DH200 and DH500 fittings are sometimes known as gland and collar fittings, Autoclave fittings, or High Pressure fittings. All of these fittings use stainless steel 1/4 inch tubes. The seal is made by the end of the tube. A gland allows the tube to screw into a port. A collar screws directly onto the tube using a left hand thread. The collar allows the gland to put force on the tube, and allows the gland to keep the tube in place.
You can get DH200 and DH500 fittings and tubing from Fluke Calibration. HiP calls DH200 fittings LF4, and DH500 fittings HF4. Autoclave Engineers calls DH500 fittings F250C (9/16-18 thread) and DH200 fittings SF250CX (7/16-20 thread).
Internal threads (external on tubing) are the same for DH200 and DH500 fittings and are ¼-28 LH.
Note: A common mistake with DH200 and DH500 fittings is that the collar is not always screwed on far enough. If this is the case, then the sealing surface on the tube can not touch the sealing surface in the port. This will cause huge leaks no matter how hard you screw in the gland. Install the collars properly and they work great.
In no case should a tube or nipple threaded for a DH500 collar be used with a DH200 collar and gland. There is a possibility, either during initial assembly, future maintenance or trouble shooting, that the collar may back out and that the gland may engage by only a few threads. This would significantly lower the pressure capability of the connection and could create a hazardous condition.