Kudos to Peters
I work with NERC doing an educational outreach on cybersecurity. Personally, I found Mr. Peters article ("Do Firms Expect Too Much Cyber Threat Data?," online in April, www.controlglobal.com/articles/2011/CyberData1104.html) both impressive and informative.
Ralph Anderson
[email protected]
Temperature Measurement Compensation
In the responses to the compensation question submitted by Mr. Nahan in the May issue ("Ask the Experts," www.controlglobal.com/articles/2011/weather-compensation-transmission-standardization.html), the methods of compensation were not addressed.
Consider the case where we are using a platinum 100-Ω RTD at about 200 Ω (360 ºC) to measure temperature with a Wheatstone bridge ratio of 3:1; i.e., 200 Ω to 600 Ω, and in two-wire configuration.
If we assume that we have field connection wiring such that the resistance of each leg is 2 Ω, then we have effectively added 4 Ω to the 200 Ω value of the RTD, thus creating a +4-Ω (about 10.2 ºC) error.
We could probably make a zero adjustment error, but, since the resistance of the leads will vary with ambient temperatures, that is not really very satisfactory. What we could do is use a third cable core to the arrangement.
What we have now accomplished is to add equalization where 1:1--200 Ω. But in our case, with a 3:1 bridge ratio, what we have, in fact done, is to change the ratio from 600:200; i.e., 3:1 to 602 to 602:202, which is approximately equal to 2.98:1. We have reduced the error from about 10.2 to something very much less than that, depending on the full-scale measurement span. That would probably be acceptable in most cases. Where it was not, we could revert to four-wire measurement, not simply add a fourth wire, as was suggested.
In such a measurement configuration, a bridge is not used. A precisely controlled fixed value current is simply passed through the RTD, and the voltage drop (pd) across it is measured at the remote (sensing) location by the second pair of wires.
Since any pd in the pair would be unacceptable, the method requires use of high-input-impedance amplifiers, so the current in the two sensing wires may be regarded as effectively zero.
W. Brown, c. Eng Miet
Katy, Texas
