Before you read this - I'm pretty much feeling my way in this area - so have pity if my errors are obvious and basic...

I have a load cell, which provides its data to an LED display. I want to also log the load it registers using my DT800. The LED unit has terminals: Signal +/- and Excitation +/- Excitation is 10 VDC.

If I connect Signal +/- to the dataTaker on a channel type V, I get good, linear readings on the dataTaker between 0 and 5 mV in the range I am interested in.

This is good and solves my problem of capturing data from this load cell. However, my problem is, when dataTaker is logging, it "spikes" the LED unit readout, as though it is introducing voltage into the line. i.e When I send a HA command, the LED is stable. once I send GA, I receive spikes (which seem related to the 2 second acquisition cycle) (RA2S).

dataTaker itself does not "spike", its data is good, the LED unit will spike at different (randomly varying ?) amounts equating to 0-2 mV. A voltmeter across the dataTaker +/- terminals also registers the spike. Is dataTaker filtering out something on itself?

I have tried connecting the Excitation (-) to Common Analog Ground on the DT in case this was ground related - no change. In case it has any bearing, There are also 4 channels reading strain gauges: Full schedule as below:

RA2S 
1BGI(120,I1000) 
2BGI(120,I1000) 
3BGI(120,I1000) 
4BGI(120,I1000) 
5V(FF4)

Channel 5 being the relevant one here... What basic error am I making here ?

Jim

PS: I can also hear interference on AM radio coinciding with the DT acquisition cycle - I don't expect that's relevant, but I mention it for completeness.

You have connected an additional (and active) component into a sensitive circuit (the load cell). Each time the DT800 samples you are changing the basic characteristics of the circuit, so the display runs through a short error cycle (the spikes).

You need to see if the display has an output signal and connect that to the DT800 rather than the load cell itself. If the display doesn't have an output, I would replace it with one that does.

Probably other approach you could take, but that is what I would be doing.

Thanks for answering, and I'm sure you are correct.

Unfortunately there are no "Output" terminals available at the LED display. Is there no way to make the DT800 measure the voltage "passively" rather than "actively" ?

Connecting and disconnecting a voltmeter across the signal does not cause this effect - and that's all I really want the DT to do with this channel.

Sorry if I'm being thick here...
jimdunn

Good morning Jim,

Sorry but no. In all probability the multimeter will also be affecting the readout. It just the multimeter is on line all the time so you if you are getting an error it will be a constant one. The dataTaker reads the signal for about 30 mSec so you are seeing spikes.

You have the chose of either using the display or the dataTaker or get a load cell with a duel bridge so the display can be on one bridge and the dataTaker on the other.

Cheers,
Roger

Thank you both.

I'm going to go with Roger's solution of a dual bridge load cell, since this has the advantage of some redundancy and cross checking of data.

Jim