In all my years of training, responding, and now teaching others, my NO-GO has always been 10,000 ppm. But why 10,000 ppm? Is this a Haywood thing, or is this based on science and the limitations of most direct reading instruments (DRI)? Well, here are the facts as to why 10,000 ppm should be your team’s NO-GO limit!
Once NH₃ gets above 10,000 ppm, we need to be very careful about treating a direct-reading instrument (DRI) as giving a quantitatively accurate concentration. Although there is no single accuracy specification for “most NH₃ direct-reading instruments,” it depends heavily on the sensor technology and, especially, the rated measuring range of the particular sensor. For many common portable industrial-hygiene instruments using electrochemical NH₃ sensors:
| NH₃ concentration | What you can generally expect |
|---|---|
| 0–100 ppm | Usually reasonably quantitative |
| 100–1,000 ppm | Generally good quantitative measurement if within sensor range |
| 1,000–5,000 ppm | Increasingly dependent on sensor/model |
| 5,000–10,000 ppm | Near the upper end of many sensors; accuracy becomes more questionable |
| >10,000 ppm | Often outside the specified measurement range |
| Very high concentrations | May read over-range, saturate, or potentially behave nonlinearly |
For example, one Membrapor NH₃ sensor has a 0–10,000 ppm nominal range, but its specifications include an output linearity of <5% of full scale and a maximum overload of 15,000 ppm.
Other NH₃ electrochemical sensors are actually designed for much lower ranges. A SemeaTech NH₃-5000 sensor, for example, is linear through 5,000 ppm but specifies 10,000 ppm as the MAXIMUM overload, not as its measurement range.
And Dräger has an NH₃ sensor with a MAXIMUM measuring range of 10,000 ppm, while its default range is only 1,000 ppm.
What happens above 10,000 ppm?
This is the important distinction: 10,000 ppm = 1% NH₃ by volume
If you put an instrument with a 0–10,000 ppm range into 15,000, 20,000, or 50,000 ppm NH₃, we cannot assume that a displayed number is accurate within ±X%.
The sensor may:
- go into an over-range condition
- saturate
- become nonlinear
- display its maximum value
- take considerably longer to recover
- experience a temporary zero shift
- potentially suffer sensor damage depending on the sensor design
For example, DD Scientific has an NH₃ sensor with a 0–10,000 ppm measurement range, while another version is only 0–5,000 ppm with 10,000 ppm specified as the maximum overload.
Suppose an instrument specifies:
Accuracy = ±5% of full scale and the NH₃ sensor has a 10,000 ppm full scale.
That means: ±500 ppm —not ±5% of whatever concentration you’re measuring. So at 8,000 ppm, a ±5% FS specification could mean approximately: 7,500–8,500 ppm. And once you’re above the 10,000 ppm range, that accuracy specification no longer applies.
This becomes especially important with ammonia because OSHA/NIOSH’s IDLH is 300 ppm, so 10,000 ppm is more than 33× the IDLH concentration. At those concentrations, I would treat a typical portable electrochemical NH₃ DRI primarily as an alarm/over-range instrument.
If you need to know whether you’re dealing with 10,000 vs. 20,000 vs. 50,000+ ppm, I’d want an instrument specifically designed and calibrated for that range—potentially using a high-range NH₃ sensor, dilution system, or optical/laser technique, depending upon the application.
Interestingly, a recent evaluation comparing electrochemical NH₃ sensors with high-end laser instruments found good agreement at concentrations up to 200 ppm, but that study doesn’t establish accuracy at 10,000+ ppm.
I would not assign a generic accuracy such as ±10% to an NH₃ DRI above 10,000 ppm. If the instrument’s specified range ends at 10,000 ppm, concentrations above that should generally be regarded as over-range rather than quantitatively accurate measurements.
NOTE: I used ChatGPT and Google Gemini to help with this post.