In addition, the junction of any thermocouple is an inherent AC rectifier that is out of control by the designer. Any impedance unbalance between the two thermocouple wires (including the PCB and OPAMP) will cause current to flow in the thermocouple junction and give rise to (large errors) I personally found error from 120 degrees Kelvin at room temperature using thermocouple meters for the process industry. This inherent sensitivity of thermocouples disqualifies most designs for reliability purposes.
Gert Gremmen ce-test, qualified testing http://www.cetest.nl -----Original Message----- From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of cherryclo...@aol.com Sent: maandag 7 januari 2002 12:49 To: ken.ja...@emccompliance.com Cc: emc-p...@majordomo.ieee.org Subject: Re: EMC-related safety issues Sorry everyone! When I replied yesterday to Ken's posting I didn't spot an error he had made. He had assumed an incubator compliant to 1V/m close to a laptop, whereas the question I originally posed concerned an incubator such as the one I had tested that had full-scale temperature errors at 1V/m from 30 to 1000MHz. So I hope you'll all forgive me if I redo part of my reply. Here goes... Ken said (06/01/02 06:56:46 GMT Standard Time)... Would I feel comfortable placing a CISPR compliant PC next to a medical device qualified to 1 V/m? There is an inherent (not planned) margin of safety here that is many orders of magnitude. The answer is absolutely yes. If there were a problem, I would expect it more to occur below 30 MHz, at the power supply switching frequency, IF the medical device processed extremely low levels of electrical signals and was poorly shielded. But I believe there are separate immunity requirements which cover this eventuality as well. Firstly – my original question concerned how close one would be prepared to place a fully-compliant laptop to the unmodified incubator, which as you will recall I found to give full-scale temperature errors at 1V/m field strengths from 30 to 1000MHz. An incubator that was qualified to 1V/m (as per Ken's reply above) would be at least 28dB less sensitive to RF fields (assuming a square-law relationship for error voltage versus field strength) and would be much more robust. In the EU such medical devices are expected to work properly in fields of at least 3V/m (you can't rely on the CE mark as any guarantee), but I posed the question about the unmodified incubator because I understand that outside of the EU few countries have mandatory immunity regulations. Note that thermocouples have an output of between 3 and 50 uV/degreeC, so if you want to achieve ±0.1C accuracy you are looking to keep error voltages below 0.3 to 5uV. It doesn't take much RF ingress to cause that level of error. Note also that traditional thermocouple amplifier design (such as commonly seen in the 1960s and 1970s, and still lingering on in some products) brings the thermocouple wires straight into an opamp. No shielding, no filtering, and no CMR in the opamp at RF. Worst-case RF demodulation performance is almost a certainty with such a design. Many other 'traditional' transducers using microvolt signal levels used to use amplifiers designed just as badly for EMC, and I sincerely hope there are none of them left any more. Aside: A typical comment from a UK EMC test lab manager (this one from a personal communication in 1998): "I was testing a temperature control system for immunity yesterday. As usual, I found that I could get any temperature I wanted simply by varying the RF frequency." Just so you don't think my incubator example was a one-off. Secondly – maybe when you wrote the above you weren't thinking of the previous correspondence in this thread about the proximity of the low-energy lamp to a bedside radio. Yes, I know, this concerned a radio receiver, what I mean to draw your attention to is the discussion about the intention and validity of the EMC standards – they simply do not cover situations where devices are placed close to each other – so they cannot be relied upon to provide compatibility in such situations. Military EMC standards are more thorough in this respect. And as I have already said, commercial EMC standards were not written with safety issues in mind, and most safety standards have not been written with EMC-related issues in mind (see my IEEE 2001 EMC Symposium paper and my longer article in ITEM UPDATE 2001 for details). Regards again, Keith Armstrong In a message dated 06/01/02 15:51:27 GMT Standard Time, cherryclo...@aol.com writes: Subj:Re: EMC-related safety issues Date:06/01/02 15:51:27 GMT Standard Time From: cherryclo...@aol.com Sender: owner-emc-p...@majordomo.ieee.org Reply-to: cherryclo...@aol.com To: ken.ja...@emccompliance.com CC: emc-p...@majordomo.ieee.org Ken, replies below. Regards, Keith Armstrong In a message dated 06/01/02 06:56:46 GMT Standard Time, ken.ja...@emccompliance.com writes: Subj:Re: EMC-related safety issues Date:06/01/02 06:56:46 GMT Standard Time From: ken.ja...@emccompliance.com (Ken Javor) To: cherryclo...@aol.com, cortland.richm...@alcatel.com CC: emc-p...@majordomo.ieee.org What an EMC engineer who understands the physics of field-to-wire coupling would say is that the operation of non-antenna connected electronics associated with one subsystem will not be degraded by close proximity with the non-antenna connected electronics of another subsystem. Forget 10 meters. Are the PCs in your office separated by 10 m? Would you expect two PCs stacked side-by-side or one on top of the other to interact in any manner? These are rhetorical questions. I don't see the relevance of this paragraph to my example. This might be because I am a little slow. I would appreciate more explanation. About the blood pressure monitor example. Not enough info here to back out what is wrong, but basic logic theory says when the conclusion is impossible, you must re-examine your assumptions. If 92 dBuV/m were enough to make the device malfunction, it would malfunction a lot and there would have been enough trouble reports to get it fixed or withdrawn. And 92 dBuV/m at 10 meters is SCREAMING!!! I am on location and don't have FCC regs easily available, but the limits stair-step around 40 dBuV/m at 3 m, per my recollection. This is exactly why I emphasised that the front-panel display of the blood sample incubator (not a blood pressure monitor) continued to read the set-point temperature (37.1C) even though the actual temperature could be way off. I am confident that in actual operation the incubator temperature was quite often at least a few degrees C different from what was displayed on its front panel, probably affecting the performance of the reagents. And I don't think that 92dBuV/m is a high field strength to be emitted by a PC placed nearby, or for a non-compliant laptop at 10 metres. Would I feel comfortable placing a CISPR compliant PC next to a medical device qualified to 1 V/m? There