating or noticeable drift, the thermocouple should be checked or replaced independently of the regular calibration interval.
Protecting cables against heat and mechanical damage
Cable routing is frequently the most mechanically vulnerable part of a furnace measurement. Cables can be crushed by doors, routed across sharp edges or damaged by hot workpieces.
Important measures include:
- using suitable furnace feedthroughs
- not clamping cables between the door and seal
- observing the minimum bending radius
- protecting hot cable sections with suitable insulation
- avoiding tensile loads on the probe and logger connection
- assigning cables clearly to the measuring channels
- positioning connectors outside the hottest zone
Metal braiding and glass-fibre insulation improve mechanical or thermal protection, but they also have defined temperature limits. The complete cable construction must be suitable for the hottest location that actually occurs.
Typical errors in high-temperature recordings
| Error | Possible consequence | Better approach |
|---|---|---|
| The logger is placed directly next to the hot furnace door | The permissible instrument temperature is exceeded | Install the logger outside the hot zone with thermal separation |
| An arbitrary type-K probe is used up to 1,000 °C | Damage to the cable, insulation or measuring tip | Check the complete probe construction |
| Type N is connected to a type-K input | Systematic measuring error due to the incorrect characteristic curve | Use a suitable thermocouple input |
| A copper cable is used as an extension | Additional temperature-dependent errors | Use a suitable thermocouple or compensating cable |
| Only the furnace air is measured | The workpiece or core temperature remains unknown | Define the measuring points according to the process objective |
| The probe rests directly against a heating element | An unrealistically high local value is measured | Select a representative position with defined attachment |
| The measuring interval is too long | Brief temperature peaks are not recorded | Adapt the measuring interval to the process dynamics |
| An old thermocouple is reused without testing | Drift and an incorrect temperature profile | Provide for calibration, comparative testing or replacement |
Practical example: Temperature profile of a heat-treatment furnace
Steel components are heat-treated in a batch furnace at a target temperature of 850 °C. The operator wants to check whether all workpieces remain within an approved temperature window of 840 to 860 °C for at least 45 minutes.
A four-channel thermocouple logger is positioned outside the furnace. Four type-K probes are arranged as follows:
- channel 1 on a workpiece in the front section of the furnace
- channel 2 on a workpiece in the rear section of the furnace
- channel 3 inside the core of a representative solid component
- channel 4 freely positioned inside the furnace chamber for comparison with the furnace air
The cables are routed through a dedicated feedthrough. The logger remains in a cool area protected against radiant heat. A measuring interval of ten seconds is selected.
The evaluation shows that the furnace air reaches 850 °C after only 35 minutes. The surface of the front components follows a few minutes later. However, the core probe does not reach the lower limit of 840 °C until 58 minutes have elapsed.
The previous process control system had started the soak time when the furnace-air temperature reached its target. As a result, the workpiece core remained within the required temperature window for only 22 minutes.
The process is adjusted so that the soak time only begins once the slowest representative workpiece probe reaches 840 °C. A repeat recording then confirms a sufficient soak time in all monitored areas.
The example shows why a single furnace-controller value may not be sufficient for evaluating heat treatment. Only the multichannel profile reveals the actual thermal loading of the workpieces.
Which measuring instruments / products are suitable?
The testo 176T4 temperature data logger records up to four thermocouple channels simultaneously. It is suitable for types K, T and J and covers a range up to +1,000 °C with a type-K input.
The large display shows current values, limit-value violations as well as minimum and maximum values. Up to two million measured values can be stored and subsequently evaluated or exported using PC software.
The logger itself must be operated outside the hot furnace zone and within its permissible ambient-temperature range.
Thermocouples for furnace and process measurements
The thermocouples category contains different versions for air, surface, immersion, penetration and process measurements.
For selection, the thermocouple type and maximum temperature are not the only important factors. The sheath material, diameter, measuring-junction type, insertion length, connecting cable and process atmosphere must also be specified.
Straight thermocouples for stationary high-temperature measuring points
The straight thermocouples in accordance with EN 50446 are designed with a metallic thermowell for stationary industrial high-temperature measurements.
