You can make your own Mode C test kit and easily correct it yourself if you are the type of guy who likes DIY ( do it yourself ). Note this will not get you out of inspections, but can save you money at official test and calibration times. ( it takes the shop less time to test and mark O.K. than to calibrate.) Check out the limits as per documentation for your equipment... some require +/- 300 ft. accuracy other countries differ. Read the calibration specs for your specific encoder.

METHOD

All the syringe is a cheap vacuum pump. You will need 60 cc size per +/-4000 ft. altitude! SO if you want to "climb" to 8000' a 120 cc syringe will work best. If you can't get big ones T in a few in parallel.

In order to stop the vacuum from pulling the syringe back to "neutral" drill some holes at equal spaces to peg through some nails or rivets long enough to catch at the mouth of the cylinder.

Pre-arrange with ATC to get their assistance in verifying your readings, they may only do it at certain times if your field is busy. Some request you phone them rather than use the field freq.

Now:

1. Set the Altimeter to the field elv. or get the QNH from ATC.
2. Then with the tube disconnected depress the syringe/s to get rid of all the air space in it before connecting to the tube.
3. Connect the syringe/s in place of the static port... it must be a "closed system" for the test with your Altimeter as your guide ref.
4. Now pull on the syringe piston at a medium pace ( disconnect the ROC and plug the tube if you want ) and "climb" to a specific altitude as specified in your calibration guide. "Hold " this Altitude by inserting rivets or nails in the holes drilled in the stem of the syringes to stop the vacuum pulling it back. Watch the altimeter for "leaks" if it is steady call ATC with the transponder on ALT and ask them for the reading. Your encoder should be within +/- 300' depending on requirements in your country. Should adjustment be required conform to the manufacturers specifications. There is a high and low adjusting screw that only works between ranges.
5. Now descend to a lower altitude by slowly releasing the syringe from say 8000' to 4500' replace the nail to peg it... check with ATC what the reading is now. Tabulate these readings and date it. I suggest you do this every six months or at annuals and compare or note any changes, this will give you some idea if there has been any change in the encoder's accuracy.

My last check on a Brand New encoder showed it to be withing +/- 150'..

Simple ... I think.

see image.... visual guide.