Thermocouple signal conditioners
Thermocouple signal conditioners
(OP)
I am using PLC to monitor temperature 500-900 c using K type thermocouple. PLC only takes 0-10V analogue signal.
I found a thermocouple transducer to convert thermocouple 0-1000C to 0-10V however I would like to maximize accuracy over range I am looking at 500-900C
Can anybody recommend signal conditioning thermocouple transducer that might do this without having to do too much electronics building?
Thanks
I found a thermocouple transducer to convert thermocouple 0-1000C to 0-10V however I would like to maximize accuracy over range I am looking at 500-900C
Can anybody recommend signal conditioning thermocouple transducer that might do this without having to do too much electronics building?
Thanks





RE: Thermocouple signal conditioners
It's a good principle that the measuring system should have plenty of head room at the top, and same thing at the bottom. One should beware of what happens to the control system and/or user displays if the variable happens to get out of range.
RE: Thermocouple signal conditioners
TTFN

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RE: Thermocouple signal conditioners
VE1BLL
What do you mean room at top and bottom? Does that mean if I am interested in a range I should have capability be reading 20% above and below in my PLC?
RE: Thermocouple signal conditioners
8 bits can give a resolution of approx. 0.4% once calibrated, so if you have a signal conditioning unit that can have an offset voltage applied to it of -5V then add a gain of 2 your input to the PLC could give you 500-1000 deg C range for 0-10V input. The only downside is that you can't then tell any difference between 500C and a lower temperature. If this never occurs in your process it may not be a problem, otherwise you may have to programme you PLC to recognise a low A-D count near zero reading as being = 500C and anything from there down to all bits = 0 as out of range or sensor failure.
RE: Thermocouple signal conditioners
Adding to what BrianG already provided:
In my line of work, we pay very close attention to safety as a science unto itself. There have been accidents (in many fields) where having a sensor/display system bottom-out, or max-out, has made a bad situation much, much worse. It should be standard policy that the sensors and displays cover a range that provides useful information even in the event of failure modes.
Of course, there may be situations where one needs to make a trade-off. The point is to not allow oneself to instinctively narrow the sensor and display range, without at least taking a moment to also consider failure modes.
8-bits is pretty low-end in this day-and-age.
One old trick is to convert the signal-of-interest to the frequency domain (V/F converter), and then measure the frequency. There's PLENTY of resolution (accuracy is another topic) in the frequency domain, even in the low kHz range. This assumes that your PLC has a discrete input and enough uC horsepower to measure frequency. Also, think about the counting time.
RE: Thermocouple signal conditioners
Dan - Owner

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RE: Thermocouple signal conditioners
I'm sure there are other modules with the same capabilities that might be cheaper.
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RE: Thermocouple signal conditioners
When I get my hands on a good electronics engineer I will probably use a PIC based controller whish can take in 14 bit temperature values. Much smaller, cheaper, and capable.
RE: Thermocouple signal conditioners
You might consider: http://www.sparkfun.com/products/10294 While it has only a 10-bit ADC, it comes with a bunch of other devices that might prove interesting, and the tools are either cheap or free.
TTFN

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RE: Thermocouple signal conditioners
When you drive 4-20mA through 500 ohms and you get 2.0 -10.0Vdc suitable for your 0-10Vdc analog input.
Yes, you lose 20% of the scale by having a live zero at 4mA, but with a 400°C span (900-500) you lose 600°C of the 0-1000° range that's not of interest.
No, 2 wire loop powered transmitters can not be scaled for 0-20mA because the transmitter uses about 3.5mA run its own electronics.
Google loop powered temperature transmitter for 1.3 million hits
Be sure to inquire of the vendor about getting the range done for you, so you don't have to buy software or a special cable to configure the range. Either the manufacturer or the distributor should be able to range it for you.
RE: Thermocouple signal conditioners