Re: shift in RTD Reading

Dan Wright wrote:
> 
> Hey all,
> Using an RTD to measure temperature in a fluid flow line.  When a VFD
> gets turned on to start a fan (600V) the temperature reading drops by
> ~4°C.  Using National Instruments Fieldpoint RTD module to read
> temperature. It is my understanding that noise that causes an offset
> like this has a DC component.  Is this true?  If so, Why (or how)would a
> variable frequency drive cause DC level noise?  Any ideas on how to
> remedy this?
> 
> Dan W

I have seen this symptom, not only with temperature transmitters, but
pressure and other transmitters, also.  It was very scary to see
almost every measured signal in the power house shift somewhat the
first time we turned on the first PWM drive in the old power house. 
Apparently the sensitive input circuits act as AM radio receivers and
detect the high frequency noise and add some of that DC to the
measured signal.

We ended up adding quite a bit to the price of the drives by including
both line and motor lead filters (transformers and 5% line chokes on
the input and custom made three phase common mode chokes on the T
leads), in order to eliminate or reduce this distortion to tolerable
levels.  But we did experiment with some transmitter filters with fair
success.  We tried things like putting a ferrite bead on both the RTD
cable and the 20 ma line, both at the transmitter.  These are
available in two piece cores with a plastic snap closure, so they are
easy to try.  But wound common mode chokes worked better.  However, in
out situation, it was impractical from a time and labor standpoint to
add a pair of common mode chokes to each of hundreds of transmitters. 
So we spent money on big hardware to stop the noise at the source.  
-- 
John Popelish