Website service announcement

The online community is now in a ‘read only’ mode whilst we transfer our forum and blog content to their new home in IET EngXTM 

Update: The launch of our new online community IET EngX is taking a little longer than anticipated!

We want to make sure that everything is tidy in the new community  so Engineering Communities will remain in read-only mode until w/c 6th December.

Read our latest blog post for more information. 

Log in to the online community

Want to post a reply? You'll need to log in  Having trouble logging in? Read our help guide
4-20mA signal magnitude comparator and alarm
Rob Eagle
170 Posts

I have a safety system with 3 independent fan sets operating in a duty/standby/back-up configuration, each fan set has an associated pressure differential transmitters (PDT), the PDT control the fan speed to maintain a depression in a common area.

The PDTs also collectively initiate a duty/standby/back-up switching operation on detection of excessive depression based on a 2oo3 voted decision. 

For diagnostic purposes I want to compare the output of each of these PDTs with each other and raise an alarm if one is out of whack with respect to the other two, whether it be reading higher or lower, by a predetermined margin.

So basically I am looking for a 4-20mA signal magnitude comparator with trip function, anyone know of a suitable device to do this, ideally able to support a SIL 2 safety function?

7 Replies

Amelec? Used to use these decades ago, very reliable. ADT 132 range from memory. They call them "trip amplifiers", or have I got this all wrong?

Rob Eagle
170 Posts

Thanks Roger, I am wanting a trip amplifier that trips on comparative value between two 4-20mA signals rather than an absolute value on one.

I think the ADT139 does that. It has been at least 35 years since I played with one though!

Rob Eagle
170 Posts

Wow!  That's the one, thank you Roger.

Roger Bryant
378 Posts

There is also this from E+H:




Attached files
Rob Eagle
170 Posts

Thank you Roger that's a good one to know too.

Log in

Want to post a reply? You'll need to log in  Having trouble logging in? Read our help guide