diff --git a/src/shearwater_predator_parser.c b/src/shearwater_predator_parser.c
index c7a529c..ce71771 100644
--- a/src/shearwater_predator_parser.c
+++ b/src/shearwater_predator_parser.c
@@ -375,9 +375,11 @@ shearwater_predator_parser_samples_foreach (dc_parser_t *abstract, dc_sample_cal
 			sample.temperature = temperature;
 		if (callback) callback (DC_SAMPLE_TEMPERATURE, sample, userdata);
 
-		// PPO2
-		sample.ppo2 = data[offset + 6] / 100.0;
-		if (callback) callback (DC_SAMPLE_PPO2, sample, userdata);
+		// PPO2 -- only return PPO2 if we are in closed circuit mode
+		if (data[offset + 11] & 0x10 == 0) {
+			sample.ppo2 = data[offset + 6] / 100.0;
+			if (callback) callback (DC_SAMPLE_PPO2, sample, userdata);
+		}
 
 		// CNS
 		if (parser->petrel) {