Bad thermocouple? - Page 1
Forum section: electronics / beginners

author: danb35
author_rank: Contributor
timestamp: December 22, 2012, 12:29:14 am
content: I recently bought a used Fluke 87V off eBay. Thinking it might be handy to use the temperature measurement capabilities, I also bought a Fluke 80AK-A thermocouple adapter, which arrived here yesterday, and a generic K-type thermocouple, which arrived today. Plugged the thermocouple into the 80AK-A, and the 80AK-A into the 87V, and set the 87V to the temperature mode--it says "OPEN". Do I have a bad thermocouple, a bad adapter, or am I doing something wrong?

author: Jeff1946
author_rank: Regular Contributor
timestamp: December 22, 2012, 01:05:31 am
content: Use ohmmeter to measure your TC, should be little resistance. If broken you can twist ends together then strike with a hammer against a hard surface to "fuse" together. I believe it is done industrially using a hydrogen rich flame to melt together. Good test is to put into a ice-water slush in a insulated container (Styrofoam cup for example) and see if you get the correct temperature. You also can use boiling water but will need to adjust for altitude.

author: SLJ
author_rank: Frequent Contributor
timestamp: December 22, 2012, 01:38:34 am
content: Mine all read 5 to 6 ohms except the two POS ones from china I just wasted $3 on 1=open, other 17 ohmsAnyone got a temperature to thermocouple voltage comparison chart if it exists?Never mind, found one:http://instrumentation-central.com/pages/thermocouple_reference_table.htm

author: danb35
author_rank: Contributor
timestamp: December 22, 2012, 01:53:38 am
content: 15 ohms, sounds higher than it should be. Looks like time for a return.

author: SLJ
author_rank: Frequent Contributor
timestamp: December 22, 2012, 02:19:40 am
content: I don't know if 15 is too high or not as it's the voltage they generate at temp. The thermocouple that came with my 87V reads 4.7 ohms. The china one that's not open reads 17 oms and is off by a couple of degrees compared to the rest.

author: SeanB
author_rank: Super Contributor
timestamp: December 22, 2012, 04:37:03 am
content: Normally thermocouples are pretty much a wire resistance wise on a DVM. They should read as a low resistance, and if they have a bare tip ( most do) then you can measure each leg and check that they are the same resistance. If the tip breaks off it is simple to clean it off and then weld the ends together again. I use an acetylene torch normally, or for quick work just clean them and twist together tightly. To check the adaptor plug it in without the thermocouple and check the voltage at the thermocouple socket. It should have a voltage of around 2V or so at a very low current, deliberately sent through the thermocouple to detect an open circuit by driving the reading to off scale high or low ( depends on polarity of the bias voltage and the desired effect of the sensor failing) on the display. Of no voltage then the wiring inside the adaptor is broken ( common) or the resistor that feeds this voltage is open circuit.

author: ftransform
author_rank: Frequent Contributor
timestamp: December 22, 2012, 12:35:05 pm
content: Is twisting them and smacking with a ball peen hammer good enough compared to a weld?Does the performance between real twisting and welding differ?

author: SeanB
author_rank: Super Contributor
timestamp: December 22, 2012, 12:48:10 pm
content: Twist and peen works until the surfaces oxidise again, how long depends on the temperature and if it is an oxidising or reducing region. No difference otherwise, though the welded join is better. If you have no way to weld then it is simple to DIY with a carbon rod ( old zinc chloride battery not alkaline) and then use a 12V car battery to strike the arc between the carbon rod and the joined thermocouple metal. A second of arc and they are joined permanently. Otherwise of you are not going much above 300C silver solder works very well, the 2 thermocouples created cancel out as they are so close together that they can be considered to be at the same temperature. Adds extra mass to the joint though which reduces the response time.

author: johnwa
author_rank: Frequent Contributor
timestamp: December 24, 2012, 01:48:51 am
content: I have had good results using capacitor discharge welding to form thermocouple junctions. To do this, first disconnect the thermocouple wire from the measuring instrument. Then strip the ends at the far end of the cable back about 50-100mm. Connect a large electro up, somewhere between 1000-10000uF, and charge it up to somewhere around 20-40V. Then short the two wires, they should fuse together. You may need to give it a couple of goes. It is probably best to start off with lower voltages, and increase until you get an acceptable result. You should wear safety glasses too. Note that the capacitor is connected to the same end of the cable that you will be forming the junction on, in order to minimise the effects of cable resistance.

author: G7PSK
author_rank: Super Contributor
timestamp: December 24, 2012, 04:27:32 pm
content: Clean the wires and then dip in a paste of borax or even baking soda if you have nothing else then heat in a gas flame to red heat and then hit with a hammer. This is forge welding.