Multi-parameter Water-Quality Probes (YSI/Hydrolab)

Lecture 4

Multi-parameter probes

• PROS– Collect multiple metrics of water quality at the

same time– Internal data logger – can collect in

unattended (log) mode

• CONS– Require calibration and maintenance– Expensive ($5-10K and up)

YSI (Yellow Springs Inc)

https://www.ysi.com/ysi/Products

YSI sensors

HACH - Hydrolab

http://www.hydrolab.com/sondes.asp

Some Hydrolab sensors

Calibration

• For every sensor (except temperature)• Follow manual and fill out log (keeps track

of sensor condition/status)• Required before every deployment and

should be tested again after retrieval• Should test standards using a separate

meter/test strip if possible• Most calibrations are one-point, some 2-

point

2 point calibration

Data in this range (interpolated)

Sequence

• Change D.O. membrane day before!!!• Check temperature against thermometer• Conductivity first – 1 pt cal (typically 50mS)• pH second – 2 pt cal (typically pH 7 and 10)

should be about 180mV difference• Turbidity and/or chlorophyll – 2 pt cal, see

manual for correct protocol and standards.• D.O. last, needs to equilibrate in H2O-

saturated air. Most likely to drift over time.

Calibration tips

• ALWAYS RINSE between calibration standards with water and some old standard. AVOID contamination.

• Save $$$ by knowing your chemistry…BUT check self-made standards with a different instrument.

• pH sensors go bad – CHECK for a 180mV difference, if less than 165mV then replace.

D.O. issues

• Frequent membrane changes ensure good data! Become good at this procedure!

• No bubbles! No wrinkles! Membrane should be firm, but not

• Allow membrane to stabilize overnight for best results.

• Calibrate in water-saturated air, membrane must be dry.

D.O. tests

• Check D.O. sensor function by:– Disconnect power and wait 2 mins. Readings should start high

and drop towards 100% saturation (see D.O. sat table for mg/L value at that temperature)

– Sensor should read 100% in water-sat air, and drop to 30% or less in STILL water.

• Sensor/sonde must be in moving water for accurate measurement. At least 1 ft per sec (30cm per sec). Use stirrer attachment, or raise-lower sonde the necessary amount during measurement.

• During this period the D.O. reading will increase to a plateau, this is the correct reading. Should be within 80-120% of saturation value for that temperature (see D.O. sat table!)

• Keep sonde cool and wet (wrapped in a wet white towel) to easily check if sensor needs to be recalibrated.

Pre-deployment

• Calibrate at least 24hrs before deployment.• Set up to log at your desired interval (usually

every hour on the hour)• Leave instrument logging in the water-filled cup

in the lab. This is a pre-deployment dataset.• Do this again after retrieval for a 24 hr post-

deployment dataset. The two should be similar, if not, there were sensor problems that occurred during the deployment.

Lab

• Calibrate 2 YSI’s.

• Setup for unattended monitoring (hourly)

• Leave in lab overnight for pre-deployment data

• Deploy one at pier for 1 weeks.