I have a DT80 connected to a CEM20 (id#2). The CEM20 has one 4 wire RTD and six 3-wire RTDs connected to it, reading in at about 1Hz.
They are Pt385 RTDs with 100 ohm resistance at 0C. I'm using the II parameter to excite at 2.5mA and, for the moment I am just trying to read ambient temperatures.

The 3 wire RTDs all read between 16 and 17C while the 4 wire reads 22C, when they should all be the same. I can't figure out what I'm doing wrong!

The three wire RTDs are from two different manufacturers, and I recently had a local calibration lab check them all and they all behaved within spec in the range of 0 to 100C.

For each of the 3W channels I jumpered the * and + terminals with a very short (maybe 2cm?) piece of 18AWG copper wire. I then connected the single wire to the *, and then connected the two common leads to the - and #.

Any ideas as to what I'm missing or doing wrong? The DT80 / 3W setup (hardware or software) seems to be the only common factor.

Example readins;
203PT385(II,3W,"Temp_50kW~degC",=205CV,LM,NA,100)

209PT385(II,4W,"Temp_Anemometer~degC",=211CV,LM,NA,100)

Thanks!

Edited to add: Figured it out. I had replaced the original CEM20 cables with longer cables. I didn't think anything of it because I thought the CEM did the measurement and relayed digital signals, but I've now realized that's not true. The CEM20 is a multiplexer that sends the raw signals through to the DT80. The added resistance in my longer wires was thus not compensated for and introduced an offset into my measurements. The 4W rtd setup didn't suffer from this because, well, it's 4W.

I am curious whether the resistance of the factory-supplied cable is accounted for during a measurement or not. Unfortunately I don't have it with me so I can't check at the moment. To be confirmed later...

Hi fireandlasers,

Your observation is spot on; however, there is one more thing. The current series 4 logger is compatible to read 4-wire only, the 2-wire or 3-wire will give you an incorrect reading because of the extra resistance that we have in the internal relay (solid state relay).

You will not see similar issue on any older series.

Best regards,
dataTaker Expert