You're lucky that the output is actually giving a value that you want. The KA4558 has an input offset voltage of +/-2mV typical (6mV max), which could give an output error in the range of +/-800mV.OK, using 18F4580 and Type T thermocouple (TC) to make a thermometer to be used in laboratory setting. I have the thermocouple interfaced with a difference amplifier using KA4558 and a gain of about 400.
Yes. The value from the thermocouple reflects the difference in temperature between the tip and the "cold/reference" junction. You therefore have to offset the temperature by the cold junction temperature.Do I convert both ressults to temperature and then subtract the CJ temp?
If the temperature of the cold junction is room temperature, e.g. 25^C, then by ignoring it, the temperature you report will be 25 degrees too low.What is the result if I ignore the CJ temp?
If the temperature of the cold junction is room temperature, e.g. 25^C, then by ignoring it, the temperature you report will be 25 degrees too low.
You're lucky that the output is actually giving a value that you want. The KA4558 has an input offset voltage of +/-2mV typical (6mV max), which could give an output error in the range of +/-800mV
Yes. If you assume 25^C ambient, but it's actually 18^C ambient, your uncompensated temperature will be 7^C off (at least).That is, if I assume ambient is 25°C and create my lookup table based on that then find the ambient is really 18°C, my lookup table has the incorrect values for each A/D result.
You can either null the offset of the opamp, or use a low-offset-voltage opamp, e.g. LTC2050, or probably any that you see in these search results: https://www.google.com.au/search?q=thermocouple+amplifier&safe=off&source=lnms&tbm=ischYou've got me worried on this one. Is there an op amp that you would recommend for this application that I can find in a dip?
To get reasonable stability you have to use OP07 or 714 op amp. I'm not updated, there could be op amp with lower drift nowadays.Ok. I've been looking around and trying to find a decent dip amplifier is not easy. So, I'm thinking of using a 24 bit a/d to avoid the need. Analog Devices AD7714 is what I'm thinking of using. What do you think of this choice? I will already be using SPI for datalogging on an SD card, so it seems like a good fit, but I've not done anything like this before.
I am thinking of preconditioning the TCvoltage with a thermistor so that the CJ temp is included ion the A/D, but I'm not really sure how to accomplish that yet. I have looked at using the LM35 for CJ compensation, but I',m not real sure how to use that one either (at least not in this fashion). There is a reference voltage input on the AD7714. Would taking the output of al LM35 to the + reference do that trick?
Any advice?
Yes, you got it, if you connect 803uV to the -TC at 18 centigrade the +TC will give you the correct voltage. The difficult part is to arrange the thermistor to give you the right voltage for 0 to 50 centigrade.I've been tinkering with a NTC thermistor (15k) but I just don't get it. Question: At room temperature of 21°C, my T-Type TC should have a voltage of 0.000803V when properly compensated (referenced to 0°C). So, if I use a thermistor, does the voltage I am supplying FROM the thermistor (I am assuming I will be using a thermistor resistor divider network here) as a biasing voltage need to bring the TC voltage to that point? Is THAT how this is supposed to work? I'm referencing page 4, figure 14 of the attached pdf.
OK. That is clear enough on the conversions. In another thread I asked about these lookup tables. I'm still fuzzy on this. I don't understand what values actaully needt o be in the tables. If the table is composed of integer temperatures in the range of interest (0-250°C in this case), and the A/D result is used to lookup the temperature, why do I even need a table?
Let me see if I can walk myself through this and get some guidance on it - this is just not making any sense to me.
Let's assume that the Type T TC is properly compnesated. The range of temps I am interested in is 0-250°C. The max TC voltage then will be 12.013mV. Let's also assuem a 10-bit A/D instead of the 24 bit version I mentioned earlier. So, if I want the voltage into my 10-bit A/D to be 0-5V, I will amplify the compensated TC voltage by 416. Let's assume my amplifier is doing exactly that. From this I know 0v(a/d input)=0(a/d result)=0°, 1v=205(rounding)=58.52°C(interpolation), 2v=410=111.21°C, 3v=614=160.05°C, etc.
Ok, I see from this that my results from the A/D will not be linear with respect to the temperature (expected since TCs are not linear elements) so that I can't just create a scaling factor and multiply the A/D result to directly get the temp.
So, tables...... I'm sorry, but I don't understand what to do here. What values are in the tables? I know I get an A/D result in the range of 0-1024. If my temp is in the range of 0-250°C, then each bit is approximately 0.244°C. Again, knowing this, it seems logical to simply multiply to get the temp, but it's not a linear function, so I know my temp will be incorrect. I'm stumped.
If I make a table of values, the only table that makes sense to me is one made up of temperatures related to the A/D result. For instance, A/D 10 = 3°C, 20 = 6°C, 50 = 14.95°C, 100 = 29.46, 200 = 57.22, etc. This means to me that my table can have at most 64 values as they will all be single point values (4 bytes each). Each individual A/D result will need to be interpoltaed if it does not fall on one of the specified values. No biggie there. If I need a table for both the thermistor (small range there) and the TC (wide range), I will need to be careful in choosing the values placed in the tables to afford me the best chance at a correct interpolation.
Is THIS how it's done?
Is this really the best solution? This seems very .... brute force. By that I mean it seems to lack sensitivity, precision and accuracy all at the same time.
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