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USP 797 and Environmental Monitoring

Posted in: Pharmacy

The United States Pharmacopeia (USP) publishes general chapters regarding safe production and handling of various compounds in order to protect and improve health. We at Primex sometimes get questions from labs who use OneVue Sense about our compliance with these important standards.

While we encourage our users to whom these chapters apply to read them carefully, there are some general notes I can provide. We covered USP 795 in August and want to continue with USP 797.

797 mentions our major compliance services in OneVue Sense – differential pressure, air temperature and humidity and refrigerator temperature. The ability to run reports on schedule or on demand, to receive alerts, to monitor the use of the system and to respond to alerts with corrective actions or resolutions makes OneVue an excellent fit to stay safe and prove compliance.

USP 797 – Pharmaceutical Compounding – Sterile Preparations

For ante-rooms, positive pressure must be maintained in some buffer rooms, with negative pressure in others. Classified rooms should monitor differential pressure and have a line of demarcation to note clean from dirty, such that pressure always flows from clean to dirty so as not to contaminate clean rooms.

OneVue monitors differential pressure. So you can have access to all the alerts and reports that provided in OneVue! We also offer an optional contact closure with the sensor that only alarms when the door separating the spaces is closed and pressure can normalize.

If you’re confused about how differential pressure works, you’re not alone. Let’s take this step by step. If we want to keep contaminants out of our clean room, we want the clean room to have higher pressure than its surroundings. Then when a door opens, air will flow from high to low, meaning out the door. As air flows out, dirt or germs are blown away from the clean room entrance.

If you’ve ever opened a door and felt a rush of air against your face, you were entering a higher pressure space. But in a clean room, before you open the door, you want to ensure your space is properly pressurized. Or be able to provide reports if asked to demonstrate compliance to USP 797.

So instead of opening the door to provide air flow from one space to another, we drill a small hole for a tube that leads through the sensor. Air will flow from high pressure (H on the sensor) to low (L on the sensor input) and the sensor uses a bypass configuration to characterize the flow, which is then converted to differential pressure and calibrated to known differential pressure levels to ensure accuracy.

So just like you may feel the air flow against your face, the sensor feels the air flow across the sensing element. And because the difference in pressure changes when the door is open, you can use a contact closure to only send alerts when the door is closed and pressure is allowed to properly normalize in each space.

I’ve had sites ask if they need two sensors since there are two spaces – one clean and one dirty. The answer is usually no – the differential pressure sensor measures the difference in pressure between the two spaces. So the value in OneVue or on the local display is H-L (where H is high and L is low – both are noted on the input at the bottom of the sensor). If you need a local display on both sides of the door, you could install a sensor on the clean side and one on the dirty side, but they will show the same value within the accuracy tolerance (0.02 Pa + 3% of the reading, which converts to 0.0008 inches of water + 3% of the reading). But the measured quantity is the same regardless of which side you measure from because the flow from high to low pressure is the same physical quantity. (Note that if you do change the tubes and have the one labeled H going to the low pressure space, the differential pressure reading will be negative but still quantitatively correct.)

In terms of how to adjust your differential pressure, that’s generally under the control of your HVAC expert who can adjust settings to provide more or less pressure in various rooms. We understand this gets confusing so please us know if you have questions!

OneVue monitors ambient temperature and humidity via the A100Q service. Clean rooms should be at 20⁰ C or cooler and at relative humidity below 60%, which is well within the range of our system. USP mandates daily documentation but also notes continuous recording devices. By offering continuous monitoring, OneVue automates those records and adds the alerting functionality to let your HVAC expert know when temperature or humidity goes out of regulated range.

Knowing your monitoring system remains accurate is important. USP 797 requires that temperature and humidity monitoring devices be verified for accuracy at least every 12 months or as required by the manufacturer. Primex manufacturer guidelines are to replace the sensor every 3 years in an abundance of caution. Our sensors are remarkably stable over time unless dropped or otherwise physically damaged. In cases of physical damage, we do recommend replacement immediately.

Documentation needs also require temperature logs for refrigerators that are used. Our Temperature Sensors, or T101Q service, can handle that! CertiTrak probes show an accuracy within 1.0⁰ F between -40-221⁰ F (and within 0.5⁰ C between -40-105⁰ C).


We know you’re busy. You have to be compliant with regulations and it’s a priority to be safe for your staff and for the patients who use your services. OneVue provides a full environmental monitoring portfolio to handle your reporting and offers visibility to alerts and dashboards when you need to pay attention to an HVAC or refrigeration problem. Please contact us if you have questions or we can provide additional details.

This blog post was written by Primex's Healthcare Product Manager, Katie McMillian, and Product Management Associate, Connor Hutson.

October 28, 2019
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