M631 and M641 are programmable real-resistance decades with parameters designed specifically for RTD sensors' simulation and testing of RTD evaluation units like temperature regulators, transducers, etc. The core function is still resistance so you can as well calibrate ohmmeters and other resistance based meters easily.
 
The main difference between M6x1 RTD simulators is combination of accuracy and load capacity. M641 with its 5 W load capacity is suited for industry where M631 with its 0.01°C accuracy and 0.001°C resolution is rather a laboratory reference. Containing some of the most stable (and expensive) foil resistors available, the M631 has temperature coefficient as good as 1 ppm/°C. RTD simulators can be used for AC application as well, see datasheet for typical frequency responses.
 
M6xx series was made to make resistance calibration as easy as it gets. Large LCD shows all related parameters including total accuracy. And there is no residual resistance or hidden absolute error so you don't have to calculate it by yourself, accuracy you see is accuracy you get. And that's not the only thing that firmware sorts out for you. Would you like the resistance shown in distance or force units? User function will do this for you. Complete recalibration? Ten minutes and off you go.
 
RTD simulators' functions can be remotely controlled via RS232, USB, LAN or GPIB interface. This way you can introduce calibration/test stage directly into production line of any resistance based sensor and reduce time required for final quality tests dramatically.