is an inherent (not planned) margin of safety here that is many orders of magnitude. The answer is absolutely yes. If there were a problem, I would expect it more to occur below 30 MHz, at the power supply switching frequency, IF the medical device processed extremely low levels of electrical signals and was poorly shielded. But I believe there are separate immunity requirements which cover this eventuality as well. Maybe when you wrote the above you weren't thinking of the previous correspondence in this thread about the proximity of the low-energy lamp to a bedside radio. Yes, I know, this concerned a radio receiver, what I mean to draw your attention to is the discussion about the intention and validity of the EMC standards – they simply do not cover situations where devices are placed close to each other – so they cannot be relied upon to provide compatibility in such situations. Military EMC standards are more thorough in this respect. And as I have already said, commercial EMC standards were not written with safety issues in mind, and most safety standards have not been written with EMC-related issues in mind (see my IEEE 2001 EMC Symposium paper and my longer article in ITEM UPDATE 2001 for details). So I cannot see that there is any 'inherent' margin of safety in the above situation, as you claim there "absolutely is". (And may I suggest that anyone who thinks that the statement in all (or most) IEC EMC immunity standards: "Products shall not become unsafe as a result of these tests." means that that products which pass those immunity tests are necessarily free from EMC-related safety problems, needs to think a little bit harder about the subject?) Regards again, Keith Armstrong on 1/5/02 12:23 PM, cherryclo...@aol.com at cherryclo...@aol.com wrote: Dear Cortland People can't simply say: "ordinary semiconductors won't demodulate RF levels produced by an unintentional radiator" ­ even the smallest amount of RF can be demodulated ­ there are no hysteresis or threshold effects in a PN semiconductor junction or FET that is biased into its conduction region (at least not until you get below signal levels equivalent to less than a single electron). What I am sure most engineers would really mean to say is: "ordinary semiconductors exposed to RF levels from an information technology product which is fully compliant with all relevant EMC emissions standards and is at 10 metres distance will generally not demodulate a sufficient level of interference to make an appreciable difference to most electronic systems." Now we have a statement which has some scientific rigor and some engineering validity to it. (Although I do worry that in Europe our harmonised EMC standards only test emissions up to 1GHz, so what does that say about the possible emitted fields strengths from a PC with a 1.2GHz clock frequency?) Let's see if we can put some meat into this discussion with a real-life example... I once tested a blood sample incubator for RF field immunity. The incubator was used during screening programs (for cancer and other diseases) and kept about 100 test tubes at 37.1C (normal blood temperature), while the reagents in the test tubes changed colour. After 24 hours of incubation medical staff would inspect the test tubes and write letters to people telling them they were sick, or that they were clear of the disease. I don't know what temperature tolerance the reagents had to give an accurate medical diagnosis, so assume ±0.1C. On the front panel of the incubator was a display of its temperature, which was of course 37.1C. We found that field strengths as low as 1V/m would cause the incubation temperature to range over full scale, from heaters fully off (in which case the temperature would decline to ambient) to maximum (in which case the water used to incubate the test tubes would boil). We could use the RF test frequency to control the temperature between plus and minus full scale over the frequency range 80 to 1000MHz at 1V/m (and did not test beyond 1GHz). Most worryingly, the front panel display would only show temporary variations from its 37.1C when the RF field was turned off or on, and would continue to show 37.1C even when the water in the incubator was stone cold or actually boiling. Most demodulation effects in bipolar and FET devices approximate to a square law - for example a 1dB fall in the field strength (keeping everything else constant) would typically result in a 2dB fall in the demodulated 'interference' error signal, as John Woodgate has recently pointed out. If we assume that the 1V/m field strength was causing a 60C temperature error, how low would we need to make the RF field to get down to the 0.1C accuracy of the front panel display? Assuming square-law characteristics for the device doing the demodulation I calculate a field strength of around 40mV/m or 92dBmicrovolts/metre. You will notice that I have been generous to the incubator and assumed that the 1V/m field just about caused its temperature error to increase by 60C to boil the water, whereas it could have been overdriving the internal circuits by a considerable margin and still suffered a 60C error at 0.1V/m. We didn't test this possibility as our focus was (as in most of these cases) on quickly modifying the product so it passed the immunity test - which we did.. 92dBmicrovolts/metre is not a very high RF field level for a PC without any EMC precautions at a distance of 10 metres. How many people reading this would be now be quite happy to place even a fully-compliant PC (compliant at 10 metres distance, that is) right next to the unmodified incubator? If it helps, imagine that it is your young daughter whose blood sample is in the incubator to discover which drugs she needs to survive. Shall we have a vote on how close we would be prepared to place the PC? Might be interesting. Let's not even think about the problems of proximity to cellphones and other intentional radiators. I didn't mention that the incubator was a small model used for mobile screening, for installation in a truck adapted for medical screening purposes which travels to various communities and parks there for a few days while it tests the local people for disease - hardly a very well controlled electromagnetic environment. What does the above imply for similar incubators in countries that do not have mandatory EMC immunity standards? Or for older incubators in the EU that have never had to meet the EMC directive? (Please don't reply with the old chestnut that "we haven't heard of any problems so far, so everything must be OK" - people who should have known better were using that phrase before September 11th. It is just not an acceptable argument where safety issues are involved, as any expert in safety law will tell you. Try: "I've been driving past that school at 40mph for ten years and haven't hit a kid yet, so it must be safe mustn't it?" as a test of the concept.) Regards, Keith Armstrong
