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Sensor tolerances from ASHRAE Guideline 13
How close should a BAS reading be to your meter? These are the values many specifications use, with the source, and a plain note on what is not covered.
What Guideline 13 is
ASHRAE Guideline 13, "Specifying Building Automation Systems," is a guide that engineers use when they write a BAS specification. It is not a code. One of its tables lists how accurate a reported value should be, and many project specifications copy that table.
The numbers are end to end. They include the sensor's own accuracy, the controller's analog-to-digital conversion and any loss in transmission. So the fair test is to compare what the BAS shows with a reference meter held at the sensor.
The reporting accuracy values
| Measured variable | Reported accuracy |
|---|---|
| Space temperature | ±1 °F |
| Ducted air temperature | ±1 °F |
| Outside air temperature | ±2 °F |
| Dew point | ±3 °F |
| Water temperature | ±1 °F |
| Delta-T | ±0.25 °F |
| Relative humidity | ±5% RH |
| Water flow | ±2% of full scale |
| Airflow, terminal units | ±10% of full scale, from 10% to 100% of scale |
| Airflow, measuring stations | ±5% of full scale |
| Airflow, pressurized spaces | ±3% of full scale |
| Air pressure, ducts | ±0.1 in. w.g. |
| Air pressure, space | ±0.01 in. w.g. |
| Water pressure, absolute and differential | ±2% of full scale |
| Electrical: amps, volts, watts, power factor | ±1% of reading, not counting utility meters |
| Carbon monoxide (CO) | ±5% of reading |
| Carbon dioxide (CO2) | ±50 ppm |
Source: ASHRAE Addenda g, h and i to Guideline 13-2007 (ashrae.org). The guideline describes these as values for typical HVAC applications and notes that industrial and process work may need tighter accuracy. Check the current edition and your project specification before you sign off on a point.
Reading "percent of full scale"
Percent of full scale is based on the top of the sensor's range, not on the reading in front of you.
- A terminal unit flow sensor scaled 0 to 1,000 CFM at ±10% of full scale is allowed ±100 CFM anywhere from 100 to 1,000 CFM.
- A water pressure transmitter ranged 0 to 100 psi at ±2% of full scale is allowed ±2 psi.
Percent of reading is different. ±1% of reading on 40 amps is ±0.4 amps.
How to check a point
- Put a reference meter at the sensor, in the same air or water the sensor sees.
- Let both readings settle. Averaging sensors need several readings across the coil, then an average.
- Read the BAS value at the same moment and write down both numbers.
- Compare the difference with the tolerance in the project specification, or with the table above when the specification points to Guideline 13.
Your reference meter has its own accuracy. A meter that is only good to ±1 °F can't prove a sensor is within ±1 °F, so record which meter you used.
Where the workbook and the app use field practice instead. Not every pass criterion in the Test Guide comes from Guideline 13. These are common field values, and they are labeled that way:
- Motor amps: within ±5% of a clamp meter. The guideline's electrical value is ±1% of reading, which suits power meters more than a clamp meter check at startup.
- Valve and damper position: matches the command within ±5%.
- VFD speed: matches the command within ±2%.
- Generic 4–20 mA points: value matches the scale within ±2% of span.
The project specification always wins over both lists.
Let the app do the comparison
Enter the BAS reading and your meter reading in the free Panelside app and it tells you pass or fail. The checkout workbook carries the same Test Guide for all 21 point types.
Coming soon. The app and the store are not open yet.