XPost: sci.electronics.equipment   
   From: presence@MUNGEpanix.com   
      
   In sci.electronics.basics Ralph Mowery  wrote:   
   > In article , presence@MUNGEpanix.com   
   > says...   
   >>   
   >> > So the object was precision and not accuracy.   
   >>   
   >> If the goal was keep the needle on their marks it does't have to mean   
   >> anything was precise. Maybe your guages had no faces, or read mA instead   
   >> of degrees, and bent needles. As long as your +/- 3 degree thermocouples   
   >> and controllers did not jump up and down + and then -3 degrees all the   
   >> time, you were good.   
   >>   
   >>   
   >   
   > One good example of what we  had is this.   
   >   
   > In a vat of material is a test hole.  In that hole is a rod about 3/8   
   > inch in diameter and a foot long.  At the end there are two   
   > thermocouples and two RTDs.  The thermocouples wires go about 100 feet   
   > to a PLC (similar to a computer) card that converts the milivolts to   
   > digital that is then displayed on a compute screen.  The RTDs go about   
   > 10 feet  to a converter that converts the change in resistance to a 4 to   
   > 20 miliamp signal.  That goes to a card on the PLC and then to the   
   > computer display.   
   >   
   > While the computer will display to 3 decimal places at 300 deg C from   
   > the lowest to the highest temperature shown on the display can be around   
   > 3 deg  differnet and all 3 be within the limits of the equipmnet.   
   >   
   > At a certain time a sample is sent to the lab and one of the computer   
   > displays is set as a standard and the object of the PLC is to keep the   
   > actual temperature , whatever it actually is, to that 'standard'.  Not   
   > too accurate as to temperature, but very precice.  The operators only   
   > needed to keep that one computer display as close to that 'mark' as they   
   > can if for some reason the PLC messes up and they have to adjust  the   
   > control manual.   
      
   What's the control loop if the PLC dies? How do people control temperatures   
   manually? Is there a foot pedal to stomp on to switch the heaters on and   
   off?   
      
      
   There's a couple machines I fuss with that use platinum junction RTDs and   
   we have alarm limits set. If the machine drifts into an alarm state,   
   outside of a warmup period that's pretty much the end of the day and   
   everything stops until it can be fixed. The loops on these machines are   
   tuned to maintain and hold a set point of less than 1 degree F. The   
   displays are all wrong, show fake levels of precision, and read in C, but   
   are wrong by several degrees, even if you do the math. We gave up trying to   
   calibrate the displays against what the real temperature with the offset   
   features when the probes were last changed. It just isn't worth the time.   
   Those machines are not accurate, they're not precise (as measured with   
   their own instrumentation), but they will absolutely hold a stable   
   temperature if you can determine the set points yourself.   
      
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