I want to be able to monitor two pipes with a single display using 'type 'K' thermocouples. Temperatures involved lie in the range 10-45 deg C.
Now I know it is naughty to have additional junctions involved in thermocouple work, and really everything should be terminated on an isothermal plane, but in the real world should I expect problems if I run both thermocouples to a change over toggle switch and then on to the display in the type K wire ? The display (a West N8010 which does cold junction compensation) I'm not looking for super accuracy and the display only has 1 deg C resolution anyway.
Andrew The introduction of numerous unknown inter-metallic junctions is likely to introduce errors which may be quite large unless you can supercool your switch ;-) Comark used to (and may still do) manufacture an expander box for their digital thermocouple meters, these allowed up to 20 or more thermocouples on
1 meter and are designed to add very little uncertainty to the measurement; certainly worth a try on eBay or the surplus places such as Greenweld. Martin
...mmm ... if the components of the switch are all at the same temperature will not the 'going in' effective junction not be compensated for by the 'going out' junction where the metal transition will be of the opposite sense ?
And it's one hell of a machine that needs me to monitor CLOSELY the temperature of the cooling water flowing in and out, which is the application in question
If the thermocouples aren't in insulating sheaths, use a double pole double throw switch, if there is no conductive path between them then a single pole double throw switch will do. Put the switch in a box with a bit of insulation inside to keep draughts off it. This will keep the three or six copper to chromel or alumel junctions that you create at about the same temperature. You ought to be able to get within a tenth of a degree quite easily this way and it will take up much less space than the thirty way Cropico switches we used to use before we got data loggers.
We used to have some Fluke monitoring equipment which included very expensive thermocouple switching boxes, curiosity got the better of me and they appeared to contain nothing more than a bog standard wafer switch!, of course the contacts could have some exotic plating that I couldn't see, but I very much doubt it.
'Proper' thermocouple switches have contacts and connections that are made of the same metals as the thermocouple itself so are rather expensive. For this reason it is important to use the right switch for the type of thermocouple that you are using and also to connect it the right way round! I don't think plating the correct metal would work, as there is a junction created between the plating and the base metal. When I worked in process control the problem was overcome by having all thermocouple wires from the plant run to a single cabinet which was temperature controlled (an isothermal plane) and had its own embedded thermocouples going back to the computer, that was running a complex cold junction compensation algorithm that took the isothermal plane temperature into consideration. But then they had money to play with to get it right. Most of the systems were in the oil industry, some in food processing , but one was the development fermentation setup that did the initial production of Viagra !
Thermocouples work because a temperature gradient along a wire generates a voltage along it, which depends only on the total temperature difference between the ends of the wire and the type of wire.
This is true of all normally conductive wires.
Two different kinds of wire are used in thermocouples because the amount of voltage along each wire differs depending on the kind of wire, and the difference between these voltages is what is measured - the junction just connects them electrically.
Say the cold end of one wire is at at 0 volts, the hot end of that wire (and the junction, and the hot end of the other wire) might be at 10 mV - the other wire might only develop 8 mV, so there will be a difference of 2 mV at the cold ends. That 2 mV is what is measured.
Any connection where there is no difference in temperature along its length does not create any voltage, and therefore doesn't change the reading, no matter what it is made from.
If you use chromel/alumel wires to connect from the switch to the meter you should be okay [1].
There shouldn't be much if any temperature difference along the switch bit of the path [2], and although the switch and the reference in the meter will likely be at different temperatures the Alumel/Chromel pair will generate the exact voltages required to compensate.
[1] If you use copper the effective reference temperature will be the Alumel/Chromel / copper junctions (switch) temperature, but the meter will think the reference temperature is different (ie the reference inside the meter) and things may go pear-shaped.
You can be clever and compensate for this by using a "zone box" (wherein the temperature is uniform - it doesn't matter what it is, just that it is uniform) and an extra thermocouple inserted in one copper lead from the box to the meter with it's measuring end in the box - then overall the effective cold end is the one in the meter. You would only do this if you wanted to connect very many thermocouples though.
[2] One reason why the temperatures of the Chromel/switch and Alumel/switch junctions may differ is because the thermal conductivities of Chromel and Alumel are different, and at high measured temperatures heat leaking back along the wires can cause one junction (can't remember which) to be hotter than the other - but at the temperature ranges you mention you should have little or no problem with this effect.
{ - of course this can all go haywire under extreme conditions, eg junctions between different metals also give rise to small voltges - these will typically cancel out, but the voltages are temperature-sensitive, and if things are done to extremes that can cause problems - but that's not likely under your conditions - }
Note that even precision grade K thermocouple wire is only accurate to about
1% of the difference between the measured temperature and the reference temperature anyway, although two couples made from the same batch of wires will usually differ by much less than that.
I run small pottery kiln - good for 1400 C. The termocouple I was supplied to go with the temperature controller had plain stranded copper leads terminated in 4mm "banana" plugs to suit the 4mm sockets on the controller. Now I'm sure that the temp indication I get from that system isn't accurate in any absolute sense, but it is certainly "good enough" for that particular purpose. I'm sure that the introduction of a couple of switch contacts into that setup wouldn't have had a significant effect on the accuracy ("significant" in the sense that it would have affected the firing).
I suspect that as long as you do everything symmetrically (same switch contacts in both arms) and as others have said, keep the junction temp for each pair of junctions the same, then it should be OK unless you need sub-10 degree accuracy.
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