How to measure Formalin

Formalin/Formaldehyde
To whom it may concern,

I was asked to figure out how to take instantaneous formalin readings in a medical laboratory that conducts biopsies (there are actually quite a few areas in the lab where formalin is utilized) - how to best measure the ceiling levels of formalin.  We are currently using wonderful "gastec" color coded tubes to measure the levels of formalin.  We are wondering if there is a better way to obtain a more accurate measurement.  

From what I have been told, the problem with instantaneous readings of formalin is that methanol interferes, therefore when readings are taken; the methanol interacts with the Formalin, causing the results to not truly be representative of the actual formalin concentration.  

We are looking for a device that will measure the instantaneous formalin concentration, and that will not interfere with methanol.  Does anyone know of a device, and the name of the device?

I appreciate all help.