Velleman K8067 - slope and intercept

Kits General
1

Hi,

Sorry to bother you with this, but I could not find a satisfactory answer when searching old forum posts.

I have 2 of the Velleman K8067 kits assembled and working, but need to know the actual correlation 0-20mA to centigrade mapping.

The specification states: -20 to +70C for 0-20mA.

So, with industrial sensors one clearly assumes that 0-20mA maps to -20 to 70C.
Therefore using 250R resistor (giving a 0-5V) the following equation should therefore hold:

degC = -20 + 18 * V

However the equation in the manual suggests a constant of -23. So do I assume that the actual range for 0-20mA is -23C to 67C, -23 to 70C or something else completely?

I don’t really want to have to resort to deriving a calibration curve since this would defeat the whole object of getting a properly tested kit using a nice linear output LM335 and TLV2741 rail-rail op-amp. :open_mouth:

Initial results indicate that an offset of -30.6 and slope of 21 are closer to measured values using a Fluke 1587 and a type K thermocouple. i.e. this gives a range of -30.6 to 74C for the 0-20mA range.

Any clarity on this much appreciated. :neutral_face:

Regards
Eclectica

2

Not sure if we understand your question.
If you want to visualize the curve, you could make a spreadsheet:
Calculate the temperature for each AD-level using the given formula (101xAD-value/256)-23
(AD-value: 0 to 255)
Next, to convert to millivolt, multiply AD-value with 5000/256

3

Thanks for your reply.

Ok, so based on your adc formula the 0-20mA maps to -23C to 78C, not -20C to 70C as stated in the specification.

To see what I am getting at see the image here.

1127×614

I hope this will work as you suggest! :-
Regards
E.

4

Specs are conservative, ‘real’ outer limits can be calculated.