Thermocouple puzzle.

I had a conversation with one of our other engineers before lunch and he wondered what would be the result of the following TC arrangement - two identical T/Cs in parallel in contact with the same object but where there was a small temperature gradient. TC wires are the same length.

TC1    /----------------+-------

1000C *                 |        ?

       \----------------)--+----

                        |  |

                        |  |

                        |  |

                        |  |

TC2    /----------------+  |

1002C *                    |

       \-------------------+

I said the instrument would register the lower temperature, 1000C. He thought it might be 1001 C, the average.

Neither one of us is 100% sure.

One answer here, see 'averaging thermocouples'

If the T/Cs are the same style, you should read the average. See this link:
http://www.eng-tips.com/viewthread.cfm?qid=130117&page=48

Average

Thanks for the links guys. I wasn't sure so I went with the lowest temperature gradient which would be TC1 to cold junction.

Average is the answer Bob.

I got it wrong the first time I saw it.

I always keep my eyes and ears open and my mouth shut, as I learn something new every day. BUT, one time (and only one time, tee hee), I got it wrong.

Customer had eratic heat control on a large extruder. I connected my laptop to the AB PLC and started a Trend on actual Process Variable. It was a steady increase during cold start, then would go beserk with noise.

We started to inspect the point of temperature measument, I discovered that there were two thermocouples in parallel. One of the thermocouples was sunk (drilled) into the deep steel body of the extruder barrel. The second thermocouple was merely attached to the surface of the barrel.

I have seen in the past a second surface thermocouple for data acquisition only, and assumed a unqualified person inadvertantly connected the two together.

I instructed the customer to use only the thermocouple in the drilled thermowell. Yes it works and is stable. BUT.

We, looked it up, and did determine, that the parallel thermocouple was part of the original OEM design. The key reason was to take thermal lag and PID Integral wind up out of the picture to prevent cold start, overshoots.

When using only the deepwell TC, the thermal lag was so great, there was massive overshoot. When using only the surface TC, the PID cut it's output too soon, and cold-start time was horrendous.

By averaging the two thermocouples, the net result is to get the fastest rise to setpoint, with minimal overshoot. BRILLIANT.

We did find that one of the paralleled TC had an intermittent connection. Once we repaired the connection, thetemperature loop was stable again.

Yes, using a deep and shallow thermocouple is a common method for one extrusion OEM. The deep is responsive to the shear heat of the screw, the shallow to the effect of the heater or cooling. By controlling off of the average of the two, the PID loop works well.

A very common arrangemnet thermocouples in parallel for exhaust gas temperature for turbines ( jet engines). As many as 12 or more in parallel.

Actually if you look at the diagram T/c 2 has longer leads so it will have less influence on the result. I know, i'm nit picking LOL