Monday, Mar 09, 2009 at 10:25
Hi All,
A bit of information on using multiple thermocouple sensors with one digital temperature gauge:
. A type-K thermocouple can give adequate accuracy for coolant/engine block temperatures around 100 C. The specified accuracy for industial Class II type-K thermocouples complying with international standard ITS-90 is +/- 2.2 C or 0.75%, whichever is greater (pretty much what Auber have said).
However, most T/Cs are much better than this and it is usually possible with digital gauges to 'trim' the reading, to calibrate it to a particular thermocouple. That is, put the thermocouple into vigorously boiling clean water (~100 C at sea level) and trim the gauge to read exactly 100 C. This should give you a long-term accuracy of about +/- 0.5 C in this temp range.
. You can display multiple type-K inputs (say, EGT and coolant) on one display unit if it's done properly. Terminate the ends of each thermcouple onto a small terminal strip AS CLOSE AS POSSIBLE to the input terminals of the digital gauge - preferrably touching the case of the gauge. This is necessary, as the gauge will have a "cold
junction compensation" sensor very near its input terminals. As long as all the thermocouple wires are terminated very close to (and hopefully at the same temperature as) this CJC sensor, no measurement errors will be introduced.
From this terminal strip you can then run ordinary light-duty copper wires to and from you selector switch elsewhere on the dashboard.
. You may find the digital gauge has a lot of inherent damping and it takes quite a long time for the reading to 'ramp' from, say, EGT at ~500 C down to coolant at ~95 C when you switch inputs. If so, you may have to wire the 12V supply for the gauge via your selector switch so that the power supply is momentarily interrupted when switching inputs. This way the gauge will re-start and immediately display the new input.
Email me if you'd like more details.
PS: I can't buy Australian-made thermocouples, even in bulk, at anywhere near the Auber sell price. Hope they don't last very long (only kidding...)
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