Hello,

I am currently undergoing a project which aims to carry out load test for different materials so as to measure the amount of force exerted on the material upon impact.

I am using a DT80 datalogger and a load cell and a 2-channel transducer. The load cell I am using has details found here: http://www.pt-global.com/index.php?shop_name=pt&func=14&DPT=p&DPI=236&DCI=89&S=4781cc30ed5586f1d47bf63777db3021&rand=1241683554.

I understand that upon impact of a material; this stage lasts for a few miliseconds...and before doing such a test, I will need to set a sampling rate to the DeLogger...but my question is how?

Because if I do not set the sampling rate i.e. 500-2000 kHz then I do not think I will be able to observe any results at all since the impact process very tiny in duration.

How do I check the gauge resistance of this load cell too? Other details for the load cell- Capacity- 10,000 Kg 2mV/V Output Resistance- 705 ohms Input Resistance- 820 ohms

Any help much appreciated

Thanks
Shan

Good afternoon Shan,

DT80 is not designed to sample at these types of speed you need. The current design maximum speed for the DT80 is around 4 Hz.

Cheers,
Roger

Thanks for quick reply Roger...very much appreciated indeed

I have followed the tutorials provided by dataTaker from your website. Since I am connecting a load cell (via a 2-way transducer) to DT80, I understand I have to set the signal type to "Bridge" in the DeLogger Program right?

I connected the wiring to channel 4 of the datalogger (DT80) and followed all of the other instructions as planned. However I do not understand this stage from the tutorial pdf- "Record the zero PPM reading in the text1.dlt window"

I cannot seem to find a zero PPM reading. What's more I have spent quite a long time figuring out a controls until I can get a proper reading on the datalogger, alas i couldn't.

I tried to test the load cell by applying a force by striking the load cell surface with a hammer but I didn't get any increase in the datalogger's readings...the readings are the same as before impact...

Maybe I am missing out something simple...

I'm sorry for this long statement as I did spend quite a long time on this drop test experiment but it doesn't seem to go anywhere...please have a look at my load cell specifications (link give in my previous post). Any help is, once again, greatly appreciated.

Thanks Again

Shan

Good morning Shan,

There is no such thing as too much detail. I would rather have a long winded explanation of great detail then a simple "It doesn't work". Ok, dynamic loading introduces a whole range of new issues.

1. DT80 is never going to be fast enough for what you are trying to do. It was not designed to do the type of high speed sampling you require. The DT800 in burst mode would be ideal for this type of work. In burst mode it can sample up to 100 kHz for a short period of time. It has been designed to take a quick snap shot of just the type of measurement you are trying to do.

   To test your load cell in working correctly with the DT80 you will need to apply a static load to the load cell. A transient from a hammer blow will simply not be seen by the DT80.

2. The load cell you choose will depend upon the duration, rise times and maximum dynamic loads of the event you are trying to measure. Generally piezoelectric load cells perform much better in dynamic applications than resistance strain gauge based load cells.

   Once you are sampling fast enough if you see a loading curve that rises sharply then has a rounded peak and a falling slow that doesn't match the rising face then the load cell response frequency is slower than the event frequency. The only solution would be to use a piezoelectric load cell.

I would talk to a mechanical or civil engineer who is experienced in dynamic loading and measurement and ask them for assistance.

Cheers,
Roger

Thanks again Roger for the help.

The only problem is that I only have the DT80 datalogger at my disposal, and as a student I will have to stick to that, but i will talk to my mechanical engineer about this matter. I will also inform him of the details you have mentioned recently.

Furthermore the drop test tower expands to 8 meters in height. For now I will only limit my tests for not more than 4 meters. i.e. a drop mass will extend to 3-4 meters then will be subjected to free fall and then impacts on the test specimen.

Can you provide some scenarios where upon impact, it will be seen by the DT80? Because you mentioned that a transient blow from a hammer will not be seen or any change of value in force will be observed.

So is there any other case/scenario where DT80 will see the change and provide a peak/drop in the graph/table of results..so i can test for that and deduce

Thanks so much for the assistance.

Shan

Hi Shan,

Which Uni are you attending? There is simply no chance at all that a DT80 will go even close to what you need. You simply need a much faster data acquisition system.

I have how ever done this with a DT800. The Drop rig was some 4 to 5 m high with a Variable weight tup. But we use Piezoelectric load cells and high frequency (10 kHz) LVDT's. All instrumentation had signal conditioning modules.

Cheers,
Roger

University of New South Wales

That's great ..real swift response u made...umm..I think I will work on it tonight and see what I can do...if not then i will talk to my supervisor about this....and pass on the information you provided to him....

But of course before I do that, I need your approval first whether its ok or not to pass the information u mentioned to my supervisor. One more thing so under what conditions is the DT80 best suited for a test for load cell (drop mass)experiments?

Thanks again for the assistance

Shan

Hi Shan,

This is a public forum that any one can access so no problems with passing any information on. The only unit in our product range is the DT800.

If you want to discuss any of this with me Just phone or email me.

Cheers,
Roger

Just out of curiosity.

Was wondering if I use the newly proposed DT800 with the CURRENT load cell I was using before? Is there a chance it will work and pop out a reading?

Oh and we're working on acquiring all new instrumentation...hopefully

cheers,
Shan

Hi Shan,

You can read the load cell you have with a DT800. But I think you will find that the load cell will not react fast enough to the impulse for the drop.

You can try it if you want but I would only use a Piezoelectric load cell in a drop rig.

Cheers,
Roger

"...the load cell will not react fast enough to the impulse for the drop....."

In terms of the signal, how much information are we NOT able to obtain using this load cell? i.e. accuracy etc..

Cheers
Shan

Hi Shan,

Can't tell. You will need to look at the shape of the impulse.

Cheers,
Roger

Kk no worries I'll check that out