Sciencemadness Discussion Board

Thermocouple Advice

MrHomeScientist - 20-3-2013 at 11:11

I am looking into getting a nice thermocouple to add to my lab equipment, but I know very little about them. The reason I want one is for the potassium synthesis, in which I will need to measure the sand bath temperature (which will be ~200C). This isn't too demanding, but it would be nice if I could find one that is chemical resistant so I can use it to directly measure reaction mixes in the future. I did some preliminary searching, and it looks like K-Type probes go up to about 1300C. That would be neat to have for metallurgy of low melting point metals (Bi, Sn, etc.) as well.

Does anyone have any recommendations for probes that work well in different lab situations? Minimum requirements are that it work for the K synthesis (measuring a sand bath at ~200C), but it would be nice if it was robust enough to use for other things.

I've seen quite the range of products in a search on eBay, from this cheap one: http://www.ebay.com/itm/251221097301?_trksid=p5197.c0.m619
to this nicer-looking model: http://www.ebay.com/itm/270381494094?ssPageName=STRK:MEWAX:I...

but I'm not sure what to look for in terms of quality.

Magpie - 20-3-2013 at 12:24

I can recommend Omega Engineering - they have a very wide selection. Here's what I use, and like:

http://www.omega.com/ppt/pptsc.asp?ref=KQXL_NQXL

I buy a 12" length, 1/8"diameter, "K" type. I use a PID for readout.

MrHomeScientist - 22-3-2013 at 05:38

Thanks Magpie, that does look nice.

That sort of reinforces what I've been seeing when searching for these things - nearly everyone sells the probe, but I never see what it actually plugs into. Is there a particular device to use with thermocouples, or do different things work? I know you said you use a PID, but I have no experience with those beyond what google tells me. I have an EE friend that could probably build one, but I imagine we'd still need some data sheet correlating probe voltage to temperature to get it working.

watson.fawkes - 22-3-2013 at 06:26

Quote: Originally posted by MrHomeScientist  
That sort of reinforces what I've been seeing when searching for these things - nearly everyone sells the probe, but I never see what it actually plugs into. Is there a particular device to use with thermocouples, or do different things work?
What they plug into, after all is said and done, is a voltmeter that converts the voltage across the thermocouple into a temperature reading. Thus there are lots of things they plug in to, or are wired into, for fixed applications. You posted two such handheld meters in your original post. I have a twenty-odd dollar Chinese unit, and it measures temperature just fine. The PID controllers Magpie mentions have screw terminals on the back (as a rule). You can take off the plug of a thermocouple assembly and screw the lead wires down, or alternately get a thermocouple socket (Omega also sells them) and install it.

Unless you want something mighty special, I would not recommend trying to roll your own circuit. Too much work. If you want something custom, I'd recommend osPID, which was built to be modified.

Magpie - 22-3-2013 at 07:49

Quote: Originally posted by MrHomeScientist  
Thanks Magpie, that does look nice.

That sort of reinforces what I've been seeing when searching for these things - nearly everyone sells the probe, but I never see what it actually plugs into.


As you can see the mating connector comes with the Omega thermocouple. But you do have to buy some thermocouple wire, probably in some minimum length. I think I made my hookup to the PID controller with about 10 feet of wire. Give yourself plenty of wire.

MrHomeScientist - 22-3-2013 at 10:16

Quote: Originally posted by watson.fawkes  
What they plug into, after all is said and done, is a voltmeter that converts the voltage across the thermocouple into a temperature reading. Thus there are lots of things they plug in to, or are wired into, for fixed applications. You posted two such handheld meters in your original post. I have a twenty-odd dollar Chinese unit, and it measures temperature just fine.


Well I suppose "never" was too strong a word, since I did find those two! More correct is that it seems to me that the vast majority of sales are for the probe only. Thanks for the good info, too.

=======================

@Magpie: You're right, the probe you linked doesn't have any wire so it would just stick directly out of the meter, thanks for pointing that out.

I think what I may do is buy the meter in my first eBay link, and also grab the probe you pointed out. As for extension wire: according to the description on the eBay item it accepts "standard miniature thermocouple connectors." If I'm reading the Omega site correctly, that means I need a male SMPW-type connector.
They sell ready-made extension cables here, but there is no male SMPW down in the list of connector codes. Do you know if SMP is the same as SMPW? The page was under the mini connectors heading, so I suspect they are. Option 14 (male SMP to stripped leads) would be the one I want if so.

If you don't know off the top of your head, I can of course just ask the company.

Magpie - 22-3-2013 at 11:24

Quote: Originally posted by MrHomeScientist  
=======================

If you don't know off the top of your head, I can of course just ask the company.


I don't know about connectors on the meter end of the wire as the PID controller just has screw-down connections. I would call Omega.

metalresearcher - 22-3-2013 at 11:50

Type K is rated 1300 C but only for short periods (a few seconds). For continuous use, don't heat it over 1000-1100°C otherwise it will wear quickly. Consider a Type S (Pt-Pt/Rh) which can be used till 1600°C for a long time.But they are more expensive.