effect of column length on pressure

[mk12]

I was using 19x100mm 5 micron C18 waterxbridge column with 10ml/min, even with 100% water backpressure was 1000 psi.
i just bought another 19x250 mm ,5 micron, C18 Waterxbridge column..back pressure is more than 4000psi( my pump maximum limit) with 95% H20:5% ACN 10ml/min flowrate.
I can not use it now cause most of my method need 95% aqueous as starting gradient. Why the backpressure is so high? i thought it would be around 2500 psi with 100% water before I bought it. In the papers they sent with column they have used 80:20 ACN:water with 16ml/min flow rate and reported backpressure was only 1000 psi.

Can somebody explain me why the back pressure is so high when the colum lengght is only 2.5 time more.

please help

[bisnettrj2]

Ok, stupid question time:

1. What's the instrument backpressure without the column installed?

2. What's the backpressure without the column hooked to the detector?

3. What's the backpressure with 100% ACN?

4. Have you called Waters about this yet?

[juddc]

2 questions:

Has the temperature of the column changed?
What's the pressure with 100% water?

FWIW, I have occasionally seen relatively large changes in back pressure from NEW columns of the same type run under identical conditions from a number of manufacturers, Waters included.

[mk12]

Hi! I apologize for posting this question without checking all the possible mistakes first..i checked again and what i found the outlet peak tubing was of smaller size than regular peak tubing for this machine. I sincerely apologize to people who answered my question for wasting their time

[bisnettrj2]

Don't worry about it. We'll all file this away as something to check if we find ourselves in a similar situation.

[HW Mueller]

mk12, that´s a pleasant surprise, coming back with further tests results which remove another strange questionmark.