spec transducers

guys

i have to spec some 0-30 psi transducers for a current project. my question is what is the best transducer voltage or currrent and out of the two which are the best in each class ie. 0-5 volt ,1-5 volt ,0-10 volt , -10-+10 volt ,

In this case , what are you connecting the devices to ? that would be the most factor to start with .

A lot depends on what your do and how close the instrument device the you are. If you are close I like to use 0 to 10 volt, but it the device is a fair distance away I like to use 4 to 20 ma. The 0 to 10 volt you do not have to break the loop to check, but it is hard to calbrate over longer distances. The 4-20 ma loop has to be broke to check it, but it is more forgiving over long distances. The 4 to 20 also has the plus of the 4 ma low signal, this help if you get a broken loop for troubleshooting.

Hope this answers your question.

jjsscram

Phil,


I'd definitely opt for the 4-20mA for the same reasons jjsscram mentioned. One way around the "breaking" the circuit issue is to build the circuit with a built-in resistor terminal. A-B makes an off-the-shelf 249-ohm two-tier terminal (two on each side of the resistor, but takes only one terminal position in the block). It is pricey at $11.00 and you could, in the words of Bernie Carlton, roll your own. The benefit is that any time you need to read the current, you measure the voltage drop across the resistor and calculate. All of our new panels with 4-20mA I/O have one knifeswitch terminal and one resistor terminal.

Grover

The A-B part number is 1492-something... (I'm not at work right now).

application

guys i will be terminating theses to an ab 1746-ni8 analog input card . grover1 i have no experience using 4-20 transducers. do you have a drawing of the setup you have running in your panels ? . can anyone give me some more info on the problems troublshooting 4-20 .ie why does the loop have to be broken ? the distance from card to transducer will only be about 35 feet max

phil buchanan said:

can anyone give me some more info on the problems troublshooting 4-20 .ie why does the loop have to be broken ? the distance from card to transducer will only be about 35 feet max

Click to expand...

Basic Electricity 101 ..Voltage is mesured in parallel..Current is mesured in series!

So you have to break the loop to insert your meter..Not that its a big deal..The only time you have to worry about it is when you calibrate and while you are doing that the sensor is realy sending data to anything..

As for wiring..it depends on the sensor you choose..

I also like 4-20ma for troubleshooting..

D

As far as breaking the circuit for checking is concerned most PLC analogue inputs have a 250 ohm resistor so you can measure 1-5V for a 4-20mA circuit. If you are using high end sensors these also have Hart support so you can connect a PC. If you give some thought to the terminal layout in your panel you can arrange things to provide easily accessible test points for your sensors.

Andybr

Most pressure transducers are set up to do 4-20 mA as a loop powered two wire device. A voltage output transducer almost always requires 3 or 4 wires. If you look at the documentation on your analog input card it will provide wiring schematics for a typical two-wire 4-20 mA transmitter.

As far as troubleshooting or calibration i will measure the current output the transmitter is producing then, I will substitute a 4-20mA source set at the same as I measured into the loop and then remove or calibrate the instrumentthe instrument in place. I work at a water plant and cannot take instruments off line so that's how I do it.
hope that helps
shierj

For anyone who is interested, the 249-ohm resistor terminal block that I mentioned earlier is A-B part number 1492-WD4RC001.


Grover

We use terminal(s) with a manual disconnect in the middle. Just place your probes on top & bottom of the terminal, pull the disconnect and you have your current reading without disrupting the system. Works great.