Noob thermocouple question

[Phillip Brooks]

I've been asked to code and test temperature logger based on the NI-9211.

 

Done.

 

It turns out that the room is 15' x 30' and the custom 20 gauge type J thermocouple wires ordered from Omega are 12' long. I need to reach the far corners of the chamber from an adjacent control room.

 

The first thought was to order new thermocouples, but then I noticed that NI offers 'extension wire'. 

 

http://sine.ni.com/nips/cds/view/p/lang/en/nid/10492

 

Is it possible to extend the existing thermocouples using the correct type wire up to lengths of say 50' ?

 

If so, any downfalls, tricks or calibration issues? I'm monitoring an ESS chamber that cycles from zero to 60C.

 

 

[ShaunR]

50 feet should be fine. The "rule of thumb" is Less than 100 feet is "usually" OK for most thermocouples.

 

Ignoring interference, impedance matching and cold junction compensation, It basically boils down to the total loop resistance.

 

The total loop resistance is a calculation based on the diameter and length of the thermocouple wires. Thermocouples usually have an impedance per foot/meter parameter and the thermocouple loop resistance should be less than about 50 ohms (don't forget 1 foot off a thermocouple spool is 2m of loop wire). You will notice in the NI-9211 spec there are compensation values for impedances greater than 50 ohms so it doesn't mean you cannot use greater, just that compensation must be applied. There is of course a maximum limit due to the sensitivity of the input circuitry, but you should be fine as long as you are not using wires the size of a gnats whisker. 

 

So which idiot didn't order RTD if the temp range is 0-60 degC? 

[hooovahh]

I've used some other thermocouple extension wires before.  I don't know if I ended up with the cheap ones or not but I found that the temperate reading was always off by a few degrees with an extension in place.  

 

I have another situation where I had a PC and I needed to take a temperature reading 20 feet away.  We could get a 20 foot thermocouple, but since we had no hardware restrictions on what takes the measurement, and because we only needed to take one temperature reading, we ended up using a TC-01 USB device.  This meant that the USB cable would be 8 feet or so, and we could either get a USB extension cable and use a short thermocouple, or use a thermocoule extension.  I'm surprised more people don't use this little device for a simple temperature measurement, but I assume other systems need multiple temperature readings, or the system already has some extra hardware for taking a measurement.

[EricLarsen]

  On 6/6/2013 at 11:56 AM, ShaunR said:

So which idiot didn't order RTD if the temp range is 0-60 degC? 

 

Just for future reference, for 0-100 degC conditions I've used many of these:

 

http://www.digikey.com/catalog/en/partgroup/ad592/10680

 

Ridiculously cheap and ridiculously accurate. They are current sourcing, so we've used them with wire distances of several hundred feet with no noise or drop in accuracy. They do requre a voltage exication source, typically 24VDc. Wire them in series with a high precision resistor (usually 10k, 0.01%) near the data logger to convert the current to a voltage. Depending on what accuracy you'd need, the total cost of sensor + resistor is about 10-20 bucks.

[Phillip Brooks]

  On 6/6/2013 at 11:56 AM, ShaunR said:

So which idiot didn't order RTD if the temp range is 0-60 degC? 

 

I just did the coding...   

[MarkCG]

One of a TC's advantages is that they are cheap and have very low time constant, meaning you can measure rapid temperature variations. The bare TC tip is small and is easy to get into good thermal contact with the thing to be measured.  RTDs are more expensive and physically bigger. In the HVAC industry they'd twist TC bare wires together, and solder it to copper tubing of the air conditioners. When test is over, you just cut the wire off near the tip.