Probe to fit Thermometer Site guage

mcgeebc · 11-04-2009, 06:48 PM

I think I understand the difference between a thermocouple and a probe, but I want to be sure I am picking the right thing.

I am building an electric HLT that will be controlled with a PID.

I am buying the Weldless Thermometer Sight gauge from Bargain Fittings seen here:
http://www.greatbargain.net/order/shop1.html

I think I need a probe with a 1/2" NPT back thread. I need to find something like this, but with 1/2" instead of 1/4" (and hopefully a little cheaper)
http://www.auberins.com/index.php?main_page=product_info&cPath=3&products_ id=109

From what I understand, a thermocouple will not work because it will stop the liquid from filling in the site guage.

I'm sure other people have bought the same sight gauge from bargain fittings, what are other people using?

**Bobby_M** · 11-04-2009, 08:20 PM

The probe you linked to will work with a 1/2" to 1/4" NPT bushing.

Plan9 · 11-04-2009, 08:22 PM

Here is the bushing...

http://www.mcmaster.com/#4464k265/=4czxmw

**passedpawn** · 11-04-2009, 09:36 PM

Brass version of this bushing is available at Lowes for couple of bucks. I bought them there. I do like the SS one much better tho!

BTW (and I am sure this has been discussed here before, but...) you really should get the RTD probe instead of the thermocouple. Accuracy of the thermocouple depends on the PID staying at the same temperature. If you put the PID in a box that might get warm (i.e., if there are SSR's in there), the PID could potentially have an error of 2 or 3 degrees F (according to Auber... I find this hard to believe, but this is from them). The RTD is immune to this problem, but does cost a few dollars more.

In case you wonder, there is a reference voltage generated in the PID referred to as the cold junction. It is the source of the thermocouple error. You can see this diode almost sticking out of the vent holes in the back of the PID.

mcgeebc · 11-06-2009, 04:48 PM

Thanks. I guess I'll get the RTD and stainless bushing. Might as well do it the right way and not the cheap way.

Just wish it was cheaper.