Anyone know where I can get a pressure differential transmitter with 4-20mA output for a safety application that is classed as a Type A device as defined in BS EN 61508?
I am not sure you can get Type A Differential Transmitters adding to the fact that they are generally SMART and/or a complex instruments. Simple components like relays, switches, valves, etc. are normally Type A. What is important in a Safety System is the SIL level you are trying to achieve. Both Type A and B can achieve SIL 4 in theory but I would be surprised a differential transmitter is specified within a SIL 4 Safety System. I have never known of one. Yokogawa offer Differential Transmitters that can achieve SIL 2 or 3 and it is the type we have used in our safety systems that have required SIL 2. The Yokogawa.web page has a number of reports explaining this and they are very useful. It is also covered in IEC61508 part 2 section 7.4. It also defines what Type A and Type B components (elements) are. There are tables for Type a and Type B components and the properties required to achieve the SIL levels. The properties being hardware failure tolerance and Safe failure fraction.
Unfortunately this is for a nuclear application and any SMART device has to undergo an EMPHASIS assessment for use in a nuclear safety application, this is an extremely detailed assessment and cost a lot of money and time. Furthermore, even with EMPHASIS certification, which some devices have i.e. Siemens DSIII, the devices cannot be used in a redundant configuration to support a single safety function due to the nature of common cause failure in the software.
I am trying to specify a 2oo3 system to support a single safety function as such I need to know if I can get Type A devices, if not then a completely different approach will be needed, which is looking the way it is going to have to go.
Snap I also work in the Nuclear industry so I understand where you are coming from with Emphasis testing and the associated costs and the challenge you have. Just a suggestion but I am sure the Yokogawa's have been substantiated (Emphasis checked) within my company so if this is combined with the Siemens DSIII could this help in avoiding the common mode failure in the 2oo3 system i.e diverse technology. I assume this being driven by the Safety Case.
Otherwise a different approach. If it's for flow measurements then consider other diverse technologies such as Ultra Sonic or electro-magnetic Flow measurement.
Sorry probably not much use not understanding your specific requirement.
I have actually, at last, found a Type A pressure transmitter with independent SIL certification, the DMP331 from BD I Sensors GmbH, they also do the DMD331 a pressure differential transmitter, it looks almost identical to the DMP331 but without the independent SIL certification, so I assume that this could be developed further. We could do with more than just one option though.
Maybe in time as confidence in SMART devices develops we may find a relaxing of the rules.
