Nova gas analyzer 7900p series 4-20mA connection

Thread Starter

Rubayet

Joined May 15, 2023
4
I have to use a gas analyzer of nova, it has 4-20mA output. I used a data acquisition system with 250 ohm resistor. But the data eventually it shows not more than 50 percent in the laptop, data over 50% is cut. But in the display, it shows more than 50%. How can I fix it so that it will show all data?
 

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MrChips

Joined Oct 2, 2009
30,986
Can you post the actual Nova model number and serial number?
What are the gases and concentration ranges the unit can measure?

Try using a 100Ω resistor instead of 250Ω.
 

DickCappels

Joined Aug 21, 2008
10,236
I trust your math implicitly, but not my typing. The thread starter stated 250 Ohms and I used it in the calculation but then went on to type 150 ohms.

I guess the main question is what voltages were the transmitter and receiver designed to deal with, and where did the value of 250 ohms come from?
 

Reloadron

Joined Jan 15, 2015
7,546
Figure it this way. You have your 4 to 20 mA loop. Using a 250 Ohm resistor 4 - 20 mA becomes 1 to 5 volts. Using 500 Ohms it becomes 2 to 10 volts. Your gas analyzer is connected to your A/D (Analog to Digital) converter. What is the converter and please post your code for the A/D. The voltages will relate to actual gas concentration and your sensor will need calibration using a known gas concentration.

Ron
 

Thread Starter

Rubayet

Joined May 15, 2023
4
Figure it this way. You have your 4 to 20 mA loop. Using a 250 Ohm resistor 4 - 20 mA becomes 1 to 5 volts. Using 500 Ohms it becomes 2 to 10 volts. Your gas analyzer is connected to your A/D (Analog to Digital) converter. What is the converter and please post your code for the A/D. The voltages will relate to actual gas concentration and your sensor will need calibration using a known gas concentration.

Ron
I am using dataq instument di-2008 for data logging
 

MrChips

Joined Oct 2, 2009
30,986
Thanks for posting the Nova specification data.
As far as I am aware, all Nova gas analyzers are custom designed.
The spec sheet calls for 4-20mA output for 0-50% F.S. gas concentrations on all gases.
This has been programmed into the unit. There is nothing you can do to change this.

If you need 4-20mA output for 0-100% then you have to order another unit or you can ask Nova for the modifications.
 

Thread Starter

Rubayet

Joined May 15, 2023
4
Thanks for posting the Nova specification data.
As far as I am aware, all Nova gas analyzers are custom designed.
The spec sheet calls for 4-20mA output for 0-50% F.S. gas concentrations on all gases.
This has been programmed into the unit. There is nothing you can do to change this.

If you need 4-20mA output for 0-100% then you have to order another unit or you can ask Nova for the modifications.
but in the built in screen it shows more than 50%
 

MrChips

Joined Oct 2, 2009
30,986
but in the built in screen it shows more than 50%
I understand.

The display shows the gas concentration as calculated from the sensor after zero and gain correction has been applied.

Digital information is sent to the 4-20mA output DAC circuit. It is capped at 50% full scale because that is what was requested in the customer order. You cannot change this without reprogramming the unit.

If you wanted 4-20mA to span 0-100% you would have had to specify this in the work order.
 

Reloadron

Joined Jan 15, 2015
7,546
I am using dataq instument di-2008 for data logging
OK, I have used quite a bit of the DATAQ stuff including their commercial stuff and their starter kits. Dataq also makes available SDK (Software Development Kits) but just using the basic Dataq included software your DI-2008 module has programmable gain of
  • 10 mV to 50 V in 12 steps
Analog Inputs The DI-2008 features eight isolated differential channel inputs located on the sixteen-position screw terminal blocks for easy connection and operation. Each channel can be programmed to acquire data as Voltage (±10, ±25, ±50, ±100, ±250, ±500mV; ±1, ±2.5, ±5, ±10, ±25, ±50 Volts)

Your sensor assuming 4 to 20 mA using a 500 Ohm load resistor will give you 2 to 10 volts so I would set up that channel on the A/D for +/- 10 volts. You should have the WINDAQ software. You can setup and calibrate using WINDAQ. How is your DI-2008 setup? Can you monitor your voltage in? Data logging is easy using WINDAQ and you can define all your limits which is nice on the charts.

I also agree with MrChips as to your sensor module. What are you using for a gas sample? What voltage do you see across a 500 ohm load? That or less the A/D measure your current loop with a good DMM. Exactly how are things setup?

Ron
 

MrChips

Joined Oct 2, 2009
30,986
It doesn't work like that. The Nova 7900 series gas analyzer is more than just a sensor. It is a multi-gas multi-sensor system that requires zero and gain calibration for all gases. It also compensates for cross-interference when one specific gas sensor would be in error when other gases are present.

In this case, the gases measured are CO, CO2, CH4, and H2. I suspect that CO2 is measured by IR spectroscopy while H2 is measured by thermal conductivity. CO and CH4 could be measured by chemical sensors but I am not certain about that.

The voltage output to the 4-20mA DAC has already been capped at 50% gas concentration because that is what the customer ordered.
 

MrChips

Joined Oct 2, 2009
30,986
I just noticed the functional block diagram of the Nova gas analyzer.
H2 is measured by a thermal conductivity cell.
CO/CO2/CH4 are measured by IR detector with 0-5V analog outputs.
You could tap off the analog outputs and get raw data from the IR detector.

All four LCD displays are driven by a common 2-wire RS-485 bus.
You could capture the serial data and interrogate the data. You would need an MCU and a bit of programming to do this.
 
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