Technical Bulletin for June 2003

Subject: Optical Cell Temperature Regulation

Relevant Models: Baird FSQ and DV6S & SN / DV6E models. Replacement parts for this Temperature Controller system are available through Antares Analytical for DV & FSQ models

General Background: Optical cells can “drift” with varying temperatures and humidity. This ‘drift” is the one of the consequences of the actual expansion & contraction of the various metals used in the optical cell. This thermal expansion / contraction can be controlled by a number of different methods including choice of metals used, construction design of the optical cell, temperature and humidity control of the lab environment and temperature control of the unit itself..

On the earlier Baird DV2 – 5, HA series A/S spectrometers, all ICP (expect ICP2070) and MOA spectrometers, thermal drift was controlled by the cell design, cell materials used for construction and customer control of the Lab environment.

For almost all earlier Baird spectrometers, cell design was based on the “A” frame design using stainless steel for construction. Using the “A” frame, or triangular design of the frame, gave very rigid strength so drift was kept to a minimum. Because the DV series spectrometers were also of a vacuum design, added Stainless Steel supports welded to the basic optical cell minimized this thermal (and vacuum pressure) induced drift making the DV2-5 optical cell a very stable optical platform.

FSQ: The FSQ was introduced as a lower cost vacuum Foundry spectrometer in 1988 and the basic “A” frame construction was used, however for cost & manufacturing reasons, aluminum was used in the optical cell rather than Stainless Steel. The FSQ was meant to be used in a foundry, rather than a “clean” lab environment so greater fluctuation in temperatures was expected. Because of these reasons, it was decided to design the FSQ with a heated optical cell bay to minimize the drift caused by temperature variation.

DV6 / DV6E: When the DV6 was introduced, earlier models did not have a temperature controller system, and this was added later for marketing reasons. The DV6 came in 2 versions (heated and unheated) and the temperature was also stabilized at 1 of 2 temperatures (explained in more detail below)

This Temperature Controller / Heater can not be retrofitted to earlier Baird spectrometers.

Basic Temperature Control System Components. The temperature control circuits have only a few components but they must work and be set correctly, to minimize optical drift, especially on the FSQ and especially in Labs with wide variations in temperature & humidity during the work day.

Heater Assy – This unit provides the actual heat for the optical cell, contains optical cell temperature sensor, and circulates heated air through the optical cell bay. The cell heater unit is a black rectangular box located on the Delta side of the FSQ optical cell and directly below the DV6 optical cell. The unit uses the same components in FSQ and DV6.

Temperature Controller This is small controller box (similar to a thermostat in a house) located on the Temp Controller Panel Assy, which controls the heater at a preset temperature. This “assembly” contains only the Controller, the TB1 for the Controller Circuit wiring and the mounting panel.

Heater Temperature Settings:

1. FSQ: The FSQ is factory preset for a target of 100 degrees Fahrenheit with an acceptable range between 97 & 103 with an approximate 1 degree temperature threshold (at 100 degrees). The threshold means the Temperature Controller will let the temperature drop 1 degree below it’s preset and then turn the heater on to bring it back up to temperature. This “threshold “ is typical for FSQ & DV6.

2. DV Series: The DV6 spectrometers with a heater are set at either 85 or 100 degrees. There is no easy way to tell except to check the temperature with a temperature probe. Your current setting should be at, or close to, either temperature, depending on the factor temperature settings. Also remember early model DV6 did not have these Temp Controllers so verify your DV6 has one before checking temperature.

Temperature Regulation Problems and Solutions: Typical problems and solutions below are relevant to both DV & FSQ except where noted.

Temperature Setting: Every 6 months a check should be made of the optical cell temperature to be sure if it’s at factory setting listed above. Use a digital thermometer with a probe. Place the probe inside center the optical cell compartment (not touching any metal surfaces) close the compartment, let it heat to preset temperature and check. Temperature should be (FSQ and some DV6) between 97 and 103 OR (remainder of DV with heating systems) between 82 & 88 degrees.

Note: At this point there will be a difference between the Temp Controller Assy on the VD6 & the FSQ. The Temp Controller on the DV6 has a light which blinks as the heater is working. The faster the light is blinking, the more the lower the temperature in the DV meaning heater is working. At the preset temperature , this light will either be blinking very slowly, or not at all.

Temperature out of range: This is a very common cause of long term instrument “drift” on a temperature controlled Baird spectrometer. If the temperature “preset” moves out of the 97-103 range … it will commonly show on the Hg alignment (FSQ) or Optical Alignment (DV6). Over time the peak alignment dial number moves farther from it’s original Factory position..

For Example: Assume when the FSQ is at 100 degrees, the Hg alignment dial peaks at 700. If the temperature drops (or rises) by 10 degrees, the Hg alignment can peak as far away as 650 or 800.
The optical spectrum does NOT shift evenly when the temperature varies. Element drift depending on their optical wavelength, so this will show C 1930 drifting more than Cr 2989 in the same Low Alloy Steel or Cast Iron matrix.

If the temperature is out of spec then an adjustment needs to be made to the Temperature Controller. On the FSQ the temperature controller is located behind the removable rear panel and is mounted on the inside wall beside the vacuum pump. On the DV6 the controller is mounted inside the back of the spectrometer, in the center bay behind the black screen.

Temperature Adjustment: There are 3 adjustment screws on the temperature controller. Access is best if the metal cover is removed. In all cases, turning the adjustment screw (COARSE & FINE) clockwise raises the temperature setting, counterclockwise lowers the temperature setting.

CAUTION: Under normal conditions, the gain adjustment should be left at the factory setting. On the GAIN turning CW widens the “spread” of temperature range when the heater turns on & off, CCW narrows the temperature range.

Fine Control - For use when the temperature needs to be adjusted by less than 5 degrees. The control itself will rotate about 3 times around end to end. Each ½ turn will raise or lower the cell temperature by about 1 degree. To raise the temperature turn clockwise, to lower turn counter-clockwise

Coarse Control – This control should only be used if the temperature is massively out of spec (8 or more degrees) and should be adjusted very carefully. Again clockwise will raise the temperature and counter-clockwise will lower it. With the coarse gain, one complete rotation can raise / lower the temperature as much as 20 degrees so adjustments should be made in 1/8 turns only.
After each adjustment is made, let the temperature adjust for 15-20 minutes before making the next adjustment.
If the cell temperature should be very high (above 105 degrees) it may be necessary to remove the side panel from the optical cell bay to being the cell air temperature down rapidly. Be sure to replace the panel , let the cell warm up and then monitor the temperature.
After the desired temperature is reached, the FSQ / DV should be left on overnight & the temperature again be checked in the morning. It is possible another adjustment may need to be made.
Generally the cell temperature will never be below room temperature. If necessary, because of Lab time requirements, it is better to run a cell cooler than spec, rather than hotter (ex: On the FSQ better to run at 90 degrees than 110 degrees) until the temperature can be adjusted.
Optical alignment & Standardization should always be done after adjusting the cell temperature because the optics have probably drifted from your last alignment & standardization.

This Technical Bulletin is edited by Gerry Smith, Technical Services Manager of Antares Analytical. Having spent 25 years working with Baird spectrometers (22 years with Baird Analytical in various capacities, including many years as Final Test / Calibration Manager and Senior Service Engineer, International Operations) he or any of our Technical Support Staff, are well qualified to answer any Technical or Applications problems you may have.

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