TUS (Temperature Uniformity Survey) uses multiple test thermocouples to measure the temperature at each point in the effective heating zone of the furnace to determine whether it is within the specified uniformity range; SAT (System Accuracy Test) compares the readings of the control/recording system with a calibrated reference measurement to determine the measurement chain error. Both are basic requirements for CQI-9 and AMS2750. Temperature errors not only affect the structure and hardness, but also directly cause deviations in carbon potential calculations.
- TUS: Check "whether the temperature is consistent everywhere in the furnace"
- SAT: Check "Is the temperature displayed by the meter accurate?"
- Frequencies and tolerances depend on furnace classification, instrument type and standards followed
- The test thermocouple and recorder themselves need to be calibrated and traceable
- Furnace temperature ±5 °C results in a carbon potential error of approximately ±0.02–0.03
Two tests solve different problems
| Project | TUS | SAT |
|---|---|---|
| Purpose | Is the temperature distribution in the effective heating area uniform? | Are control/recording system readings accurate? |
| method | Arrange multiple test thermocouples at specified positions in the furnace, heat up and keep records | Insert a calibrated test thermocouple near the control thermocouple and compare readings |
| result | Is the maximum deviation of each point within the uniformity requirements? | Is the system deviation within the allowable range? |
| focus on | Furnace design, circulation, heating elements, loading | Thermocouple drift, compensation wires, instruments |
TUS key points
- Number and location of test points: Determined based on the effective work area volume, usually covering the corners and center;
- Test temperature: Covers the actual temperature range used (minimum, maximum, intermediate point if necessary);
- Test conditions: Empty furnace or with load, according to standards and customer requirements;
- Recording frequency: Sufficient to capture temperature rise overshoot and steady state;
- criterion: The temperature at all test points during the holding phase is within the specified uniformity tolerance (e.g. ±5 °C, ±10 °C, etc., depending on the standard and furnace category).
SAT Essentials
- Place the test thermocouple tip as close as possible to the control or recording thermocouple;
- Use calibrated test instruments and thermocouples that are within their validity period;
- When calculating the deviation, the known correction values of the test thermocouple and instrument must be deducted;
- When the deviation exceeds the allowable range, find the cause (thermocouple aging, compensation wire, wiring, instrument) and correct it.
How to determine the frequency
The frequency of TUS and SAT depends on:
- The standard followed (CQI-9 or AMS2750, etc.);
- Furnace category (the stricter the uniformity requirements, the higher the frequency);
- Instrument configuration type (control, recording, over-temperature thermocouple configuration);
- Historical results (continuous qualifications may allow an extension of the period, as specified in the standard).
The relationship between temperature and carbon potential
In a controlled atmosphere furnace, temperature errors feed directly into the carbon potential calculation. Goss control analysis points out that under the conditions of 930 °C and CO 20%, a furnace temperature of ±5 °C will cause a carbon potential deviation of approximately ±0.03, and ±10 °C will cause a carbon potential deviation of approximately ±0.05. Therefore, TUS and SAT are not only "temperature" requirements, but also the basis for carbon potential control.
Frequently asked questions
Can TUS be outsourced?
Yes, many companies entrust qualified third parties. However, companies still need to understand the content of the report and be responsible for rectifying unqualified items.
How often should thermocouples be replaced?
Depends on type, temperature, atmosphere and frequency of use. Standards usually have regulations on the number of reusable times or usage time, and thermocouple usage records should be established.
Do I need to have a new TUS done after a furnace overhaul?
Generally required. Temperature uniformity should be re-evaluated after changing heating elements, furnace linings, fans, or control thermocouple locations.