Depending on the version, they can be used at temperatures up to 1,250 °C. Whether they can be connected directly to a mobile data logger depends on the connection head, terminal block, cable and required measuring chain.
WIKA TC40 for flexible installation options
The WIKA TC40 cable thermocouple is available for direct temperature measurement in machinery, pipes, tanks and processes.
Depending on the sensor version, ranges up to +1,200 °C as well as different cable and process connections are available. For measurement up to 1,000 °C, the specific combination of probe, cable insulation and installation method must be designed accordingly.
ICS Schneider Messtechnik assists with selecting the data logger, thermocouple type, probe design and calibration. The required information includes the maximum temperature, process atmosphere, number of measuring points, required measuring interval, measuring duration, probe position, cable route and required measuring accuracy.
Conclusion: Up to 1000 °C, the complete measuring chain must match the application
A temperature data logger with a measuring range up to 1,000 °C can reliably document high-temperature processes, provided that a suitable thermocouple and a professionally designed measuring chain are used.
The high process temperature must not be confused with the permissible ambient temperature of the logger housing. The logger generally remains outside the furnace, while only the probes and suitable thermocouple cables are routed into the hot zone.
Type K is suitable for many applications up to 1,000 °C. However, the actual operating limit depends on the probe diameter, sheath material, cable and furnace atmosphere. Type J has a smaller usable high-temperature range. Type N can offer improved long-term stability, but requires an explicitly compatible input.
Multiple measuring channels enable the furnace air, workpiece surfaces and core temperatures to be recorded simultaneously. This provides substantially more meaningful information about heating time, uniformity, soak time and cooling than a single controller value.
For quality-relevant processes, the logger, probes and connecting cables should be checked at suitable temperature points or calibrated together. Display resolution alone is not a measure of the actual accuracy of the complete measurement.
Frequently asked questions about temperature data loggers up to 1000 °C
Can the testo 176T4 be placed directly inside a furnace at 1000 °C?
No. The measuring range of the thermocouple input extends up to 1,000 °C, but the logger itself may only be operated within its considerably lower ambient-temperature range.
Which thermocouple is suitable for 1000 °C?
Type K is frequently used. However, the specific probe version, sheath material, diameter and furnace atmosphere must be suitable for this temperature.
Can the testo 176T4 evaluate a type-N thermocouple?
No. The instrument supports thermocouple types K, T and J. A different compatible measuring system is required for type N.
Why can not every type-K probe be used up to 1000 °C?
The thermocouple characteristic curve may cover this range, but the cable insulation, probe sheath, connector or mechanical construction may have considerably lower temperature limits.
Is it sufficient to record only the furnace-air temperature?
Not always. During heat treatment, the workpiece or core temperature may follow with a significant delay. The required measuring points depend on the process objective.
How many measuring points are useful for a furnace profile?
This depends on the furnace size, load and quality requirements. Several positions within the usable chamber and at least one representative workpiece are frequently monitored.
How short should the measuring interval be?
The measuring interval must be considerably shorter than the expected process changes. For normal batch processes, five to 30 seconds are often suitable, with correspondingly shorter intervals for rapid processes.
Can a normal copper cable be used as an extension?
For direct thermocouple measurement, a suitable thermocouple or compensating cable should be used. Unsuitable metallic junctions can generate additional thermoelectric voltages and measuring errors.
What does a resolution of 0.1 °C mean at 1000 °C?
It only describes the smallest displayed increment. The actual measuring deviation is considerably greater because of the logger accuracy, probe tolerance, drift and installation influences.
How frequently must high-temperature thermocouples be calibrated?
The interval depends on the temperature, number of cycles, process atmosphere and quality requirements. Shorter inspection intervals are advisable when the thermocouples are frequently exposed close to their upper operating limit.
Which information does ICS Schneider require for the system design?
The required information includes the temperature range, furnace atmosphere, process duration, number of measuring points, required probe positions, cable route, measuring interval, required accuracy and required calibration.