Low Cost Loop Powered Process Meter

wwsmith

Member
Join Date
May 2010
Location
College Station, TX
Posts
18
I want to use an analog 4-20ma output sent to a loop powered digital process meter to display the temperature set point for an HVAC zone. This set point temp will only be allowed to range from about 50F to 85F so as to accommodate night set back temperatures and other non-occupied allowed indoor temperatures. Decimals are not needed so even a two digit meter would be fine although 3.5 is much more common and perfectly acceptable.

The lowest cost loop powered process meter I have found is from Omega for $50. See details here http://www.omega.com/ppt/pptsc.asp?ref=DP720041

I need about ten of these and was wondering if anyone knew of a lower cost process meter that would work for my situation. Color of numbers and display size (smaller is better) are not important. Mainly looking for something cheap of reasonable quality.
 
You have to be careful with loop powered indicators. They usually have a diode that creates a voltage drop to power the indicator. This looks like a load to the transmitter, and you can run into problems with excessive load at 4 mA. For example, if the diode is creating a 2.5 VDC voltage this is equivalent to 625 Ohm load. If your transmitter's allowable burden is 750 Ohms and you have a 250 Ohm PLC input in series your transmitter can't send a signal until the signal gets up to 5 mA.
 
Thanks, Tom, for that observation about a diode in a loop powered meter. I was unaware . . .

Dan
 

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