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How to calibrate the 9260 vertical gradient

Tech Notes

          

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Title: How to calibrate the 9260 vertical gradient

Created: 16 Apr 2012

Last Revised: 16 Apr 2012

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Applies To: All 9260 Mini Fixed-Point Cell Furnaces

 

Problem Description: Vertical gradient needs checked or aligned

 

Resolution/Work Around: The 9260 Furnace is adjusted at the factory for a flat vertical temperature profile.  The temperature profile should be regularly checked and adjusted as necessary to maintain it within about 0.2°C over the bottom 12 cm (5 in.) with a slightly higher temperature toward the top. The following steps explain how to test and adjust the temperature profile.

 

Step 1: Measure the profile

 

The temperature profile should be measured at 660°C or at the highest fixed point with which the furnace is intended to be operated. The furnace should have the fixed-point cell installed and the cell should be completely frozen or melted. Incorrect data will result if the cell is only partially melted or frozen.  Use a quartz SPRT and thermometer readout that together provide adequate resolution and stability to allow relative temperature measurements to 0.02°C or better. You must wait until the temperature of the SPRT is stable before recording readings.  It may take 20 to 30 minutes after inserting the SPRT before it becomes stable. Measure the temperature at three locations: t1 with the SPRT fully inserted into the well, t2 with the SPRT withdrawn 6 cm (2.5 in.), and t3 with the SPRT withdrawn 12 cm (5 in.). To reduce possible effects of temperature drift repeat the measurements in the following order: t1, t2, t3, t2 again, then t1 again. Wait until the SPRT is stable (about two minutes) before recording each reading. Average the two t1 measurements and the two t2 measurements then compare the temperatures. Temperature t2 relative to t1 should be less than 0.1°C higher or lower. Temperature t3 relative to t1 should be between 0 and 0.2°C higher. If the gradient exceeds these limits adjust the end zones as explained in the next step.

 

Step 2: Adjust the end zones

 

The temperature profile can be adjusted by altering the calibration parameters "toPPCt" and "botPCt" in the controller (see Section 5.16.5). Increasing or decreasing the top zone heat by 1% typically increases or decreases t3 relative to t1 by about 0.1°C at 660°C and increases or decreases t2 relative to t1 by about 0.03°C. Likewise, increasing or decreasing the bottom zone heat by 1% typically increases or decreases temperature t1 relative to t3 by about 0.1°C at 660°C and increases or decreases temperature t2 relative to t3 by about 0.03°C. From these relationships, the following formulas are derived and these can be used to determine new settings for the top and bottom zones to produce a flat gradient with the top only 0.1°C hotter than the bottom.

 

topPct’ = topPct + (25 % / ⁰C)*(T2 - T1) - (17.5 % / ⁰C)*(T3 - T1) + 1.75%

 

botPct’ = botPct + (25 % / ⁰C)*(T2 - T1) - (7.5 * % / ⁰C)*(T3 - T1) + 1.75%

 

For example, t1 is measured in Step 1 to be 657.71, t2 is measured to be 657.83, and t3 is measured to be 658.41. The bottom zone is set at 94% and the top zone is set at 91%. The new settings are calculated as follows.

 

topPct’ =

91% + (25 % / ⁰C)*(657.83°C – 657.71°C) - (17.5 % / ⁰C)*(658.41°C − 657.71°C) + 1.75% = 84%

Or

91 + (25) * (657.83 - 657.71) – (17.5) * (658.41 * 657.71) + 1.75 = 84

 

botPct’ =

94% + (25 % / ⁰C)*(657.83°C – 657.71°C) - (7.5 % / ⁰C)*(658.41°C − 657.71°C) + 1.75% = 93%

Or

94 + (25)*(657.83 – 657.71) - (7.5)*(658.41°C − 657.71) + 1.75 = 93

 

After adjusting the end zones repeat Step 1 to check the temperature profile again. If the temperature differences still exceed the recommended limits repeat Step 2 again.

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