Bad current reading using NI 9203 with NI 9172

[EspenSH]

I have trouble with the current (4-20 mA) reading from a pressure transducer. The output from the pressure transducer is connected to the ports of a NI 9203, and this unit is further connected into the NI 9172 unit.

Using a multimeter, I measure the output from the pressure transducer to 3.9 mA at "0" pressure and about 20 mA at maximum pressure. The pressure transducer seems to be working fine.

By using the DAQassistant or the Measurement and Automation Explorer I get different current readings. At "0" pressure it gives a VERY noisy reading between about 0 to 4.2 mA (it should be stable at around 4 mA). At maximum pressure the current reading is stable at around 5 to 6 mA (whereas it should give around 20 mA signal). What am I doing wrong?

I have tried to use all of the 8 different slots in the NI 9172, but they all give the same reading. The equipment is brand new.

Thank you,

Espen

[EspenSH]

To give some more information about my problem;

The pressure transducer has 3 connection points on its backside. Let's call them connection point (1), (2) and (3). To connection point (1) and (2) are a ~30V power supply connected. To connection point (2) and (3) are the output cables connected. With these output cables a 4-20 mA signal can be read with a standard multimeter. I am still not able to get any reading using the NI 9203.

It says in the manual of the pressure transducer that only an ammeter whose internal resistance less than 10 ohms should be used. Can this be the source of my problem?

[EspenSH]

I have tried something additional, which again did NOT solve my problem. I had a NI 9205 (measures voltage) laying around, and I connected the 4-20 mA signal from the pressure transducer to the NI 9205 with a 470 ohm resistor in parallell. I have done this many times before, and never had a problem. Again, the same type of reading appeared as when I was using the NI 9203 (measures current).

Anyone has any ideas?I

[KyleBryson]

---

@EspenSH wrote:

It says in the manual of the pressure transducer that only an ammeter whose internal resistance less than 10 ohms should be used. Can this be the source of my problem?

---

Hi EspenSH,

That was my first thought.  The 9203 specs list an input impedance of 138 Ohms, and your 9205 test with 470 Ohms in parallel with a large impedance would both seem to exceed the transducer's recommendation by an order of magnitude.  This larger resistance would tend to prevent a sensor from reaching it's highest output level, though it's impossible to know how your transducer reacts outside its specified operating conditions.

You could test using the 9205 with a smaller resistor, or using the 9205 to measure across the current terminals on your multimeter (essentially using its ammeter function as an external shunt resistor).

Regards,
Kyle

[EspenSH]

I have managed to get some sort of reading using the NI 9203. The mA signal from the pressure transducer should vary between 4-20 mA. What I read in Labview or MAX is a signal from 3.2mA - about 7mA.

Some further information: the output potential from the pressure transducer at 4 mA is around 0.66V, at ~12 mA around 0.72V, and at ~20 mA 0.78V. By measuring the current with the multimeter only gives a nice 4-20 mA reading.

I tried to connect a 9205 unit in parallell with both a 470 ohm resistor as described above, and also resistors of ~4 ohms, ~ 10 ohms and ~20 ohms, but without any more success.

I would really like to see the 4-20 mA signal in Labview, since the pressure transducer is calibrated from manufactor with respect to the 4-20 mA. (I need to measure accurate pressures). If it is the case that the internal resistor in 9203 is too large (manual of the pressure transducer indicated using an ammeter with max 10 ohms) what can I do in order to still use the 9203 and get a 4-20 mA signal in Labview?

[KyleBryson]

Hi Espen,

Under what conditions were you measuring those output potentials from the sensor?

I would expect the 9205 with 4 Ohm or 10 Ohm to provide improved results over the 470 Ohm.  What measurements do you get from the 9205?  Can they be linearly scaled to the desired range?

I'm not sure that the sensor will be able to directly interface to the 9203, since the specifications seem incompatible.  Perhaps someone else will suggest another idea, but it seems like the external shunt resistor and the 9205 ought to be your best bet.

Regards,
Kyle

[EspenSH]

First of all, the problem is now solved.

The pressure transducer has 3 connection points. Let's call them connection point 1, 2, and 3. The power supply (at 24V) is connected to point 1 and 2. In the manual a ammeter can be connected to point 2 and 3 and should read 4-20 mA.

After quite some frustration I called the supplier of the transducer and a service engineer told me to connect an ammeter (or NI 9203) is series with the power supply giving 24 V. The transducer is working as a "resistor". Following from this; 4 mA is running from the power supply at 24 V at "zero" pressure and 20 mA is running from the power supply at 24 V at 100% pressure. By doing this, I aquire good readings using NI 9203 and Labview.

Connection point 2 and 3 provides a signal of 4 - 20 mA at a potential of 0.6 to 0.8 V. To example my problem; 0.8 V is not enough potential to drive 20 mA through a 469 ohm resistor or any other resistor above ~40ohms. According to Ohms law; R = U / I = 0.8 V / 20e-3 mA = 40 ohms. Using a shunt resistor of 469 ohms would give a current of I = U / R = 0.8 V / 469 ohms = 0.0017 A = 1.7 mA. In other words, the potential between connection point 2 and 3 was too low in order to use any sort of DAQ or for that matter the NI 9203 (it has a built in shunt resistor of higher than 40 ohms).

According to the service engineer I talked to connection point 2 and 3 should only be used by e.g. a standard ammeter (whos internal shunt resistor is probably very small) in order to check if the transducer is "working". The manual of transducer is very confusing regarding these connection points, and describing an ammeter connected to point 2 and 3 as a "check meter" and "external indicator"

Thanks for all replies,

Espen

[Seb14]

Hello,

I have issue to measure some current with a transducer Keller. Whatevver the pressure applied, the currant is very very low.

I have tested :

1- direct connection to the NI9203 (current 1.10e-7)

2- connection on shunt resistance of the the NI9203 0,0018A

I think understand that the transducer and NI9203 have to be wired in serie with the 24V. Is it correct?

 the transducer is conected between 1 &3.

Thanks for your help.

Sebastien

[Seb14]

nota: my DAQ is cDAQ 9185