Question and then an "illustration"/example that may help (as additional info, if needed)...
C, Does your hose have a good schrader valve at the end? I heard of some HF testers having low #'s
With the Schrader valve (check valve) at the end, the complete hose length and volume effectively becomes a reservoir or "tank" and doesn't add to the cylinder volume. So like below (even if they have the same exact compressor), the size of tank doesn't matter for the amount of pressure achieved (although the time would increase)
Btw, If the check valve is/was closer to the gauge then the hose volume would increase the cylinder volume and decrease cylinder pressure
Last edited by multihdrdr; 06-22-2017 at 10:01 PM.
The farther you get from the "Spark Plug" end of the hose with a check valve, the more you in increase the cylinder volume which decreases your MEASURED compression pressure.
Still fine for cyl to cyl comparison readings, but not for absolute measurements.
This looks like a kit similar to yours...it has a schrader valve at the end
The hose is significantly filled before the needle ever moves. The pressure fitting at the spark plug keeps the hose/meter filled with air during the recycling of the piston so that volume doesn't have to be replaced on each stroke.
The way I was taught and directions I read on using a CT was to cycle the piston till the meter stops moving. Usually that is 3 or 4 piston strokes.
Since the CT hose is reinforced rubber it tolerates pressure and doesn't expand.
The diaphragm in the meter however is designed to expand (uncoil) with an increase in pressure/volume thus moving the needle to the appropriate position.
The initial pulse is the most dramatic as the whole monitoring system has to get charged and then it's just the incremental input variances coming from the cylinder on each cycle until the diaphragm has expanded to correspond to the pressure. When it stops moving, the system is in equilibrium and you have your reading.
So, based upon that, the length of the hose would merely determine the length of time (number of strokes) the piston would have to move up and down till equilibrium was established.
Now, if you were required to do the test with a finite number of strokes of the piston, then yes the total volume would have to be considered.
Even if it did matter the volume of the hose is so small to be of much consequence. the final reading would fall within the gage accuracy. These gages are not accurate to the pound. They give a reading that is close and show the difference between cylinders.
06-22-2017, 05:59 PM
