Temperature sensor analog input

[MGuenette]

Hi,

I need to read the floor temperature of you refrigerated warehouse.
I would like to know if it's possible to send a 4-20mA current over 250' without loosing the signal.

My supplier sugests me the T775 series 2000 from Honeywell

thanks
Martin.

 

[BTalbot]

How bout wireless? Most manufactures offer a temp/humidity sensor (battery powered). That way you can easily "add-on" to your system.
Banner has 1 and I'm sure others will chime in with suggestions.

 

[Ken Moore]

Should be able to do that with most "industrial" grade transmitters.
I have several close to 1000' away. You need to check on the instrument specs. see what the maximum impedance is, then calculate you line resistance, and then add the impedance of your input module.
Worse case you may need to use slightly larger wire, I used 18 ga. twisted/shielded cable tray rated.


ken

 

[danw]

Honeywell's T775 is a remote electronic thermostat primarily designed for boiler control. It can have two sensor inputs, which Honeywell's own brochure does not define as to the type of sensor.

Rather than the unneeded unknowns and complexity of a boiler controller, why not a conventional 100 ohm RTD with a 2 wire loop powered temperature transmitter?

A 24V dc power supply at the PLC end can power the transmitter on the same pair of wires that the 4-20mA signal comes back on. You can pick the style of RTD you need. Most distributors who sell transmitters will range the output of the transmitter for you to whatever is appropropriate
0 to 50°F?
-50 to 50°F whatever . . . .

The industry standard 4-20mA that comes out of a temperature transmitter will easily go 250 feet. A recent thread on control.com discussed how to get 4-20mA to go 5 miles. 250 feet is cake, with a 24Vdc power supply.
Dan

 

[Mike Dyble]

As a rule of thumb if the resistance of the field cabling of a 4-20mA loop exceeds 100ohms then you need to starting doing some calculations. It may well still work, but you need to do the sums to make sure.

Its basic ohms law stuff.

A 0.5mm2 (roughly 20AWG) wire has a resistance of about 40 ohms per km.

 

[MGuenette]

Thanks guys,

I will take a look at a standard RTD 100 ohms..

thanks
have a good one
Martin.