variations on the TEC components

The temperature measurement of the TEC is a radiometric measurement with a temperature dependent resistor and a fixed resistor in series. An NTC behaves as:
R(T):=R0*exp(beta*((1/(T+273))-(1/(25+273))));

with beta being the material constant, R0 the resistance at 25DegC, and T being the temperature in DegC

Temperature sensing with an NTC S861

This NTC is specified as R0 = 10k +-1%, Beta = 2988 +-1%, and the fixed resistor is specifies as 10k +-1%. the following maple script

> restart;
> resv:=r0*exp(beta0*((1/(T+273))-(1/(25+273))));
> Teiler:=resf0/(resf0+resv);
> beta:=beta0*beta1; #beta1 is the uncertainity of the beta
> r:=r0*r1;          #r1 is the uncertainity of the r
> resf:=resf0*resf1; #resf1 is the uncertainity of the resf
> r0:=10000; beta0:=2988; resf0:=10000;
> beta1:=1.00; r1:=1.0; resf1:=1.00;
> resv:=resf/Teiler-resf;
> tofr:=1/(log(resv/r)/beta+(1/(25+273)))-273;
> dt:=tofr-T;
> plot(abs(dt), T=0..120);

lets us calculate the variations on the result.

The variation of the NTC resistance R0 = 10k +-1%

The variation of the NTC beta Beta = 2988 +-1%

The variation of the fixed resistance 10k +-1%

All three parameters together can thus approach 2 DegC within 0 and 100 DegC. The whole can be improved, by having a better fixed resisor with 0.1%.

As alternative component, the variation of the fixed resistance 10k +-0.1%

Plus there are better NTC, with a tolerance of 0.2 & 0.5 DegC between 0 and 70 Degrees, the S863 & the S869 Series. In addition to what has been shown, there is the temperature coefficient of the fixed resistor in the order of 100ppm/K plus the long term drift, which loosely specified as < 2% over 10k Hours.

to be continued ...

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last updated subpage : 20.june.08 or perhaps later

Copyright (99,2008) Ing.Büro R.Tschaggelar