Hi I need to connect strain gages to DT 800. Using the configuration presented at fig. 125 of the user manual (4 wire BGV input in one channel) it was impossible to balance the bridge. The command RA1S 1BGV(4W) did not result on a stable 5V excitation. In fact, voltage did oscillate between 0 and 3,33 V.

Anything ideas? Thanks in advance, VM John

Hello John,

If you look at the default settings for the bridge channel type the excitation voltage is 3500 mV. How are you measuring the excitation voltage? As the output of the bridge is ratiometric and is in PPM any variation in the power supply cancels out (PPM =(output/input)*10^6).

Have you checked for ground leaks in your setup?
For this you will need a strain gauge test unit, this will also tell you how well the bridge is balanced and check the continuity.

You should also be looking at ground loops in the system.

Roger

Hi,

I have a similar problem of vmjohn. I am trying to connect strain gages to DT800 using the command RA3S 1BGI(4W,120) in the software DTransfer. The results from DT800 are unstable. However, when using DT600 the results are stable and agree to what it has been predicted. I would like to know how to proceed since I have to use both equipments.

Thank you,
Luciana

Maybe can try running DT800 on battery without any PC and look at data after recording.

Also try with PC laptop on battery only no main power.

Sometimes problem seen with noise from PC and laptop power.

I am trying to connect a strain gauge to DT800 using the command RA1S 1BGI (4W, 350). However, I am getting so much noise. I am talking about 10 ppm variation around the mean. Furthermore, I have the same problem in laboratory as well as in a real-world SHM system.
I've tried all the tips above and I am still getting the same problem.
What do you think about it? How can I get rid of it?

Thanks. I am looking forward to hearing from you.

Eloi

Good morning Eloi,

I would start by testing the strain gauge installation;

1/ Get a strain gauge test unit and check the resistance to ground. If you don't have very high resistance to ground then the gauge has been damaged and needs to be replaced. If the gauges the gauge needs to be in a moist environment (Embedded in concrete for example) then check the environmental coating in the same way.

2/ Check the quality of the soldering. This may require using a microscope. Check for dry joints, solder causing nearly causing short circuits or close to edges of the gauge. Your solder joints should be small round pillows. Also make sure you have removed all solder flux from the joints.

3/ Check your wiring. Make sure all ends of leads are fully enclosed in the solder. Where possible use shielded twisted pair cables and only ground the shielding at one end. The end that isn't grounded check the shield is covered by the insulation and not touching or close to the other surface. I go to the extent trimming the shield cable as close to the insulation as possible the of very lightly stretching the cable insulation just enough to cover any exposed shield. Then apply the environmental coating to the end of the cable.

4/ Check your cables don't run near any power supplies or power cables.

Cheers

Roger

Dear,

I would like to stress my problem with some more comments, once I have verified the strain gauge installation:

1. I have tried foil strain gauges and embedded strain gauges as well. For the last ones I have had the opportunity to try them when they are installed or not;
2. I have tried both kind of sensors with a DT500 and the signal is stable;
3. In order to ascertain with confidence if the DT800 is out of order, I have used three different DT800 systems and the signal keeps always unstable;
4. The signal is unstable in different environmental conditions.

Based on these assumptions, is still your belief that the problem is in the wiring or strain gauge installation? Should I try another program?

Thanks.
Eloi

Good morning Eloi,

Your comment about unstable in different environmental conditions is interesting. Could you please explain what you mean.

Also what is the specification of your bridge completion resisters?
And how are you doing your temperature compensation?

Cheers,
Roger

Good morning Eloi,

Could you please clarify one point for me?

When you say the readings are unstable do you mean they change slowly with time or do they jump around?

You are using bridge completion resisters so you can read the strain gauge. It is extremely important to use the very best bridge completion resisters for this task and not just any old resister or even high quality shunt resisters. They must be matched bridge completion resisters.

The quick check is to sample your gauges and log the data. While logging the data place your figure on one of the bridge completion resisters. Does the reading change?

Roger

Dear,

The readings are unstable because they jump around the mean. But this is only true when I am using a DT800. When I am using a DT500 the readings are relatively stable (the variation is about one micro-strain)

I am using new resistors from VISHAY micro-measurements (350ohms) to complete the bridge. I am not doing temperature compensation because the strain gagues are self-temperature compensated.

When I am talking about different environmental conditions, I mean in laboratory and in the field on a real-world bridge.

Do you have more tips?

Eloi

Good morning Eloi,

You should not discount temperature just because you are using temperature compensated gauges. These gauges are set to compensate for a particular coefficient of thermal expansion (Typically 11 micro strain per DegC for steel) If you material has a different coefficient of thermal expansion then you will still need to correct for temperature.

Cheers,
Roger