Temperature gradient in 3 zone laboratory furnace?
Temperature gradient in 3 zone laboratory furnace?
(OP)
This is a surprisingly difficult control question with required details. I am putting together a stress rupture testing laboratory that uses 3 zone furnaces for elevated temperature testing. I have run into a roadblock with some nonbelievers that refuse to accept that temperature gradients occur in the furnace in certain temperature renges.
Here is the application:
1) Test coupons instrumented with 3 thermocouples evenly spaced on the coupon.
2) Coupons are typically 2-3 inches long.
3) The thermocouples also serve as control points to run the furnace.
4) Each Thermo controls its corresponding zone.
5) The control mechanism for each zone is an independent PID loop.
6) Coupon is attached to loading rods out of both sides of the furnace the do conduct heat out of the furnace.
When run with tuned PID's they perform marvelously. I have temperature deviations on the order of half degrees at +1800F temperatures. No problem here.
The problem is that it was decided to convert said 3 zone furnaces to 2 zone and run them in much the same manner. When run with 2 control thermocouples, it also performs marvelously, HOWEVER, when outfitted with a monitoring thermocouple in the center of the specimen, the temperatures are 3-5 degrees F higher.
Is there a resource that I can use to explain heat gradients to said nonbelievers to convince them that YES there can exist a 5 degree difference in temperature over 0.75 inches in a furnace?
Here is the application:
1) Test coupons instrumented with 3 thermocouples evenly spaced on the coupon.
2) Coupons are typically 2-3 inches long.
3) The thermocouples also serve as control points to run the furnace.
4) Each Thermo controls its corresponding zone.
5) The control mechanism for each zone is an independent PID loop.
6) Coupon is attached to loading rods out of both sides of the furnace the do conduct heat out of the furnace.
When run with tuned PID's they perform marvelously. I have temperature deviations on the order of half degrees at +1800F temperatures. No problem here.
The problem is that it was decided to convert said 3 zone furnaces to 2 zone and run them in much the same manner. When run with 2 control thermocouples, it also performs marvelously, HOWEVER, when outfitted with a monitoring thermocouple in the center of the specimen, the temperatures are 3-5 degrees F higher.
Is there a resource that I can use to explain heat gradients to said nonbelievers to convince them that YES there can exist a 5 degree difference in temperature over 0.75 inches in a furnace?





RE: Temperature gradient in 3 zone laboratory furnace?
Thermocouples can be sensative to radiant energy, causing them to read high.
RE: Temperature gradient in 3 zone laboratory furnace?
RE: Temperature gradient in 3 zone laboratory furnace?
TTFN
RE: Temperature gradient in 3 zone laboratory furnace?
two sensor control insures that the peak is in the middle, as you've found out.
how are you matching the sensors and what sort of stability are you seeing?
RE: Temperature gradient in 3 zone laboratory furnace?
For 2 zone control on a 3 zone furnace, what we have been doing is shunting the output signal from the center zone to the missing outter zone. This works well as long as the specimen is shifted away from the outside zone that gets the shunt.
Some other furnaces that we use are in a 2 zone configuration, so we rewired the 3 zone to be 2 zone. Hence the 2 thermocouple control configuration. Control again was fantastic, we had over a 70 hour period a maximum deviation from the setpoint of 0.7 degrees at 1100F.
RE: Temperature gradient in 3 zone laboratory furnace?
This has nothing whatsoever to do with the ability of the servo system to maintain its setpoint.
TTFN
RE: Temperature gradient in 3 zone laboratory furnace?
RE: Temperature gradient in 3 zone laboratory furnace?
Your people are confusing the ability to maintain setpoint with the other operational requirements of the system.
Obviously, the ideal thermal control system would have a much finer granularity in zoning, e.g., a 5-zone system would produce a much flatter and longer isothermal zone and a 9-zone would be even better. The limit on granularity is cost and the ability of two adjacent zones to have independence. A zone much smaller than the diameter of the furnace would probably not make sense.
Going from 3-zone to 2-zone not only increases the granularity, but makes the system asymmetrical.
A simple thought experiment would be to ask all involved if they think that a single controller could produce the desired thermal profile. If not, then ask why they would not expect that a two-zone configuration would be worse than a three-zone configuration.
TTFN
RE: Temperature gradient in 3 zone laboratory furnace?
Thanks!
RE: Temperature gradient in 3 zone laboratory furnace?
Regards,
Scotty
RE: Temperature gradient in 3 zone laboratory furnace?
TTFN