Reccomend the best relay for my application

[o_ptr]

I have a NI USB 9219, 4 Ch-Ch Isolated, 24-bit, ±60V, Universal Analog Input module which I intend to use for RTD measurement (2 wires) with three pt1000 sensors. However, it is not intended for continuous measurement. I need to keep the lines+sensor grounded when I don't measure and therefore I need a TTL (5V-DC) triggered switch/relay to connect them to the NI USB 9219 only when measuring is allowed.

Which switch would you reccomend?

Thank you!

[Now_With_Underscores]

How many wires do your RTDs have (2,3 or 4)?  How many wires do you need to disconnect on your RTDs?  If you just need to stop the current flow, we only need to disconnect one wire, regardless of the number of wires on your particular RTDs.  This will require 4 SPST relays.

9481:

4 mechanical SPST relays, 50mOhm max.

9485:

8 solid state SPST relays, 200mOhm max.

If you're trying to isolate the RTDs for some particular test, we'll need to disconnect all wires, which would require 8, 12, or 16 SPST relays (for 2, 3, or 4 wire RTDs).  This would require up to 4 9481 modules, which might be cost and space prohibitive.

Note that both of these options simply disconnect the RTD and leave it floating... if we need to ground 1 or more wires, we'll need additional SPST relays (in other words, create an SPDT using two SPSTs). 

If PXI is an option, the 31 SPST PXI-2568 is a great option.

If PXI isn't an option, you might consider purchasing the 9476 DO module, which has 32 digital outputs that we could use to drive external relays.  Said external relays could be SPDT, so we'd only need 16 channels max... now you can add more RTDs ;).

If I missed any details on your application, let us know and we'll look at other options.

[o_ptr]

I will have 2 wire RTD, 3 sensors total. That makes it 6 wires. I need to disconnect all 6 of them and to ground the wires on the sensor(s) side since they come really close to my analysis cell and will perturb my signal if I don't. I really need to ground them, deconnecting only one wire won't be enough since I have to remove any potential electostatic interaction with the trapped ions. I can't afford to have any current or voltage applied on the sensor (or the connection wires leading to the sensor) while I measure my signal. I must measure the temperature only between ion measurement series.

I will take a look at your suggestions. Thing is, I have no ideea what's the voltage applied on the sensor in the 2 wire connection with the NI 9219 USB. The manual gives only the nominal ranges, max. +-60 V and max. +-25 mA. I don't know if the solid state relays are rated for this.

[Now_With_Underscores]

If our RTDs are all floating or reference the same common mode, we can get this to fit on the 8 relays on one 9485:

The 9485 is solid state, so the leakage current is relatively high at 30uA typical.  The on resistance is also high at 200mOhm.  If either of these specs won't work for you - they will, as long as your accuracy isn't super-demanding - you'll need to look at using two 9481 modules and the above diagram.  If the RTDs for some reason aren't floating or at the same reference, you'll need 12 realys total, so three 9481 modules or two 9485 modules.

Have a great day!

[Now_With_Underscores]

If you're driving RTD100s, I'd recommend the 9217, as it's half the price, more accurate and supports 2-wire RTDs, but only supports RTDs up to 400 Ohms (i.e. RTD1000 units won't work).  Both modules drive 1mA into the RTD, so the voltage is the resistance times 1mA... for an RTD100, the maximum voltage will be under 300mV.  Solid state relays will work, but the leakage current will add some inaccuracy (30uA/1mA = 3% current leakage for 9485 versus 10uA/1mA = 1% leakage current for 9485).