PT 100 Measurement
Posted 04 January 2013 - 11:33 AM
Posted 06 January 2013 - 05:47 AM
Posted 06 January 2013 - 10:57 AM
Posted 07 January 2013 - 02:24 PM
The details for the 244R are no longer available I believe, I have not found any myself. However, the intererior workings are very similar to the newer model, and does produce an output in the 4 - 20ma range. This range I normally have no problems with and indeed I have a number of sensors already using it with the U3The temperature transmitter is connected to a three wire PT100 attached to teh appropriate input. Using an 18V power supply witht he positive output to the transmitter , the negative pole of the psi is connected via a small home constructed module as can be found at http://labjack.com/s...s-guide/188.8.131.52, figure 2.4. With this arrnagement I am getting around 1.33 volts at the output across R2. This value is quite consistent and varies with temperature. are there some settings I might have missed somewhere , or is it I am plain unlucky?
I was not able to find more details for the 244R (only found 244ER which appears to have 4-20mA current output not voltage output). Can you provide more details about the electrical specs and connections, or post a link to such information?
How do you have your 244R connected to the U3?
Posted 07 January 2013 - 03:11 PM
The reading from a floating low-voltage analog input such as EIO3 is pretty much meaningless:
Without any connection at all the channel I am using (11), is showing around 2 Volts, although very variable. I have tried other channels and they are all the same. What can I do to eradicate that voltage and measure the 1.33 Volts?
So if you connect the DVM probes to EIO3/GND on the U3, you measure 1.3 volts, but the reading reported by the U3 is varying from 0 to 2.2 volts? Could be that the voltage is actually varying and the slow DVM is giving you an average.
My problem occurs when I connect it to the Labjack U3 in that the voltage measure by teh U3 varies between 0.1 volts to 2.2 volts on a pseudo sine wave pattern ... WHilst measureing teh voltage at the terminals with a voltmeter, around 1.3 volts appears at the terminals.
I don't quite follow this. You don't want the voltage divider, you just want a shunt resistor to measure current:
Using an 18V power supply witht he positive output to the transmitter , the negative pole of the psi is connected via a small home constructed module as can be found at http://labjack.com/s...s-guide/184.108.40.206, figure 2.4.
Posted 09 January 2013 - 04:03 PM
Posted 10 January 2013 - 04:03 PM
Posted 12 January 2013 - 04:42 PM
Posted 16 January 2013 - 02:26 PM
Posted 16 January 2013 - 04:36 PM
Posted 24 January 2013 - 04:19 PM
Posted 25 January 2013 - 08:08 AM
Since you now have the LJTick-CurrentShunt, you should use Figure 6 from the LJTCS datasheet:
Make sure your 18V power supply is isolated. If it is an AC/DC supply, it will typically be isolated if you just use the +/- DC outputs and do not use a ground on the power supply.
Remove all other connections from the U3 except USB. Connect the LJTCS to a low-voltage AIN block such as FIO4/FIO5. Then connect as follows from Figure 6:
PowerSupply+ to Sensor+
Sensor- to INA+
INA- to PowerSupply-
INA- to SGND
Does this work? If it is not working, troubleshoot by putting a DMM in series between Sensor- to INA+ to measure current. Are you not getting the right current, or are you getting the right current but the LJTCS is not producing the right voltage?
If that setup does not work, then disconnect the wires going to the LJTCS that come from Sensor- and PowerSupply-, and instead put a 100 ohm resistor across those wires. You can check if this basic circuit works by using the DMM in series again, and also check by using the DMM to measure the voltage across the 100 ohm resistor.
Posted 30 January 2013 - 05:23 PM
Posted 30 January 2013 - 06:04 PM
Posted 31 January 2013 - 09:09 AM
I don't understand? You have an LJTick-CurrentShunt connected to FIO6/FIO7, correct? If you wanted to test without a signal, you could remove connetions from the LJTCS and instead jumper INA+ and INA- both to GND. This would result in something close to 0 volts, but is not a very useful test.
I was getting the varying voltage even with only the connection to FI06, and no earth connection.
A better test is look at Figure 6 from the LJTCS datasheet, but replace the sensor with a 1k resistor. That means you should have a current of 18mA in the loop, which means the output of the LJTCS should produce 2.12 volts.
Posted 31 January 2013 - 09:56 AM
Posted 31 January 2013 - 02:01 PM
The following all result in FIO6 floating:
- Nothing connected to FIO6.
- A wire hanging out of FIO6.
- A signal connected to FIO6 that is not referenced to GND in any way.
When FIO6 is floating, the voltage is undefined, so a reading is meaningless. See the following:
Back to your testing. If you have a 2-wire current loop sensor, and are using the LJTCS, you need to use Figure 6 from the LJTCS datasheet. If you are not using the LJTCS, then you need to add a load resistor and use Figure 2.6-4 from Section 220.127.116.11 of the U3 User's Guide (just use 0 ohms from the bottom of the resistor to SGND, which means a jumper wire).
Either way, a good general troubleshooting technique is to replace the sensor with a 1k resistor to get a simple fixed current.
Posted 18 February 2013 - 07:41 AM
0 user(s) are reading this topic
0 members, 0 guests, 0 anonymous users