Why can't I zero the Height Correction Unit (HCU) on my hNIBP system


The Height Correction Unit (HCU) supplied with the hNIBP system (https://www.adinstruments.com/products/human-nibp-nano-height-correction-unit-hcu)may on occassions not zero, preventing recording in LabChart.  This is generally caused by excess fluid in the system. 



The HCU adjusts the pressure recording relative to positional changes of the hand, thus accounting for orthostatic changes.  The end which attaches to the finger (rectangular shaped) is a pressure sensor, while the other end (round shaped) is a fluid bladder (Paraffin oil).  These are connected by a clear tube which is filled with the same fluid.  The volume of fluid in the system can sometimes change (reasons unknown), it’s generally this which causes zeroing issues as the pressure in the system is too high.


This easiest way to resolve this is to reduce the fluid volume.  This is a relatively easy task, but care needs to be made not to introduce air bubbles, not to burst the bladder or to damage the plastic connectors.


Please see attached document (supplied by FMS) for full procedure (click here).


Handy hints:


Use a very sharp, thin bladed, knife.  A “Stanley Knife” for example.


If air bubbles are seen near the end of the tubing, after being cut, make sure there is a small “bulb” of fluid on the end of the reservoir before refitting the tubing.  Also keep the tubing in a vertical position as to keep the air bubble at the end.


It’s usually a good idea to have a “bulb” of fluid on the end of the plastic connector before re-inserting the tubing.  This helps avoid air bubbles being formed in the tubing on re-insertion.


Multiple small sections of tubing my need to be removed to avoid “major” air bubbles.


After the system is zeroed, use a standard ruler to check height differentials between the sensor and the bladder.


If you don't feel confident in trying the above steps, we recommend contacting ADInstruments Technical Support by clicking HERE.