Howto: Use a Linksys Router as Wireless Reef Controller - WRRC

"I also picked up a second pH amplifier off ebay for use with my ORP probe. The hardware's all the same, the only difference between pH and ORP is in the software since there's no crazy conversion needed for ORP as it's just viewed in mV."

I keep looking on ebay for something to talk to my PLC (pH and ORP) and all I can find is huge old lab controllers. Am I not using the right keyword?
 
Good question. Could some of you with experience tell us exactly what sort of thing to look for? Possibly point us to a couple of good auctions. I've tried looking in the past and given up. I just keep picturing myself with a old dirty used piece of equipment from a no name brand that no one has experience with and no one can help me set up.
 
Thanks for the link. Now we're getting somewhere:) I guess I just need to know exactly what I need first. The auction link is stuff I have looked at before, but I want it small, and all in my pretty control panel:D
 
That link to the pH xmit'r with the hole in the button is kind of suspect.... Maybe it's too out of wack for calibration and the factory 'marked' it so. Dunno. GLI is now Hach. They make excellent instruments.
 
<a href=showthread.php?s=&postid=9352725#post9352725 target=_blank>Originally posted</a> by Gordonious
Good question. Could some of you with experience tell us exactly what sort of thing to look for? Possibly point us to a couple of good auctions. I've tried looking in the past and given up. I just keep picturing myself with a old dirty used piece of equipment from a no name brand that no one has experience with and no one can help me set up.
Click to expand...

I'd suggest the Vernier one. It's maybe 2"x3"x1/2" box, and if you felt adventurous you could even make it smaller since it's a tiny board inside .

Brand new they are $40 each + 5 shipping, so not too bad. In the last 6 months I've seen maybe 2 of them on ebay so you may be better off just bitting the bullet and buying them new.

http://www.vernier.com/probes/ea-bta.html

but buy them here:
http://www.schoolmart.com/index.asp?PageAction=VIEWPROD&ProdID=226
 
Being in the Instrumentation and Control field, I can honestly say that I have never encountered this Vernier device. The transmitters that you get on eBay for <$100 typically cost >$1000 when new. The good ones do, any how. They output a 4-20mA signal, which is a good robust signal and can be wired directly to a PLC. The problem that I see with impedence is, if you do not have a good connection somewhere, the marine environment will cause it to corrode and the resistance will increase. This will cause error in your readings.

Jason
 
OK, I've seen them all say 4-20mA output but what the heck does that actually mean to a layman?

Can you expand on how we'd hook that up to the DS2450 to get the probe voltage? Maybe a little schematic?
 
A pH transmitter amplifies and conditions the probe's output, then converts this voltage to a proportional 4-20mA dc-current loop. So the 4-20mA signal represents 0-14pH. This current loop method is pretty much standard in the industrial automation environment. I have no idea how to hook this up to a DS2450
 
IIRC, the DS2450 can accept 0 - 2.5V input.

Throw a high tolerance resistor(s), wired in parallel with you 4-20mA signal on the input of the DS2450 (put resistor as close as possible to DS2450). Make sure your transmitter can drive the impedance required for the resistor.

V=I*R :)
 
<a href=showthread.php?s=&postid=9356570#post9356570 target=_blank>Originally posted</a> by lakee911
IIRC, the DS2450 can accept 0 - 2.5V input.

Throw a high tolerance resistor(s), wired in parallel with you 4-20mA signal on the input of the DS2450 (put resistor as close as possible to DS2450). Make sure your transmitter can drive the impedance required for the resistor.

V=I*R :)
Click to expand...


That's what I did with mine. I used the span adjustment on the pH transmitter to adjust the voltage to be between 1 and 2.4 volts for ph 7 and ph 10. I think I used a 250 ohm resister, or something around that.
 
4-20ma is used within industry as it tolerates an amount of poor connections and allows very long cable runs...if they had used voltage directly from devices the resistance of the cable would degrade the signal...ie 5VDC at one end of a 1 mile cable would probably only be ~4.5VDC at the other. however having the device regulate a current at 20ma through the cable means there is 20ma flowing at all points of the cable...thus placing 250ohm at the end of the cable allows 1-5VDC to be measured closer to the input device.


placing 250 ohms in series with 4-20ma equals 1-5VDC...You will pay heaps for a precision 250 ohm resistor so just get 4 x 1000 ohms 1% and connect them in parallel.

I also placed a 10uf capacitor at the voltage input terminals of the 1wire A/D to help with smoothing.


rob
 
Bad Inferno, you've got the right idea, but I would argue that your 5 VDC would have no voltage after a mile. That's 12560 feet of cable! Even a mile might be too far for the 4-20mA, depending on the wire. The transmitter would have to be able to drive the impedance of the receiving device, plus the resistance of the wire. Honestly, I'd run something on fiber at that length.

Actually, the best way might be to use an RS-232 connection from the transmitter. Save some error in doing A/D and D/A conversions.

Jason
 
In Australia we are big users of foundation fieldbus for digital communications to field devices...although that distance maybe pushing it. I know 4-20ma (with HART) can travel 1300 metres over twisted pair, we are talking DCS systems though...not quite at the level of 1wire A/D.

Now I also looked at the hobby boards HVAC board and it seems to have 1 meg resister in series with each input channel. I know all my A/D channels are noisy on the DS2450...I think its to do with the 8 bit resolution...I have ph, conductivity, temperature and a home made pressure Tx all using 4-20ma and 250ohm as inputs to my DS2450.
 
What indusry are you in, Bad? I'm in the water/waste water industry, consulting engineering though. We don't typically use foundation fieldbus here--at least in the Midwest. We're just starting to use DeviceNet and HART really. We use Ethernet/IP (I always do when I can, I'm formerly an IT person) and ControlNet though. We see Modbus, Profibus, DH+ occasionally, but we don't use them ourselves at my company.

Your 1300 meters is still less than a mile. :)

You have no problem driving those inputs w/ the meg resistor?

Got details on the home made PIT?

Thanks,
Jason
 
<a href=showthread.php?s=&postid=9364290#post9364290 target=_blank>Originally posted</a> by bad inferno

Now I also looked at the hobby boards HVAC board and it seems to have 1 meg resister in series with each input channel. I know all my A/D channels are noisy on the DS2450...I think its to do with the 8 bit resolution...I have ph, conductivity, temperature and a home made pressure Tx all using 4-20ma and 250ohm as inputs to my DS2450.
Click to expand...

The resistors on the HobbyBoards HVAC board are so it can measure the 28 volts used in HVAC equipment.
 
Jason PIT cct is on pg 6 of this thread....Its on n outside wall measuring 40" WG. I do have problems with it being effected by sudden changes in ambient temp..ie sun shining on the box for 1 hr in the morning, it drops by ~3% then recovers. Sensor from ebay, theres also a link in pg 6 or 7.

I'm more involved with oil & Gas, Power Generation & Chemical. Have worked for Yokogawa, Foxboro, Honeywell both engineering and sales. I did one water treatment project in Australia, 12,000 I/O primary treatment plant.

cheers
 
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