Huge Fluctuations in measurement. I have connected strain Gage and full bridge load cell for measurement.

noticing very high fluctuation in the measurement. If I touch the DAQ body, strain values and all other sensor values whatever connected in DAQ is varying heavily. I have done the ground connection also. still variations in strain measurements is very high.

It is not because of temperature effect on completion resistor. because I am seeing fluctuations even in fully bridge sensors like load cell. PPM values varying between 2000 - 10000.

Hi senthilkumaranms,

What is the number of wire in your gauge? 2 wire gauge will give more fluctuation because of temperature, so you must use 3 wire bridge instead.

Also the quality of completion resistor play important part in Wheatstone configuration, you must use high accuracy resistor with low temperature coefficient. So external temperature will have a little impact to the reading.

Using voltage bridge (BGV) and current bridge (BGI) will also give accuracy difference, BGV has 7 ppm accuracy while BGI has 37 ppm accuracy. However if you have long wire for the bridge, BGI will give advantage since there is no voltage drop on the wire.

Best regards,
Rudy Gunawan

I am trying to attach a new load cell to the datalogger.

Load Cell type and details can be found here: http://www.adiartech.com/productdetail.aspx?pid=105&cid=1

Its a simple compression type load cell of 10T capacity with a sensitivity of 2mV/V, with four wires.

I used the current bridge - BGI - 4W as I have a long lead wire.

As mentioned on the load cell and in the calibration report from the manufacturer:

I/P+ : Green
I/P- : Black
O/P+ : White
O/P- : Red

I have connected the load cell to DT-85 analog port number 16 as follows:

Green : *
Black : #
White : +
Red : -

The program running is :

Job Program - CONFIG
BEGIN"config"
' Generated by dEX Configuration Builder Version 1.29.1924 (Firmware Version 9.08.3932)
' Target device: DT85-2
password=""

RS1S
RA"Schedule_1"("b:",ALARMS:OV:100KB:W60,DATA:OV:2KB)
LOGOFFA
16BGI(II,4W,"Load_Cell_current~ppm",LM,NA,350)
END

The output in ppm without load is around : 6.8 ppm as of this writing, and is varying around. Also, there is no deterministic variation in the reading when I put some test weights in the range of few kgs.

What am I doing wrong here ?

Hi chintanp,

It is normal to have an offset for load cell.
In this case you need to perform re-zeroing before the reading:

BEGIN"config"
' Generated by dEX Configuration Builder Version 1.30.1949 (Firmware Version 9.10.4890)
' Target device: DT85G-3
password=""

16BGI(II,4W,"Loadcell_offset~ppm",=1CV,LM,NA,350)
RS1S
RA"Schedule_1"("b:",ALARMS:OV:100KB:W60,DATA:OV:1MB)5S
LOGONA
16BGI(II,4W,"Loadcell_raw~ppm",=2CV,LM,NA,350)
CALC("Loadcell_final~",LM)=2CV-1CV
END

The first reading is capturing zero reading which is saved into 1CV
The second reading is a raw reading, no offset is applied.
The calculation channel is the final reading.

Best regards,
Rudy Gunawan

I can understand the zero error and the need to offset it.

What I cant understand is the fluctuation in the value. I havent changed the program, only enabled logging :

BEGIN"config"
' Generated by dEX Configuration Builder Version 1.29.1924 (Firmware Version 9.08.3932)
' Target device: DT85-2
password=""

RS1S
RA"Schedule_1"("b:",ALARMS:OV:100KB:W60,DATA:OV:2MB)5S
LOGONA
16BGI(II,4W,"Load_Cell_current~ppm",LM,NA,350)
END

Pls view the files attached that show few readings for no_load case and 2kg_load. As can be seen, the values are everywhere. For no load values in ppm var from 3.7 to 6.2 and with 2kg they vary from 3.3 to 4.2, though they vary with every reading in any direction.

The connections are good, and the datalogger is on battery. What could be causing these fluctuations ?

Hi chintanp,

That is also normal, bridge measurement is detecting ratio of output and input in order of millionth.
The reading fluctuation is caused by electrical noise and external temperature effect, below 10 ppm is considered as normal.

Best regards,
Rudy Gunawan

It may happens some times you should use load cells which are approved by NTEP In measurement. If you use NTEP approved load cells you can get more accurate results as compare to others .

Good morning aparkfind,

Who are NTEP? I've made my own load cells and used many others and this is about what I would expect.

Look at the variation in the data, it's only 3 Parts Per Million. This translates to an error of about 0.1% which is the standard error for load cells.

Cheers,
Roger

Hi aparkfind,

3 ppm fluctuation (0.103% accuracy) is within NTEP (National Type Evaluation Program - US certification body for weight) class III and class IIIL classification (0.14% accuracy).

Best regards,
Rudy Gunawan

Thanks for showing this valuable info!