Métrologie
Métrologie
Going To Extremes
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GOING TO EXTREMES
When You Can't Always Measure In Your Thermal Comfort Zone
George Schuetz, Mahr Federal Inc.

  When most of us think about measurement environments, what generally comes to mind are pleasant laboratories with temperatures controlled to 68°F/20°C -- plus or minus a degree or two.  Or in the worst case, we picture a gaging shop with swings of temperature between 65° and 90° F.

  Unfortunately, there are also gaging situations where measurements must be made in temperature environments well outside of the inspector's comfort zone.  But it does not have to be outside the gage's.

1. Oh baby, it's hot in here.

  Sometimes a request like this will come across my desk:  Our research division is in need of dial indicators that will withstand working conditions of room temperature to 250°F.  They will be used in 10,000 hour tests and must remain in the oven for the duration.
  
  The customer was using an oven capable of cycling through a wide range of temperatures to test part performance.  A customized indicator with a glass dial and chrome body was supplied to meet the sustained accuracy requirements in the face of frequent thermal cycling.

  Here are some of the considerations that go into making a dial indicator worthy of high temperature applications:

Crystal.  One of the most obvious considerations is the crystal.  Most crystals today are made from plastic blends or alloys that can withstand temperatures up to 170°.  To meet the 250°F temperature requirement, a glass crystal may be substituted.
   
Bezel.  Bezels are typically made of plastic or zinc.  For extreme high-temperature gaging, a steel bezel is the most likely choice.

Paints and Coatings.  Another consideration is the paint on the indicator and dial.  Most paints won't handle this type of temperature.  We might consider a special temperature-resistant coating or, for the most extreme applications, none at all.

Lubricant.  Technically speaking, indicators are not lubricated.  However, a small amount of watch oil is applied to the jewel bearings.  At high temperatures, a special Molycote can maintain lubricity when other coatings would break down.
 
Thermal Expansion Differentials.  In dial indicators, there are some very tight clearances.  Various materials used in making the indicator will expand and contract at different rates.  So it's important for the manufacturer to use materials and clearances that will insure performance of the gage over the entire operating temperature range of the test.   
                     
  Clearances between brass bushings and the steel rack, and between the brass gears and the top and bottom steel plates that hold them in place, must all be sized to eliminate the potential binding or slop due to differential expansion or contraction.
  
  The expansion characteristics of the high temperature dial indicator should be provided to the user, making it possible to mathematically adjust measurements to compensate for different rates of expansion in the gage and the part.
 
Spring Performance.  High temperatures may also diminish the force generated by take up and pull back springs within the dial indicators.  As a result, better grades of steel and larger diameter wire may be needed to insure sustained performance of the gage.  These types of customizations can accommodate for measuring applications in high temperature environments up to 600°F.

2. Whoa! I'm turning blue.
  
  Sometimes measurements must be taken in environments that are mercilessly cold.  The same thought processes are used to determine modifications that will keep the gage, but not the user, from numbing out.   But usually lower temperatures are not the biggest problem.

  Whichever direction the thermometer is moving in, mechanical indicators still can provide an economical solution to some of the most difficult measurement problems.