Make your own precision SS temperature probe

**bull8042** · 02-02-2009, 02:23 AM

I have mentioned making my own temp probes in the past, and there has been some generic discussions but no real details. I found myself getting ready to make another sensor for my rig and decided I would throw together a quick HowTo in case anyone else was interested. I apologize in advance for the lack of a "Bobby M" quality video, but soldering 22 AWG wires and holding a video camera are two things that don't necessarily go hand in hand.
OK, the materials I am using is a length of 1/4" OD Stainless Steel tubing, a sensor cable with M8 (pico) 3-pin connectors, and a National Instruments LM34CAZ temperature IC. It is a molded TO-94 transistor case sensor that fits quite nicely in the tubing, almost a perfect fit as a matter of fact.
I purchased the sensors from Newark Electronics for around $5 each. The cables can be found from any automation/electronics supplier. The ones I am using are from Phoenix Contact, but any sensor cable will do. You can actually use any small multi-conductor cable as long as it will fit into the SS tubing. I wanted the connectors so I could remove or swap out sensors without having to remove any cabling. I also am a huge fan of shrink tubing. Black electrical tape is fine if you are insulating split bolts on a motor connection or repairing a handlebar grip. But you can't beat shrink tubing when it comes to insulating small electrical connections as well as providing strain relief for the cable.

**bull8042** · 02-02-2009, 02:26 AM

I started by cutting the "MALE" end of the cable to the length that I wanted, which was about 6 inches. I stripped the jacket back about 1-1/2", being careful not to nick the conductors. I then stripped the conductors about 1/4" and tinned them with the soldering iron.
At this point, I cut 3 pieces of shrink tubing about 5/8" long and slipped over each conductor. Then, using a pair of vice-grips as a combination vice and work-piece holder, I clamped the temp sensor GENTLY. Remember, it is made of a bakelight material which is brittle. It won't take a huge about of abuse without fracturing.
One at a time, I laid each conductor along side its respective pin and applied just enough heat with the soldering iron to flow the solder that was already on the wire. No need to add more solder since we aren't making a structural connection. Once the soldering was complete, I slid the shrink tubing into position and whipped out the heat gun.

**bull8042** · 02-02-2009, 02:28 AM

Now you simply push the sensor into the piece of tubing until it is about 1/8" from the end. I used a thick piece of shrink tubing that is adhesive-lined to not only hold the cable in place, but also provide a strain relief. *** EDIT - I am no longer recommending the use of 5-min epoxy! I have found that it softens when left in contact with water and sanitizer. This make it totally unsuitable for our use. So, after chatting with Derrin, I ordered 4 of the Stainless Steel probe bodies that he is making. His site can be found here - http://www.stirplates.com/probeends.htm
He shipped them immediately and I had them within the week. They are great and definitely the way to go if you want to make your own sensors. Plus, he is "one of our own" here, so nice to support other HBT members when you can.

**bull8042** · 02-02-2009, 02:30 AM

Of course, having the proper "assistant" in any project like this is a must.

Now I am ready to install my new sensor....

**bull8042** · 02-02-2009, 11:42 AM

I notice this morning in the picture of the epoxy on the sensor tip, there appears to be a lot of air bubbles. But in actuality, most if not all small bubbles will migrate to the top and leave a nice almost mirror finish as it is curing.
This is the tubing I am using. It is seamless Stainless 316 with 0.028" wall thickness.
https://www.onlinemetals.com/merchant.cfm?pid=14826&step=4&showunits=inches&id= 902&top_cat=0

Jun · 02-02-2009, 12:15 PM

Does the SS tube or epoxy effect the accuracy of temp probe?

**bull8042** · 02-02-2009, 12:19 PM

Quote:

Originally Posted by Jun

Does the SS tube or epoxy effect the accuracy of temp probe?

Seems to have no appreciable effect at all. I can lay a probe on a solid surface like a table, let it sit and stabilize for a few minutes. Just by tapping the probe once with my fingertip, the temp will climb a few tenths. Very responsive.

**IrregularPulse** · 02-02-2009, 12:33 PM

Can we get detail how it attaches to your rig? where does the threaded portion come from to screw into the T?

**bull8042** · 02-02-2009, 01:15 PM

Quote:

Originally Posted by IrregularPulse

Can we get detail how it attaches to your rig? where does the threaded portion come from to screw into the T?

This is the adapter I am using from McMaster-Carr.
P/N - 5182K113 TYPE 316 SS YOR-LOK TUBE FITTING, ADAPTER FOR 1/4" TUBE OD X 1/2" NPT MALE PIPE
It has a double SS ferrule that I am not using. The adapter needs to be drilled all the way through so the probe can extend into the flow of liquid. I found an o-ring that is perfect size and used it in place of the ferrule. I snugged to nut down finger-tight and fired up the pump... no leaks at all.

**IrregularPulse** · 02-02-2009, 02:19 PM

cool, thanks. Do these temp probes go to Off the shelve controllers such as LOVE or RANCO or to custom written code?