Calibration of Chemical Pneumatic Belmonte Esthesiometer Stimuli using a Portable Carbon Dioxide Sensor.

Abstract

 

Purpose

 

To measure the concentration of carbon dioxide (CO₂) in chemical stimuli from a pneumatic Belmonte aesthesiometer, using a portable non-dispersive infrared (NDIR) CO₂ sensor positioned at the ocular surface plane.

 

Methods

 

A portable CO₂ sensor and data logger (COZIR CM-0041, CO2Meter.com, Orlando beach, FL, USA) was used to measure the CO₂ concentration at the ocular surface plane. Measurements were recorded every second using the GasLab proprietary software. The pneumatic stimuli were delivered in 60 second intervals with the sensor at 0mm (esthesiometer tip at surface), 3mm, 5mm, and 10mm from the surface of the sensor. The flow rate was maintained at 100 ml/min and the concentrations were varied from 20% to 100% in 20% steps (10% step sizes for 5mm working distance). At least 2 minute intervals were used between measurements to allow the CO₂ concentration to reach pre-stimulus levels. The effect of restricting the mixing from the surrounding environment was determined by also making measurements when enclosing the region between the esthesiometer and sensor with a 20mm diameter tube. CO₂ concentration was also measured with 100% nominal CO₂ at a distance of 5mm with flow rate systematically varying from 50 to 200 ml/min in 50ml/min steps.

 

Results

 

Fig. 1 left panel is a summary of the effect of changing working distance on %CO₂ compared to nominal CO₂ .The solid line is ideal measurement reference line. As is apparent, there are monotonic increases in %CO₂ regardless of test distance. Also apparent is that as the distance increases so the actual and nominal %CO₂ deviates from each other. When the effect mixing from the surround was limited, there was an increase in the %CO₂ reaching the sensors, although the concentrations were not ever at nominal concentration levels. Fig. 1 right panel shows that the %CO₂ declined at the highest flow rate.

 

Conclusions

 

Chemical pneumatic stimuli concentrations decline as the distance between the esthesiometer tip and the ocular surface increases. This is expected, as the greater the column of air, the more the mixing with the surrounding, and the more the dilution of the %CO₂. This highlights the importance of calibration and the standardization of %CO₂ threshold measurements. At highest flow rates, perhaps turbulence in the stimulus air column promotes mixing reinforcing the need for calibration of these pneumatic stimuli.