A Temperature Uniformity Survey is a test designed to prove the internal capabilities of manufacturing furnaces and ovens to be within specific operating temperature ranges. It establishes the acceptable work zone and qualifies the said operating temperature ranges. The equipment may have multiple qualified operating temperature ranges with different tolerances, as well. Survey intervals shall be in accordance with Table 8 or Table 9 found in AMS2750E. An initial TUS must be performed first and on an annual basis thereafter that captures the entire scope of the operation range from minimum to the maximum. Additional temperatures shall be added to ensure that no two adjacent temperatures are greater than 600° F (335° C) apart. An initial TUS shall also be performed after any furnace/oven modifications or adjustments that could have altered the temperature uniformity characteristics of the equipment. Examples of where an initial TUS shall be required can be found in AMS2750E 3.5.3 Periodic TUS temperatures shall be any temperature within each qualified operating range(s). For single operating ranges greater than 600° F (335° C), during each periodic test TUS temperatures shall be selected so that one temperature is within 300° F (170° C) of the maximum and another temperature is within 300° F (170° C) of the minimum of qualified operating range and there are no more than 600° F (335° C) increments in between.
During each survey all parameters shall reflect the normal operation of the equipment in production. (Examples: if the doors to a continuous furnace are normally open during production, they shall be open for the TUS; if slow ramp rates and stabilization temperatures are not used in production, they shall not be used during the TUS; if excess combustion air is used during production, it shall also be used during the TUS; if fans are operated during production, they shall also be operated during the TUS, etc.).
A TUS may be performed with an actual production load, simulated production load, a rack, or empty. Once a method of surveying is established during an initial TUS, subsequent surveys shall be conducted using the same method. If changes are made to the established method, an initial TUS shall be performed to validate the revised method. Your written method should state thermocouple placement location within the qualified work zone. Along with how the test/load thermocouples will be attached. (Example: directly to the survey rack, if heat sinks will be utilized to attach thermocouples to the rack, that the test/load thermocouples will be free air not directly attached to any metal at the testing tip for the thermocouple). My professional opinion is that directly attaching the testing thermocouples directly to the rack, no heat sinks, will give you the best data results for what your parts are seeing. I have a customer who twists a 3-inch testing tip and directly secures this 3-inch twisted tip of the thermocouple tightly to the survey rack. The thermocouple will bond with the rack or parts under heat giving a tight consistent reading.
The results are consistent thus giving an accurate depiction of what the parts would be seeing during production. This is our main objective during a TUS, to capture what the parts are seeing and to prove that your furnace has repeatability and the capability to operate within established tolerances. That method worked well for that customer. They don’t use heat sinks during production, so their testing method reflects how they run production loads. I’ve seen far fewer test/load thermocouple failures using this method of attachment and more importantly a better depiction of the heat uniformity inside of the vessel. This method may not be the best for everyone as there are many different techniques that have been incorporated into our methods of process during testing and production.
There is much value and insight gained through preforming periodic testing that out weighs simply receiving accreditation of compliance. By getting to know your equipment together we can make changes that will better your entire process! We can see where problematic issues occur and take the chance to make corrections prior to lost production time due to a system malfunction. Having personal interest and taking a proactive approach to periodic testing then begins to provide satisfaction to a routine task.
Ray Rowlett, Service Engineer