I would like to control the AM25T directly, and the CR1000 manual states that putting the commands in a SubScan is the way to go. Is there a CRBasic example available for this?
Thanks!
I do not think we publish any examples, apart for our very old loggers, as most people use the built in AM25T instruction (which is fast too). However, if you look at the examples of driving an AM16/32B in the manual for that product, looking at the examples where the channels are clocked through, you can use those as a basis for the AM25T. There are a couple of things you need to understand though. Firstly when you enable the AM25T it automatically switches in the internal PT1000 circuit, so after measuring that you need to clock to the first input channel. Also you need to send two clock pulses to move to the next channel not one like the AM16/32 multiplexers.
Once you have this working you can reduce some of the delays in the AM16/32 programs as it is possible to clock the solid state relay of the AM25T much quicker.
Thanks for this advice.
I would like to scan several 4-wire Pt100 sensors with the AM25T. They would be connected in series with the excitation provided directly from the CR1000. 4-wire half-bridge configuration.
With the AM25T command, I can measure the voltage drops, but I cannot turn on the excitation. If I turn it on before the AM25T command with ExciteV, will it stay on for the first measurement? For all the measurement?
If I control the AM25T manually, it seems I could use the BrHalf4W instruction.
Any thoughts on the best approach?
The excitation will not hold between measurements. You will either need to drive the AM25T at a lower level as you asked about before calling the bridge instruction multiple times or power the half bridge from a continuous supply.
You could use the 5V output of the logger for this supply as the critical thing for this measurement is to measure the voltage drop across the reference resistor which senses the current flowing through the PT100s. However, that is not ideal as you lose the option to reverse the excitation, which can remove small offset errors and there is a risk of self heating of the PT100 sensors.
Connecting the PT100 sensors in series also adds the risk that one wire break will take out all the sensors. The preferred option is to multiplex 4-wire PT100 sensors in using an AM16/32B set in 4-wire mode. If the multiplexer is dedicated to PT100s you can use one just bridge module on the logger side of the multiplexer shared between all sensors.
* Last updated by: aps on 3/6/2015 @ 3:37 AM *
Would the excitation hold for the first Pt100 measurement on the AM25T?
If you just measure voltages and avoid bridge measurement instructions (including the option to measure the AM25T temperature) and the AM25T instruction is called in the main sequence (scan) the excitation should hold throughout. It does get briefly switched off during each A/D conversion so you need to allow some extra settling time in the voltage measurement so the signal settles back to the correct value before the next reading is made.
Another thought, you could use a control port to power the sensor chain, switching it on and off to avoid excess current drain and sensor heating. Clearly you would need to limit the current drawn by ensuring there are enough sensors or additional current limiting resistors as a control port can only source a couple of mA.
As I said before you can get away with doing this as you would sense the current flowing by measuring the voltage drop across a precision resistor which is in the same chain.
I had thought about using a control port to switch the +5V, but using one directly as excitation is simpler of course. Are they equally stable?
They will be about as stable as the 5V output but the output will vary with the load due to internal series resistance.
However, as I said before it will not matter if you make the measurement across the series precision bridge resistor at the same time as any variation in the voltage output will compensated for by using the actual current flowing through the sensor in the calculation of the sensor resistance.