The following post in an excerpt from CSE live.
By David Sellers
Sorry for the break in the string; we just changed the software platform that drives the blog and its taken me some time to get everything set up and working with the new system. But hopefully, at this point, I am good to go and can pick up where I left off and keep moving forward.
My previous posts before the transition to the new system had been exploring 4-20 milliamp current loops, including why we use them in the first place , what they are and how they work , and how to interpret the information they provide . In this post, I’ll begin to discuss how you can hook a current loop up to a typcial data logger. I’ll be discussing how I hook them up with the Onset HOBO H8 and U12 families.. But the concepts can be applied to loggers by other manufacturers.
Most loggers accept temperature and ac current inputs directly. Temperature measurement is typically accomplished by measuring a resistance element such as a thermistor in a bridge circuit using very little power. The current transformers (CTs) typically used to measure ac current are actually self-exciting; i.e. the current they are measuring generates the measured signal via induction. Using a current loop with a logger is a bit more challenging because current loops need a power source to drive them.
That’s the down side. The up side is that being able to pick up a 4-20 milliamp signal opens the door to measuring just about anything from flow to pressure to carbon dioxide. You can even “piggy-back” onto an existing 4-20 milliamp signal by simply inserting your load resistor in series with it as I discussed previously . So, if you are in the data logging business for the long haul, as is the case for most commissioning folks and a lot of operations folks, then investing in a DC power supply panel or two can make a lot of sense. And, if you are like most field people, you will enjoy the process of putting one together.