Hi,

I need help. What is the maximum length of thermocouple (J, K,or T) can be read temperature in acceptable error (2%)? I want to monitor temperature big warehouse using multichannel data logger.

Waiting for reply. Thanks a lot.

budi
engineer

 

You are carrying mV's with a TC cable and it is very delicate. Difficult to give a distance cos it depands on the EMC, power cables nearby, etc. However, I would never use a cable more than 10 meters long. In such a case, I would place a transmitter inside the head of the TC to change mV signals to 4-20 mA.

 

I would not use a multi-channel data logger. I would use FoundationT Fieldbus transmitter located at each sensor instead. The FF network provides their power and ties them all together and brings the data to your PC or control system.

We (SMAR) make such a transmitter and the interfaces:
http://www.smar.com/products/tt302.asp

If you are not familiar with FF take a look at the book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" (buy online in hardcopy or download immediately in softcopy): http://www.isa.org/fieldbuses

If you can't buy the book now, you can download chapter 1 (overview) for free in softcopy form. It's free, but you must register an account. If your email does not support this hyperlink feature correctly, please copy the entire link and paste it into your Internet browser. Mind the line wrap, make sure to get the complete path all the way to the 4585:
http://www.isa.org/Template.cfm?Sec...=/Ecommerce/ProductDisplay.cfm&ProductID=4585

Jonas Berge
SMAR
===========
[email protected]
www.smar.com
Learn fieldbus at your own pace: www.isa.org/fieldbuses

 

I suggest that you consider an RTD - purchase cost is mildly higher, installation cost mildly lower (no special extension wire requirements), accuracy higher.

If you use 4 wire RTDs (no current in sensing pair) and 18 AWG, I reckon you could run 20m without any noticable degradation in accuracy. If you increase the wire gage to 16 AWG you could double the length. Otherwise, consider the suggestion made earlier that you locate a transmitter at the sensor.

 

we used 55 to 60 meters long themocouples in liquified natural gas storage tanks. other than this better you can go for a temp. transmitter mv to 4-20 ma. you will get more information about temp. control from http://www.tc.co.uk

 

Hello

In our application in power station, we have used thermocouple extension cable of up to 100meters, without any sacrifice in accuracy. Normally, the thermocouple from any maker come with standard (or specified) length and pocket insertion. Whether to use type K, type J, it depend very much on the temperature range that you would like to measure. On this information,it can be found from any maker like Autronica, Pentronic or others. Coming back to the length limit, I really do not know the actual limit. But to achieve accuracy, thermocouple extension cable of the same material as the thermocouple must be used. There are many suppliers of such cables, and these cable comes in various AWG size, numbers of cores-pairs, whereby the per pair is normally twisted and shielded to eliminate any EMF, especially when laid near to power cables.

Of course such extension cables are not cheap taking into consideration the benefits it provides.

Brgds

 

For environmental type accuracy (warehouse temp) it might be much cheaper and easier to use IC sensors. They are linear, require no signal conditioning and are even available with a serial interface. They can be configured in a current loop for long distances, etc. One analog card
and a mux can monitor a whole warehouse since temps don't change very quickly. Multiple TC or RTD inputs get really expensive.

Regards

cww

 

You can now get self contained wireless temp units, send data where ever you need it, no wires, no conduit, no worries.

Otherwise go with RTD or TC with a 4-20ma transmitter, shielded wires to the data logger inputs. Anyway you do this plan on spending some money and time getting it installed and working properly.

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