Hello,
i wonder if there is a difference between:
ExciteV (Vx1,2500,0)
VoltDiff(XX,1,mV2_5,1,True,0,_50Hz,1,0)
and
BrFull (XX,1,mV2_5,1,Vx1,1,2500,True ,True ,0,_50Hz,1.0,0)
what are possible deviations between the two commands?
Kind regards.
Hi,
I was confronted to the same question, some weeks ago, regarding the BrHalf command.
I wanted to measure a temperature through a home-made thermistor.
I used first the BrHalf command used classically to measure voltage at the pins of a thermistor with a reference resistor.
BrHalf (AirTemp_V,1,mV5000,15,Vx1,1,2500,False ,0,_50Hz,0.001,0)
It didn't work. I had to explicitly detailed the command lines to work it well:
ExciteV(Vx1,2500,0)
Delay(0,500,mSec)
VoltSe(AirTemp_V,1,mV5000,15,True,0,_50Hz,0.001,0)
?!?
Maybe because there is no delay control in the BrHalf command?
A quick explanation.
Using the ExciteV will turn on the excitation and it will stay there up until the point that the logger needs to use the D/A circuitry for another purpose. One of those purposes is to make an A/D conversion for a measurement.
So generally there is no difference in the steps between ExciteV, VoltDiff compared to BrFull BUT only if both the VoltDiff or BrFull are set only to make one single measurement.
The reason for the BUT is that if a second measurement is made in Voltdiff, e.g. due to using the option for input reversal, there is some risk of an error as the excitation will have been turned off for a brief period at the end of the first measurement. Whether or not you see an error due to this depends on the settling time of the sensor being measured.
Otemohu shows an example where this also could be the case, even for the example that "work it well". The main difference as to why the ExciteV/VoltSE may have worked better than the BrHalf is that they are forcing a 500 msec delay after turning on the excitation, but before taking the measurement. In the BrHalf alone, with the specified 0 settling time the instruction will only wait 3 msec after turning on the excitation and making the measurement. This may not be long enough for long/high capacitance cables.
Because of the issue with the excitation switching off when using ExciteV, if you want to use input or excitation reversal you are best to stick with the BrHalf and increase the settling time parameter within the instruction. 10-20 ms is normally long enough for most thermistors. This will force the logger to wait for that settling time after each stage of the measurement. The only reason not to do this is if a very long settling time after switching on the excitation is required as there are limit to the time you can enter in the measurement instruction.